Chapter 11 — Aldehydes Ketones And Carboxylic Acids

Fig 1 — Educational Diagram: Nomenclature

What is nomenclature? Nomenclature is the systematic method of naming organic compounds so that each name uniquely identifies a structure. For aldehydes, ketones and carboxylic acids we follow IUPAC rules (with common names still widely used).

General principles (IUPAC)

• Select the longest carbon chain that contains the functional group (carbonyl for aldehydes/ketones, carboxyl for acids).
• Number the chain so that the principal functional group gets the lowest possible locant. For carboxylic acids the carbon of the –COOH group is always C-1.
• Use the appropriate suffix for the principal functional group: -oic acid (carboxylic acids), -al (aldehydes), -one (ketones). If multiple functional groups are present, the highest priority group becomes the suffix and others are named as prefixes.
• Indicate the position of the carbonyl in ketones by a number (e.g., pentan-2-one). Aldehyde carbonyl is terminal and does not need a number in an open chain (the carbonyl carbon is C-1).
• For cyclic compounds: use names like cyclohexanecarboxylic acid (for a ring bearing –COOH) or cyclohexanone (for a ketone on a cyclic ring). For an aldehyde attached directly to a ring, use the suffix -carbaldehyde (or commonly the trivial aromatic name, e.g., benzaldehyde).

Specific rules & examples (short)

• Aldehydes: general formula R–CHO. Suffix -al. Example: CH3CHO = ethanal (common name: acetaldehyde). For a benzene ring with an aldehyde, the name is benzaldehyde (C6H5CHO).
• Ketones: general formula R–CO–R'. Suffix -one. Number the carbonyl position: CH3COCH3 = propanone (common name: acetone); CH3CH2COCH3 = butan-2-one.
• Carboxylic acids: general formula R–COOH. Suffix -oic acid. Example: CH3COOH = ethanoic acid (common name: acetic acid). If the acid is attached to a ring use names like benzoic acid (aromatic) or cyclohexanecarboxylic acid.

Numbering priority (short list)

• When several functional groups are present, priority for suffix naming (common order relevant here): carboxylic acid (>) aldehyde (>) ketone. The highest priority group gets the suffix and C-1.

How to name step-by-step (example: CH3–CH(CH3)–CH2–CHO)

1. Longest chain containing CHO has 4 carbons → parent = butanal.
2. Number from the aldehyde end so CHO is C-1.
3. There is a methyl substituent at C-3 → 3-methylbutanal.

Common vs IUPAC names (examples)

• Formaldehyde = methanal (HCHO)
• Acetaldehyde = ethanal (CH3CHO)
• Acetone = propan-2-one (CH3COCH3)
• Acetic acid = ethanoic acid (CH3COOH)

Tips

• Always ensure the carbonyl-bearing carbon gets the lowest possible number.
• For aldehydes carbonyl is terminal; for ketones it is internal and must be numbered.
• Learn common trivial names for everyday compounds (acetone, acetic acid, benzaldehyde) because they appear often in problems.

Fig 2 — Educational Diagram: Structure and Bonding

Overview: Aldehydes, ketones and carboxylic acids all contain the carbonyl group (C=O). The structure and bonding around the carbonyl (and the carboxyl in acids) determine their geometry, polarity, reactivity and physical properties.

Carbonyl group (C=O):

• Hybridisation & geometry: The carbonyl carbon is sp2 hybridised. Three sp2 orbitals form sigma bonds to two substituents and one oxygen; the unhybridised p orbital overlaps with oxygen’s p orbital to form the π bond. The geometry around the carbonyl carbon is trigonal planar (approx. 120°).
• Bond lengths & strengths: Typical C=O bond length ≈ 1.20 Å; stronger and shorter than C–O single bonds. In isolated C–O single bonds (e.g., alcohols) length ≈ 1.43 Å.
• Polarity: C=O is strongly polar because oxygen is more electronegative than carbon. The carbon bears a partial positive charge (δ+) and is electrophilic; the oxygen bears a partial negative charge (δ–). This polarity explains nucleophilic addition reactions to carbonyls.

Resonance and mesomeric effects:

• In simple carbonyls (aldehydes/ketones) resonance contributor O(–)=C(+)-R is less important than the neutral form but explains some charge separation.
• In carboxylic acids and carboxylate ions resonance is key: the carboxylate ion (R–COO–) has two equivalent resonance forms, delocalising the negative charge over both oxygens. This delocalisation stabilises the conjugate base and is the main reason carboxylic acids are significantly acidic (pKa ≈ 4–5 for simple aliphatic acids).

Acidity and α-hydrogens:

• Carboxylic acids: acidity arises from resonance-stabilised carboxylate anion and inductive effects of substituents. Typical pKa for acetic acid ≈ 4.76.
• α-Hydrogens in aldehydes/ketones: hydrogen on the carbon adjacent to C=O (α-carbon) is acidic relative to alkanes because the enolate formed after deprotonation is resonance-stabilised. Typical pKa of α-H in simple ketones ≈ 19–21 (much less acidic than carboxylic acids).

Hydrogen bonding and aggregation:

• Aldehydes and ketones have C=O which can accept hydrogen bonds but cannot donate; they have moderate boiling points (higher than hydrocarbons but lower than alcohols).
• Carboxylic acids can both donate and accept H-bonds via –OH and =O; they commonly form cyclic dimers in nonpolar solvents (dimerisation increases boiling point and reduces volatility).

Conjugation and effect on properties: Conjugation of the carbonyl with a C=C or aromatic ring (for example in α,β-unsaturated carbonyls or benzaldehyde) lowers the C=O stretching frequency, alters reactivity (conjugate addition possible) and changes colour/UV absorption.

Summary of important structural consequences:

• Electrophilic carbon (carbonyl) → susceptibility to nucleophiles.
• Resonance in carboxyl group → equalisation of C–O bond character and higher acidity.
• Alpha-hydrogen acidity → enol/enolate chemistry and tautomerism.
• Hydrogen bonding/dimerisation for carboxylic acids → higher boiling points and distinct IR signatures.

Fig 3 — Educational Diagram: Physical Properties

Overview: Aldehydes (R–CHO), ketones (R–CO–R') and carboxylic acids (R–COOH) contain the carbonyl group (C=O). Their physical properties are governed mainly by polarity of the C=O bond, ability to form hydrogen bonds, molecular size/shape and specific intermolecular associations (e.g., dimerization of acids).

Polarity and dipole: The C=O bond is strongly polar (C is partially positive, O partially negative). This gives aldehydes and ketones appreciable dipole–dipole interactions and makes them good hydrogen-bond acceptors (they can H‑bond with water or alcohols). Carboxylic acids have both a polar C=O and an O–H, so they are polar and capable of stronger H‑bonding.

Hydrogen bonding: Aldehydes and ketones cannot form intermolecular O–H…O hydrogen bonds (they lack an H on oxygen) so their intermolecular H‑bonding is weaker than alcohols. Carboxylic acids can form strong hydrogen bonds and typically exist as cyclic dimers (two molecules joined by two H‑bonds) in non‑polar solvents and vapour; this dimerization raises boiling points and lowers vapour pressure.

Boiling point (bp) and melting point (mp): General trends—bp and mp increase with molecular weight because London dispersion forces grow. For a given molecular weight: hydrocarbons < ethers ≈ ketones/aldehydes < alcohols < carboxylic acids (due to hydrogen bonding and dimerization). Branching lowers bp by reducing surface area (weaker dispersion). Conjugation with aromatic rings can alter polarity and packing, affecting bp/mp.

Solubility in water: Small aldehydes/ketones (up to ~C3–C4) are miscible or highly soluble in water because they accept hydrogen bonds from water. Solubility decreases rapidly with increasing alkyl chain length (hydrophobic effect). Carboxylic acids are soluble in water when small because they H‑bond and can ionize (form R–COO−) — ionization increases water solubility; solubility still falls with longer chains.

Vapour pressure and volatility: Aldehydes and ketones tend to be more volatile than comparable alcohols and acids. Carboxylic acids have lower vapour pressures because dimers behave like larger particles in the gas phase.

Spectroscopic signatures (physical identification): The carbonyl stretch appears strongly in IR near ~1700 cm−1 (aldehydes/ketones). Carboxylic acids show a broad O–H stretch (≈2500–3300 cm−1) and a carbonyl near 1700 cm−1 shifted by H‑bonding/conjugation.

Summary of key causes: (1) C=O polarity → dipole interactions and water solubility for small members; (2) Presence/absence of O–H → ability for H‑bonding (strong in acids, absent intermolecularly in aldehydes/ketones); (3) Molecular weight and shape → dispersion forces and packing; (4) Specific associations (dimers) → large effects on bp/mp and vapour pressure.

Fig 4 — Educational Diagram: Spectral Identification

What is Spectral Identification?
Spectral identification uses spectroscopic techniques (IR, NMR, Mass, UV‑Vis) to recognise functional groups and molecular structure. In Class 12 (Aldehydes, Ketones & Carboxylic Acids) the main goal is to identify the carbonyl group (C=O), O–H (in acids) and characteristic protons or fragments.

Main techniques & what they show

• Infrared (IR) spectroscopy – gives vibrational frequencies (wavenumbers in cm⁻¹) characteristic of bonds: sharp C=O stretch, broad O–H, aldehydic C–H bands.
• 1H NMR (proton NMR) – chemical shifts (δ, ppm) show types of hydrogens: aldehydic H, carboxylic acid H, α‑hydrogens next to C=O and aromatic H.
• 13C NMR – carbonyl carbons appear at very downfield shifts; helps distinguish ketone/aldehyde vs. carboxyl/ester carbons.
• Mass spectrometry (MS) – molecular ion (M+) gives molecular mass; fragmentation patterns (e.g., McLafferty rearrangement) give structural clues.
• UV‑Visible – useful for conjugated carbonyls: n→π* and π→π* absorptions shift to longer wavelengths on conjugation.

Characteristic spectral features (useful ranges)

• IR
  • C=O (aldehydes/ketones): ~1700 ± 50 cm⁻¹ (typical aliphatic ketone ≈1715 cm⁻¹; conjugation lowers to ≈1680 cm⁻¹)
  • Aldehydic C–H: weak bands near 2720 and 2820 cm⁻¹ (two bands)
  • Carboxylic acid C=O: ≈1700–1725 cm⁻¹ plus very broad O–H stretching 2500–3300 cm⁻¹ (wide, strong)
  • Alcohol O–H: broad 3200–3600 cm⁻¹ (distinct from COOH which is broader and lower)
• 1H NMR
  • Aldehydic proton (–CHO): δ ≈ 9.0–10.5 ppm
  • Carboxylic acid proton (–COOH): δ ≈ 10.5–13 ppm (very broad, exchangeable)
  • α‑H (next to C=O): δ ≈ 2.0–3.0 ppm (depends on substitution)
  • Aromatic H: δ ≈ 6.0–8.5 ppm
• 13C NMR
  • Ketone/aldehyde carbonyl C: ≈ 190–220 ppm
  • Carboxyl/ester carbonyl C: ≈ 160–185 ppm
• Mass spectrometry
  • Molecular ion peak (M+) gives molecular mass; common neutral losses: 15 (CH3), 18 (H2O), 28 (CO)
  • McLafferty rearrangement: characteristic for carbonyl compounds with γ‑hydrogen, gives a specific fragment.

How to identify a carbonyl compound in practice

1. Record IR: look for a sharp strong C=O band near 1700 cm⁻¹. If present, confirm O–H (broad 2500–3300) for acids or aldehydic C–H bands for aldehydes.
2. Record 1H NMR: an aldehyde will show a singlet ≈9–10 ppm; a COOH proton appears as a broad peak ≈11–13 ppm. Check α‑H chemical shifts and integration for the skeleton.
3. Record 13C NMR: confirm carbonyl carbon at ≈190–220 (aldehyde/ketone) or ≈160–185 (carboxyl/ester).
4. Use MS to confirm molecular mass and look for diagnostic fragments (M−15, McLafferty fragment etc.).
5. UV‑Vis: use for conjugated carbonyl systems (λmax shifts indicating conjugation or aromatic substitution).

Notes on common chemical tests (complementary to spectra): 2,4‑DNP (orange precipitate) indicates a carbonyl; Tollens' (Ag mirror), Fehling/ Benedict (red ppt) are specific for aldehydes; NaHCO3 effervescence indicates carboxylic acids.

Fig 5 — Educational Diagram: Preparation of Aldehydes and Ketones

Overview: Aldehydes (RCHO) and ketones (RCOR') are carbonyl compounds widely prepared in the laboratory and industry by oxidation, reduction or functional‑group transformation of other organic compounds. The main laboratory and industrial methods differ in choice of reagents and control of reaction conditions to give aldehyde vs. further oxidation to acids.

Main methods:

• Oxidation of alcohols
  • Primary alcohol -- controlled/mild oxidation → aldehyde. Reagents: PCC, Dess–Martin periodinane, Cu/HCl (distillation) or MnO2 (for benzyl alcohol). Over-oxidation to carboxylic acids is avoided by mild oxidants or by removing the aldehyde as formed.
  • Secondary alcohol -- oxidation → ketone. Typical reagents: acidified K2Cr2O7, PCC, or KMnO4 (careful control).
• Partial oxidation/cleavage of alkenes (ozonolysis)
  • Ozonolysis (O3 followed by reductive workup such as Zn/H3O+) cleaves C=C to give aldehydes/ketones depending on substituents: RCH=CHR' --[O3/Zn]--> RCHO + R'CHO (or R'COR if disubstituted).
• From carboxylic acid derivatives
  • Rosenmund reduction: Acid chloride → aldehyde using H2/Pd-BaSO4 (poisoned Pd) to stop at aldehyde: RCOCl --[H2, Pd(BaSO4)]--> RCHO.
  • Reduction of esters or nitriles with DIBAL-H: At low temperature (≈ −78 °C), DIBAL-H (diisobutylaluminium hydride) reduces esters or nitriles to aldehydes (quench after partial reduction). Example: RCOOR' --[DIBAL-H, −78 °C]--> RCHO.
  • LiAlH4 fully reduces esters/acid derivatives to alcohols (not to aldehydes) unless carefully controlled.
• Friedel–Crafts acylation (for ketones)
  • Aromatic ketones are prepared by acylation of benzene/arenes with acyl chlorides and AlCl3: ArH + RCOCl --[AlCl3]--> ArCOR + HCl.
• Grignard or organolithium reagents with acyl compounds
  • Reaction of R'MgX with an acyl chloride (1 equiv) followed by hydrolysis gives a ketone: R'COCl + R"MgX --[ether]--> R'COR" (workup).
• Hydration/hydrolysis of alkynes
  • Acid-catalyzed hydration of terminal alkynes in presence of Hg2+ (Markovnikov) gives methyl ketones: RC≡CH --[H2O/HgSO4/H+]--> RCOCH3.
  • Anti‑Markovnikov hydration (hydroboration–oxidation of alkynes with disiamylborane then H2O2/NaOH) can give aldehydes from terminal alkynes.
• Industrial processes
  • Hydroformylation (oxo process): Alkenes + CO + H2 (with Rh or Co catalyst) → aldehydes (important industrial route to higher aldehydes).
  • Catalytic oxidation of methanol → formaldehyde (industrial), cumene process produces acetone (industrial-scale methods differ from simple lab oxidations).

Important practical notes:

• To obtain aldehydes from primary alcohols, use mild oxidants or stop the reaction early and remove the aldehyde (distillation) to prevent oxidation to carboxylic acids.
• DIBAL-H at −78 °C is widely used to form aldehydes from esters/nitriles; Rosenmund reduction is the classical method for converting acyl chlorides to aldehydes.
• Choice of reagent and conditions (temperature, solvents, catalyst poisons) determines whether you obtain an aldehyde or proceed further to a carboxylic acid or alcohol.

Representative reaction equations (short):

• RCH2OH --[PCC]--> RCHO (primary alcohol → aldehyde)
• R2CHOH --[K2Cr2O7/H2SO4]--> R2C=O (secondary alcohol → ketone)
• RCH=CHR' --[O3, Zn/H3O+]--> RCHO + R'CHO (ozonolysis)
• RCOCl --[H2, Pd(BaSO4)]--> RCHO (Rosenmund)
• RCOOR' --[DIBAL-H, −78 °C]--> RCHO (ester → aldehyde)
• ArH + RCOCl --[AlCl3]--> ArCOR + HCl (Friedel–Crafts acylation → aromatic ketone)
• RC≡CH --[H2O, HgSO4/H+]--> RCOCH3 (hydration of terminal alkyne → ketone)

Fig 6 — Educational Diagram: Preparation of Carboxylic Acids

Overview
Carboxylic acids (RCOOH) are prepared by several laboratory and industrial methods starting from different functional groups. Key laboratory methods include oxidation of primary alcohols/aldehydes, hydrolysis of nitriles, carboxylation of organometallic reagents, and hydrolysis of derivatives (esters, acid chlorides, amides). Industrially, catalytic oxidation and specific carboxylation procedures (e.g. Kolbe–Schmitt, methanol carbonylation) are important.

• 1. Oxidation of primary alcohols and aldehydes

  Primary alcohols are oxidised first to aldehydes, then to carboxylic acids if strong/aqueous oxidants or prolonged conditions are used.

  Examples of oxidants: KMnO4 (aq, heat), K2Cr2O7/H2SO4, H2O2 with catalysts. Conditions determine whether oxidation stops at aldehyde or proceeds to acid.

  General equations:
  RCH2OH + [O] -> RCHO (aldehyde)
  RCHO + [O] -> RCOOH (carboxylic acid)

• 2. Oxidation of alkyl side chains of aromatic compounds

  Methyl or benzylic CH bonds attached to an aromatic ring are oxidised to carboxyl groups (even if other substituents present), e.g. toluene -> benzoic acid using KMnO4 or catalytic O2/metal catalysts industrially.

  Equation: C6H5CH3 + 2[O] -> C6H5COOH + H2O

• 3. Hydrolysis of nitriles (R–C≡N)

  Nitriles hydrolyse to amides and then to carboxylic acids. Two routes are common:

  • Acidic hydrolysis: R–C≡N + 2 H2O + H+ → RCOOH + NH4+
  • Alkaline hydrolysis (followed by acidification): R–C≡N + 2 H2O + OH− → RCOO− + NH3 ; then RCOO− + H+ → RCOOH
• 4. Carboxylation of Grignard reagents / Organolithium reagents

  Dry CO2 (solid CO2 / dry ice) is used to insert CO2 into an organometallic reagent, giving a magnesium (or lithium) carboxylate which on acidic work-up yields the carboxylic acid. This is a versatile method for preparing substituted carboxylic acids.

  Equation: R–MgX + CO2 → R–COO–MgX ; R–COO–MgX + H3O+ → R–COOH + Mg2+

• 5. Hydrolysis of derivatives (esters, acid chlorides, anhydrides, amides)

  Simple hydrolysis gives carboxylic acids:

  • Ester: RCOOR' + H2O (acid/heat) → RCOOH + R'OH
  • Acid chloride: RCOCl + H2O → RCOOH + HCl
  • Acid anhydride: (RCO)2O + H2O → 2 RCOOH
  • Amide (requires harsh conditions): RCONH2 + H2O (acid/heat) → RCOOH + NH4+
• 6. Oxidative cleavage of alkenes / Ozonolysis (oxidative work-up)

  Oxidative cleavage of C=C (e.g. using O3 followed by H2O2) can produce carboxylic acids when the alkene carbons are substituted by alkyl groups.

• Important industrial/large-scale methods

  • Kolbe–Schmitt reaction: phenoxide ion + CO2 (high pressure and temperature) → salicylate (used to prepare salicylic acid; precursor to aspirin).
  • Catalytic oxidation of toluene (Co/Mn/Br catalysts, O2) → benzoic acid (industrial).
  • Methanol carbonylation (Monsanto/Cativa): CH3OH + CO → CH3COOH (industrial manufacture of acetic acid).

Mechanistic notes & practical tips

• Strong oxidants (KMnO4, K2Cr2O7/H2SO4) and heating favour full oxidation to acids; mild oxidants may stop at aldehydes.
• Grignard carboxylation requires strictly anhydrous conditions; CO2 is introduced as dry ice and the reaction is quenched with acid.
• Nitrile hydrolysis proceeds via amide intermediate; prolonged hydrolysis is needed for complete conversion to acid.
• Oxidation of benzylic positions gives carboxylation even if the side chain is long (benzylic C must have at least one H).

Summary
Multiple complementary methods allow preparation of carboxylic acids depending on the starting material: oxidation (alcohols, aldehydes, benzylic CH), hydrolysis (nitriles, esters, amides), or carboxylation (organometallics, Kolbe–Schmitt). Choice depends on functional groups present, scale, selectivity and available reagents.

Fig 7 — Educational Diagram: Keto–Enol Tautomerism

Definition
Keto–enol tautomerism is a reversible chemical equilibrium between a carbonyl compound (keto form) and its corresponding enol (an alcohol with a C=C double bond). These two forms are structural isomers (tautomers) that interconvert by transfer of a proton at the α‑carbon and migration of a double bond.

General equilibrium
R1–CO–CH2–R2 ⇌ R1–C(OH)=CH–R2
(keto) ⇌ (enol)

Important point
Keto and enol tautomers are not resonance forms; they are distinct constitutional isomers that rapidly interconvert under acidic or basic conditions.

Mechanisms

• Acid-catalyzed: Protonation of the carbonyl oxygen increases the acidity of the α‑hydrogen → removal of α‑H by a base (often solvent) → gives the enol after deprotonation of the oxygen. Steps: (1) protonate C=O; (2) deprotonate α‑C → C=C; (3) deprotonate/release to form OH on α‑carbon.
• Base-catalyzed: A base (B:) abstracts an α‑H to form an enolate ion (resonance stabilized: negative charge delocalized between α‑C and oxygen). Protonation of the enolate oxygen (or carbon) yields the enol. Enolate resonance: R1–C(=O)–CHR2 ⇌ R1–C(–O–)=CHR2 (delocalized).

Factors controlling keto vs enol

• General stability: The keto form is usually more stable (favored) because of the strong C=O double bond vs C=C and O–H in the enol.
• Stabilization of enol: Enol form is favored when it gains additional stabilization by (a) intramolecular hydrogen bonding (e.g., 1,3‑diketones like 2,4‑pentanedione), (b) conjugation with an aromatic ring, or (c) aromaticity on tautomerization (rare). Example: acetylacetone (2,4‑pentanedione) exists largely in its enol form due to strong intramolecular H‑bonding and resonance stabilization.
• Solvent and catalysts: Polar protic solvents and acids/bases affect the position and rate of equilibrium. Base strongly favors formation of enolate (reactive nucleophile) used in synthesis (aldol, Claisen condensations).
• Kinetic vs thermodynamic control: Under irreversible, low-temperature conditions with a bulky, strong base (e.g., LDA), the less substituted (kinetic) enolate forms faster. At higher temperature or reversible conditions, the more substituted (thermodynamic) enolate (or tautomer) predominates.

Reactivity and importance
Enols and enolates are key intermediates in many organic reactions: aldol condensations, Claisen condensations, Michael additions, halogenation at α‑carbon, and stereoselective alkylations. In biochemistry, tautomerism of nucleotide bases (rare enol forms) can cause mispairing and mutations.

Spectroscopic differences (how to distinguish)

• IR: Keto: strong C=O stretch ~1700 cm⁻¹. Enol: O–H broad band ~3200–3600 cm⁻¹ and reduced carbonyl intensity.
• 1H NMR: Keto: α‑CH protons ~2–3 ppm (for simple ketones). Enol: vinyl proton (C=CH–) appears ~4.5–6.5 ppm; O–H proton appears downfield and often broad (~10–15 ppm if involved in H‑bonding).
• 13C NMR: Carbonyl carbon ~200–220 ppm (keto); C=C carbons for enol appear ~100–160 ppm.

Summary
Most simple carbonyl compounds exist predominantly in the keto form, but enols become important when stabilized (H‑bonding, conjugation) or when enol/enolate chemistry is exploited in synthesis. Keto–enol tautomerism is fast and catalyzed by acids and bases; understanding it is central to many organic reactions.

Fig 8 — Educational Diagram: Acidity and Factors Affecting Acidity

What is acidity? Acidity is the tendency of a species (HA) to donate a proton (H+). In quantitative terms, acid strength is expressed by the acid dissociation constant (Ka) and its negative logarithm pKa:

Ka = [A⁻][H⁺]/[HA]    and    pKa = −log Ka.

Why some organic compounds are acidic
An acid is stronger when its conjugate base (A⁻) is more stable. In organic chemistry, stability of the conjugate base is influenced by:

• Resonance (delocalization): Delocalization of the negative charge over several atoms stabilizes A⁻. Example: carboxylate ion (RCOO⁻) has two equivalent resonance forms — that makes carboxylic acids much stronger (pKa ~4–5) than alcohols.
• Inductive effect: Electron-withdrawing groups (EWG) such as −Cl, −NO₂ pull electron density away and stabilize A⁻ by delocalizing/withdrawing negative charge. The stronger the EWG and the closer it is to the acidic centre, the greater the stabilization.
• Hybridization: Negative charge on an atom with greater s-character is held more tightly (sp > sp² > sp³), so acids whose conjugate base has the negative charge on sp-hybridized carbon (alkynes) are more acidic than alkenes and alkanes.
• Atom size and electronegativity: For acids where the proton is attached to different atoms in the same group, larger atoms stabilize negative charge better (down a group). For example, HI > HBr > HCl > HF.
• Resonance donation / electron donating groups (EDG): Alkyl groups or electron-donating substituents destabilize A⁻, decreasing acidity.
• Solvent and hydrogen bonding effects: Solvation stabilizes the conjugate base; intramolecular hydrogen bonding can reduce the ability of solvent to stabilize A⁻ and thus lower observed acidity in solution.

Specific contexts from the chapter

• Carboxylic acids (RCOOH): Strong acidity among common organic functional groups because the carboxylate anion (RCOO⁻) is stabilized by resonance. Electron-withdrawing substituents (e.g., −Cl, −NO₂) at the α-, β- or aromatic positions increase acidity (chloracetic > acetic; trichloroacetic is very strong). Intramolecular hydrogen bonding (ortho-substituted benzoic acids) and steric hindrance to solvation can modify acidity.
• Phenols vs alcohols: Phenol is more acidic (pKa ~10) than aliphatic alcohols (pKa ~16) because the phenoxide ion is resonance-stabilized. Alkyl groups on phenol (electron donating) reduce acidity; electron-withdrawing groups (−NO₂) increase acidity.
• Aldehydes and ketones (α-H acidity): Hydrogens on the carbon adjacent to the carbonyl (α-hydrogens) are acidic because deprotonation produces an enolate ion, where the negative charge is delocalized between oxygen and α-carbon (resonance). Typical pKa values: aldehyde α-H (~17), ketone α-H (~19–20) — approximate and solvent dependent. Greater alkyl substitution on the carbonyl carbon (as in ketones) usually decreases α-acidity compared to aldehydes because alkyl groups are electron donating and destabilize the enolate.
• Keto–enol tautomerism: Acidity of α-H is intimately related to the ease of forming enols/enolates; reactions such as aldol condensation and Claisen condensation proceed through enolate formation (base-induced deprotonation at α-position).

How to predict acidity quickly

• Identify the conjugate base and check whether the negative charge can be delocalized (resonance) — resonance > no resonance.
• Look for nearby electron-withdrawing groups that can stabilize the conjugate base via the inductive effect — closer > farther.
• Consider hybridization and the atom bearing the negative charge.
• Account for solvent and intramolecular hydrogen bonding which can raise or lower effective acidity in solution.

Takeaway: Acidity in organic molecules is not just about the H—X bond strength but primarily about stability of the conjugate base. Resonance and electron-withdrawing substituents are the most important factors that increase acidity; electron-donating groups and poor solvation decrease it.

Fig 9 — Educational Diagram: Reactions of Aldehydes and Ketones — Nucleophilic Addition

Overview
Aldehydes and ketones contain the polar carbonyl group (C=O). The carbonyl carbon is electrophilic (partial positive) and is attacked by nucleophiles. The general result is nucleophilic addition at the carbonyl carbon to give a tetrahedral intermediate (an alkoxide), which is then protonated to give an addition product (usually an alcohol derivative).

Key features of the mechanism

• Polarisation: O is more electronegative than C, so Cδ+ and Oδ−. This makes C susceptible to nucleophiles.
• Two main catalytic modes: Base-catalyzed (nucleophile is activated / generated; attacks carbon directly) and acid-catalyzed (carbonyl oxygen is protonated, increasing carbon electrophilicity).
• Steps (general):
  1. Nucleophilic attack on carbonyl carbon → formation of tetrahedral alkoxide intermediate.
  2. Protonation of the alkoxide → stable addition product (alcohol or its derivative).
• Rate and selectivity: Aldehydes are generally more reactive than ketones (less steric hindrance and often less electron donation to C=O). Electron-withdrawing substituents increase reactivity; conjugation or electron-donating groups decrease reactivity. Attack on a planar carbonyl creates a new stereocenter when applicable, usually giving racemic mixtures unless chiral control is present.

Common nucleophilic additions

• Reduction (hydride addition): NaBH4 or LiAlH4 donate hydride (H−) to form alcohols. (Base/neutral conditions: fast with aldehydes; LiAlH4 needed for esters as well.)
• Cyanohydrin formation: HCN (or KCN/HCl) adds CN− to form cyanohydrins (useful synthetic intermediates and precursors in Strecker synthesis of amino acids).
• Hemiacetal and acetal formation: Alcohols add to carbonyls. Under acid, hemiacetals (one RO and one OH on same C) form; with excess alcohol + acid and removal of water, acetals (RCH(OR)2) form — widely used as protecting groups for carbonyls.
• Imines (Schiff bases), oximes, hydrazones: Primary amines give imines (RCH=NR'), hydroxylamine gives oximes (RCH=NOH), hydrazine/phenylhydrazine give hydrazones. These are formed by nucleophilic addition followed by dehydration (loss of water) under acidic conditions.
• Grignard & organometallic addition: RMgX or RLi adds carbon nucleophiles to give, after acidic workup, alcohols (primary/secondary/tertiary depending on starting carbonyl).
• Conjugate (1,4) vs direct (1,2) addition: α,β-Unsaturated carbonyls can undergo 1,2-addition at C=O or 1,4-addition (Michael addition) at the β-carbon depending on reagent and conditions.

Mechanistic note (example: base-catalyzed addition)
Nuc− + R2C=O → R2C(−O−)(Nu) (alkoxide intermediate) → protonation → R2C(OH)(Nu).

Why this matters
Nucleophilic addition is central to synthesizing alcohols, protecting groups (acetals), C–C bond formation (Grignard), and many functional-group interconversions used in pharmaceuticals, polymers and dye manufacture.

Fig 10 — Educational Diagram: Oxidation and Reduction Reactions

Definition (Class 12 level): Oxidation is the process in which the oxidation number of an atom increases (loss of electrons) or a species gains oxygen/loses hydrogen. Reduction is the opposite: the oxidation number decreases (gain of electrons) or a species gains hydrogen/loses oxygen. (Mnemonic: OIL RIG — Oxidation Is Loss, Reduction Is Gain of electrons.)

Carbonyl context — aldehydes, ketones and carboxylic acids:

• Aldehydes (R–CHO) are readily oxidized to carboxylic acids (R–COOH) by mild oxidizing agents because the carbonyl carbon in an aldehyde has a hydrogen atom that can be removed.
• Ketones (R–CO–R') are resistant to mild oxidation because the carbonyl carbon lacks a hydrogen; strong oxidants can cleave C–C bonds to give carboxylic acids or smaller fragments.
• Carboxylic acids are generally the highest oxidized common organic functional group (short of CO2) in this context; further oxidation requires cleavage to CO2 or inorganic products.

General reactions:

• Oxidation of an aldehyde: R–CHO + [O] → R–COOH
• Reduction of an aldehyde: R–CHO + 2[H] → R–CH2OH (primary alcohol)
• Reduction of a ketone: R–CO–R' + 2[H] → R–CHOH–R' (secondary alcohol)

Common reagents:

• Oxidizing agents: Tollens' reagent (Ag(NH3)2+), Fehling/Benedict solutions (for aliphatic aldehydes), KMnO4 (acidic/alkaline), K2Cr2O7/H+ (dichromate).
• Reducing agents: NaBH4 (selective, reduces aldehydes and ketones), LiAlH4 (strong, reduces carboxylic acids, esters as well), catalytic hydrogenation (H2/Pt, Pd).

Mechanistic idea (brief):

• Oxidation of aldehydes often proceeds via nucleophilic addition of an oxygen-containing species to the carbonyl and removal of hydride from the carbonyl carbon (in terms of electrons, the carbon loses electrons → oxidation).
• Reduction of carbonyls generally proceeds by hydride transfer (H-) from a reducing agent (NaBH4 or LiAlH4) to the electrophilic carbonyl carbon, followed by protonation to give the alcohol.

Oxidation states (illustrative): Example half-reactions illustrate electron transfer. Oxidation (in water): RCHO + H2O → RCOO– + 2H+ + 2e–. Corresponding reduction: RCHO + 2H+ + 2e– → RCH2OH. These show a two-electron change for conversion between aldehyde and alcohol and for aldehyde to carboxylate.

Selective behavior important for lab tests: Tollens' reagent (Ag(NH3)2+) oxidizes aldehydes to carboxylates and reduces Ag+ to metallic Ag (silver mirror). Fehling's/Benedict's give red Cu2O precipitate for aliphatic aldehydes. Aromatic aldehydes (like benzaldehyde) are oxidized by stronger agents (KMnO4) to benzoic acid.

Practical notes for CBSE students:

• Remember that NaBH4 will not reduce carboxylic acids or esters appreciably, whereas LiAlH4 will.
• Aldehyde — positive Tollens' and Fehling's tests; ketone — generally negative (except alpha-hydroxy ketones or special cases).
• In nomenclature and redox balancing, track oxidation numbers of the carbonyl carbon to decide whether oxidation or reduction has occurred.

Fig 11 — Educational Diagram: Condensation and C–C Bond Forming Reactions

Overview: Condensation reactions in aldehydes and ketones often form new carbon–carbon (C–C) bonds. These reactions proceed via enolate (or related) intermediates and give products such as β‑hydroxy carbonyls, β‑diketones, β‑ketoesters or α,β‑unsaturated carbonyls. Common class‑12 reactions: Aldol condensation, Crossed (mixed) aldol, Claisen condensation, Dieckmann cyclisation (intramolecular Claisen), Perkin reaction, Knoevenagel condensation and Benzoin condensation.

Key principles and mechanism types:

• Enolate formation: A base (or acid in some cases) removes an α‑H forming an enolate or enol which acts as a nucleophile.
• Nucleophilic attack: The enolate attacks an electrophilic carbonyl carbon of another molecule, forming a C–C bond (addition step).
• Workup/dehydration: The initial addition product (e.g., β‑hydroxy carbonyl) may be isolated or undergo dehydration to give α,β‑unsaturated carbonyl compounds.
• Regioselectivity: Only carbonyl compounds with α‑hydrogens form enolates (benzaldehyde lacks α‑H so undergoes other reactions like Cannizzaro instead).

Main reactions (concise descriptions):

• Aldol condensation (base or acid catalysed): Two aldehydes/ketones (at least one with α‑H) give β‑hydroxy aldehyde/ketone (aldol). Under heating or strong base, dehydration yields α,β‑unsaturated carbonyl (conjugated product). Example: 2 CH3CHO → CH3CH(OH)CH2CHO → CH3CH=CHCHO.
• Crossed (mixed) aldol: Aldehyde/ketone A (with α‑H) reacts with B (may lack α‑H) to give a selective product — often controlled by using one reagent without α‑H (e.g., benzaldehyde) or by preforming enolate (LDA) for high selectivity.
• Claisen condensation: Condensation of esters (or an ester with a carbonyl compound) under strong alkoxide base to give β‑ketoesters or β‑diketones. Requires the alkoxide to match the ester leaving group to avoid transesterification. Intramolecular Claisen (Dieckmann) gives cyclic β‑ketoesters.
• Perkin reaction: Aromatic aldehyde (usually benzaldehyde) + acid anhydride in presence of base (alkali salt of the acid) → α,β‑unsaturated carboxylic acid (e.g., cinnamic acid). Useful for preparing conjugated acids used in fragrances and intermediates.
• Knoevenagel condensation: Active methylene compounds (malonic ester, cyanoacetate, etc.) condense with aldehydes/ketones using weak base (amines, piperidine) to give substituted alkenes (often conjugated). Widely used in synthesis of pharmaceuticals and dyes.
• Benzoin condensation: Two molecules of aromatic aldehyde (e.g., benzaldehyde) are coupled under cyanide (or thiamine) catalysis to give benzoin (an α‑hydroxy ketone). It proceeds by umpolung of one aldehyde to a nucleophilic species.

Practical notes and selectivity: Aldol condensations are reversible under mild base — driving to condensation product often requires removal of water or heating. Claisen requires strong, nonprotic base (alkoxide) and the ester alkoxy group must match the base. Use of preformed enolates (LDA) gives regiochemical control in mixed condensations. Many of these condensations are key C–C bond forming steps in multi‑step synthesis.

Fig 12 — Educational Diagram: Special Tests and Identification

This topic covers the classical qualitative tests used to identify and distinguish aldehydes, ketones and carboxylic acids in the laboratory. These tests are based on characteristic chemical reactions (oxidation, condensation, halogenation, acid–base) that give distinctive colour changes or precipitates.

• 2,4-Dinitrophenylhydrazine (2,4-DNP / Brady's) test: Detects the carbonyl group (C=O) in aldehydes and ketones. A yellow/orange/red precipitate of the corresponding 2,4-dinitrophenylhydrazone forms. Positive for both aldehydes and ketones (not for carboxylic acids).
• Tollens' test (Ag mirror): Distinguishes aldehydes from most ketones. Aldehydes (R–CHO) are oxidised to carboxylate and the diamminesilver(I) complex [Ag(NH3)2]+ is reduced to metallic silver, producing a silver mirror or black precipitate. Ketones generally do not give this test.
• Fehling's test: Aqueous alkaline Cu(II) solution that is reduced by many aliphatic aldehydes to red Cu2O precipitate. Aromatic aldehydes (e.g. benzaldehyde) often do not react. Ketones normally give negative result.
• Schiff's reagent: Decolourised fuchsin (Schiff's) regains magenta/pink colour with aldehydes. Useful to identify aldehydes (including some aromatic ones) and to distinguish them from ketones.
• Iodoform test: Positive for methyl ketones (structure R–CO–CH3), ethanol (CH3CH2OH) and secondary alcohols with CH3CH(OH)– group (e.g. isopropyl alcohol). Produces a yellow crystalline precipitate of iodoform (CHI3) with characteristic antiseptic smell.
• Carboxylic acid tests: React with carbonate/bicarbonate (e.g. NaHCO3) to evolve CO2 (effervescence) — a simple test for the presence of a carboxylic acid. They also form water-soluble salts with bases.

How to use these tests in identification (typical decision scheme):

1. Perform 2,4-DNP — if negative, no simple aldehyde/ketone carbonyl present.
2. If 2,4-DNP positive, perform Tollens' or Schiff's — if positive, substance is an aldehyde; if negative, likely a ketone.
3. To check for methyl ketone or ethanol/isopropyl alcohol, perform iodoform test.
4. To decide if a compound is a carboxylic acid, test with NaHCO3 for CO2 evolution and check for salt formation with base.

Brief mechanisms (conceptual):

• 2,4-DNP: Nucleophilic addition-elimination — carbonyl condenses with hydrazine derivative to form a hydrazone (solid).
• Tollens'/Fehling's: Aldehyde is readily oxidised to carboxylate while the metal ion (Ag+ or Cu2+) is reduced to metallic silver or Cu2O.
• Iodoform: Repeated halogenation at the methyl adjacent to carbonyl followed by base-promoted cleavage gives CHI3 (iodoform).
• NaHCO3 test: Acid–base reaction producing CO2 gas (effervescence).

Precautions and notes:

• 2,4-DNP and Schiff's reagents are sensitive; 2,4-DNP gives the carbonyl class but not whether aldehyde or ketone.
• Fehling's test fails for many aromatic aldehydes; Tollens' reagent is more generally applicable for aldehydes but must be freshly prepared and handled carefully (explosive silver compounds on dry glassware).
• Iodoform test detects the CH3CO group specifically — not all ketones.

Fig 13 — Educational Diagram: Alpha-Substitution Reactions

Definition
Alpha-substitution reactions are reactions in which an atom (or group) on the carbon atom adjacent to a carbonyl group (the α-carbon) is replaced by another atom or group. The α-position is activated because the α-hydrogens are more acidic than ordinary alkane hydrogens and the resulting carbanion (enolate) is stabilized by resonance with the carbonyl.

Why α-hydrogens are acidic
Removal of an α-H gives an enolate ion which is resonance-stabilized: R-CH=CO-R' <=> R-CH(-)-C(=O)-R'. Electron-withdrawing groups and conjugation increase acidity. Typical approximate pKa values: acetaldehyde ~17, acetone ~19–20, esters (ethyl acetate) ~25, active methylene compounds (malonates, acetoacetates) ~9–13.

Main pathways to give α-substitution

• Acid-catalyzed enolization and electrophilic substitution: Under acidic conditions the carbonyl is protonated and converted to its enol; the enol reacts with electrophiles (E+), giving mono-substituted product. Example: acid-catalyzed bromination of a ketone typically gives mono-α-bromoketone.
• Base-catalyzed enolate formation and nucleophilic reactions: A base (OH-, alkoxide, LDA) removes an α-H to give an enolate (or its metal enolate). Enolates act as nucleophiles and undergo:
• α-halogenation with X2 (with strong base this often gives polyhalogenation).
• Alkylation by SN2 on primary alkyl halides (R-CH2-X) to give α-alkylated carbonyls.
• Condensation reactions (e.g. aldol) where enolate attacks another carbonyl.

Typical mechanisms (summary)

• Acid-catalyzed α-halogenation: Carbonyl <--> enol (acid catalysis). Enol + X2 → α-haloketone + HX.
• Base-catalyzed α-halogenation: Base deprotonates → enolate. Enolate + X2 → α-haloketone. If excess base/X2 and a methyl ketone, sequential halogenation can give CX3 (haloform precursor).
• Enolate alkylation: Strong, non-nucleophilic base (LDA) at low T generates a kinetic enolate (favours less substituted enolate). Enolate + R–X (primary, methyl) → α-alkylated product (SN2).
• Hell–Volhard–Zelinsky (HVZ) reaction (for carboxylic acids): Carboxylic acid reacts with PBr3/Br2 to give the α-bromo acid (via acyl bromide/enol bromination) which can be hydrolysed to the α-bromo carboxylic acid.

Special case: Haloform reaction
Methyl ketones (R–CO–CH3) or ethanol oxidizable methyl groups when treated with excess halogen (Cl2/Br2/I2) in base produce a carboxylate (RCOO-) and haloform (CHX3). This is both a test for methyl ketones and a preparative/degradative transformation.

Control of selectivity
Kinetic vs thermodynamic enolates: LDA, low temperature (−78 °C) → kinetic enolate (less substituted, formed faster). Weaker bases, higher temperature → thermodynamic enolate (more substituted, more stable). Choice of base, solvent and temperature controls mono- vs poly-substitution and alkylation regioselectivity.

Practical notes
- Acid-catalyzed routes favor single substitution (useful for selective halogenation).
- Base-catalyzed enolates give powerful C–C bond formation tools (alkylation, Claisen, Michael additions).
- Avoid secondary/tertiary alkyl halides for enolate alkylation (elimination competes).

Summary sentence
Alpha-substitution reactions rely on enol/enolate chemistry to replace an α-hydrogen with electrophiles or to react with electrophiles, and are central to building complexity in carbonyl chemistry (halogenation, alkylation, condensations, HVZ and haloform reactions).

Fig 14 — Educational Diagram: Chemistry of Carboxylic Acids

Definition and General Structure

Carboxylic acids are organic compounds containing the carboxyl functional group, -COOH. General formula: R-COOH (R = alkyl, aryl, H). The -COOH group contains a carbonyl (C=O) and a hydroxyl (O-H) on the same carbon.

Resonance and Bonding

The carboxylate anion (R-COO-) is resonance stabilized: the negative charge is delocalized over two equivalent oxygen atoms. This delocalization increases stability of the conjugate base and is the main reason for carboxylic acids being significantly more acidic than alcohols.

Physical Properties

• Boiling points: relatively high due to strong intermolecular hydrogen bonding (often form dimers in vapour/solution).
• Solubility: short-chain carboxylic acids (C1–C4) are miscible with water; solubility decreases with increasing alkyl chain length.
• Characteristic IR bands: broad O-H stretch 2500–3300 cm-1, sharp C=O stretch around 1700–1725 cm-1.

Acidity

Acidity is measured by Ka (or pKa). Carboxylic acids are weak acids (pKa typically 3–5 for simple monoacids). Factors affecting acidity:

• Resonance stabilization of R-COO- increases acidity (vs alcohols).
• Inductive effect: electron-withdrawing substituents (eg. halogens) increase acidity; electron-donating groups decrease it.
• Solvent and hydrogen bonding also influence observed acidity.

Preparation (Important Methods)

• Oxidation of primary alcohols or aldehydes: R-CH2OH or R-CHO + [O] (KMnO4, K2Cr2O7/H+) → R-COOH.
• Hydrolysis of nitriles (acidic/alkaline) or amides: R-C≡N + 2H2O/H+ → R-COOH + NH4+.
• Grignard reagent + carbon dioxide: R-MgX + CO2 → RCOOMgX → R-COOH (after acid workup).
• Hydrolysis of esters: RCOOR' + H2O/H+ → R-COOH + R'OH.
• Oxidation of alkylbenzenes (for benzoic acid): alkyl side chain fully oxidized by KMnO4 to -COOH.
• Kolbe electrolytic oxidation of sodium salts (important historical method): 2 RCOO- → R-R + 2 CO2 + 2 e- (gives hydrocarbons by decarboxylative coupling).

Chemical Reactions

• Acid-base reactions: R-COOH + Base → R-COO- + H2O (reacts with carbonates to produce CO2).
• Esterification (Fischer): R-COOH + R'OH ⇌ R-COOR' + H2O (acid-catalysed; reversible).
• Formation of acid derivatives: R-COOH → R-COCl (using SOCl2 or PCl5), R-CO-O-CO-R (anhydride), R-CONH2 (amide via activation), esters.
• Reduction: R-COOH + 4[H] (LiAlH4) → R-CH2OH (reduction to primary alcohol).
• Decarboxylation: R-COOH → R-H + CO2 (thermal or via salts under certain conditions).
• Alpha-halogenation (Hell–Volhard–Zelinsky): R-CH2-COOH → R-CHX-COOH (via formation of acyl halide then enolization; X = Cl/Br).

Uses and Importance

Carboxylic acids and their derivatives are widely used: acetic acid (vinegar, industrial solvent), benzoic acid (food preservative), salicylic acid (pharmaceutical precursor), fatty acids (soaps, detergents), polymers (polyesters from dicarboxylic acids).

Summary Equations and Concepts

Key concept: resonance-stabilized carboxylate anion gives appreciable acidity; derivatives are formed by nucleophilic acyl substitution of the carboxyl carbon.

Fig 15 — Educational Diagram: Derivatives of Carboxylic Acids

Overview

Derivatives of carboxylic acids are compounds that contain the acyl group (R−C(=O)−) bonded to another atom or group X. They retain the carbonyl functionality but differ in the substituent X. Common derivatives are acyl halides (RCOX, X = Cl), acid anhydrides ((RCO)2O), esters (RCOOR'), and amides (RCONR'2).

General structures

• Acyl halide: R−C(=O)−X
• Anhydride: R−C(=O)−O−C(=O)−R
• Ester: R−C(=O)−O−R'
• Amide: R−C(=O)−N(R')2

Preparation (important routes)

• Acyl chloride: RCOOH + SOCl2 → RCOCl + SO2 + HCl
• Anhydride: 2 RCOOH ⇌ (RCO)2O + H2O (or from acyl chloride + RCOOH)
• Esterification (Fischer): RCOOH + R'OH ⇌ RCOOR' + H2O (H+, heat)
• Amide formation: RCOCl + NH3 → RCONH2 + HCl (or from ester/amides via transamidation with activation)

Reactions and reactivity

Most reactions of acyl derivatives proceed by nucleophilic acyl substitution: nucleophile attacks the carbonyl carbon → tetrahedral intermediate → leaving group departs. The ease of substitution depends on the leaving-group ability and resonance stabilization:

• Reactivity order: RCOCl (acyl chloride) > (RCO)2O (anhydride) > RCOOR' (ester) > RCONR'2 (amide) > RCOO− (carboxylate)
• Hydrolysis: Under acidic or basic conditions, derivatives convert back to carboxylic acids (or salts in basic hydrolysis). Saponification refers to base-catalyzed ester hydrolysis to give soap (carboxylate salt) and alcohol.

Mechanistic points (Class‑12 level)

• Nucleophilic attack on C=O forms a tetrahedral intermediate (sp3 carbon).
• Stability of intermediate and leaving-group ability determine reaction rate.
• Amides are least reactive because of strong N→C=O resonance (partial double-bond character).

Spectroscopic clues

• Carbonyl (C=O) stretch in IR differs: acyl chlorides and anhydrides show higher ν(C=O) (≈1800–1750 cm−1), esters ≈1740 cm−1, amides lower ≈1650–1690 cm−1 with N–H bands ≈3300 cm−1.
• 13C NMR: carbonyl carbon shifts vary (esters/amides ~160–180 ppm; acyl chlorides often downfield).

Applications & importance

Derivatives are widely used as intermediates in organic synthesis: esters in fragrances and plastics, acyl chlorides and anhydrides as acylating agents, amides as peptide bonds in proteins.

Summary (key takeaways)

• Derivatives = R−C(=O)−X; they undergo nucleophilic acyl substitution.
• Order of reactivity (fast → slow): acyl chloride > anhydride > ester > amide.
• Preparation and interconversion commonly use activating reagents (SOCl2, DCC, etc.) or catalysts (acid/base).

Fig 16 — Educational Diagram: Interconversions and Synthetic Uses

Scope: This topic covers how aldehydes, ketones and carboxylic acids (and their derivatives) are interconverted and used in synthesis. Emphasis is on typical reagents, conditions, important named reactions and strategic uses in multi-step synthesis.

General principles

• Carbonyl compounds (aldehydes and ketones) are electrophilic at carbon; they undergo nucleophilic addition (e.g., hydride, cyanide, Grignard) and condensation (aldol, Wittig).
• Oxidation state changes: alcohols ↔ aldehydes/ketones ↔ carboxylic acids. Choice of oxidant/reductant controls selectivity.
• Derivatives of carboxylic acids (acid chloride, ester, amide, anhydride) are interconverted by nucleophilic acyl substitution.
• Protecting groups (acetal/ketal) are used to mask carbonyls during multi-step syntheses.

Key interconversions (conceptual map)

• Primary alcohol → aldehyde: mild oxidants (PCC, DMP). Aldehyde → primary alcohol: NaBH4 or LiAlH4 reduction.
• Primary alcohol → carboxylic acid: strong oxidants (KMnO4, K2Cr2O7/H+).
• Secondary alcohol → ketone: oxidation (CrO3, PCC).
• Aldehyde → carboxylic acid: Tollens (Ag2O/NH3), KMnO4, Benedict; aldehyde → acetal (ROH/H+) for protection.
• Carboxylic acid → acid chloride: SOCl2. Acid chloride → ester: ROH (Δ or pyridine). Acid chloride → amide: NH3 or amine (cold).
• Carboxylic acid → ester: Fischer esterification (ROH/H+). Ester → acid: hydrolysis (acidic or basic saponification).
• Ester/acid chloride → aldehyde: Rosenmund reduction (RCOCl + H2/Pd-BaSO4) or DIBAL-H on ester at −78°C.
• Carboxylic acid/ester/amides → alcohols: LiAlH4 reduction (gives alcohols); partial reductions use special reagents.

Important named conversions and synthetic tools

• Wittig reaction: converts carbonyl to alkene (useful for C=C formation with control of substitution).
• Grignard reagents: add to carbonyls to form alcohols (formal-dehyde → primary alcohol; aldehyde → secondary; ketone → tertiary). Reaction with CO2 gives carboxylic acids after workup.
• Hydride reductions: NaBH4 (selective for aldehydes/ketones), LiAlH4 (stronger; reduces esters, acids, amides, nitriles).
• Cannizzaro reaction: non-enolizable aldehydes undergo base-induced disproportionation to give an alcohol and a carboxylate (e.g., benzaldehyde).
• Aldol condensation/Claisen-Schmidt: builds C–C bonds between enolizable carbonyls and electrophilic carbonyls forming β-hydroxy carbonyls and α,β-unsaturated carbonyls.
• Baeyer-Villiger oxidation: ketone → ester (or cyclic ketone → lactone) using peracids.
• Wolff-Kishner and Clemmensen reductions: remove carbonyl oxygen (convert C=O to CH2). Useful in adjusting oxidation state in synthesis.
• Formation of imines (Schiff bases) and reductive amination: convert carbonyls to amines (key in medicinal chemistry).

Mechanistic roles in synthesis

• Carbonyls are versatile building blocks: electrophilic center for C–C bond formation (enolate chemistry, Grignard) and for functional group interconversion (nucleophilic acyl substitution).
• Protecting a carbonyl as an acetal permits reactions at other sites (e.g., organometallic additions) and is reversed by acid hydrolysis.
• Control of chemoselectivity is achieved by choosing reagents (NaBH4 vs LiAlH4; DIBAL-H for partial reductions) and reaction conditions (temperature, solvents).

Spectroscopic/analytical notes (practical synthetic checks)

• IR: carbonyl stretch ≈ 1700 cm−1 (aldehydes/ketones), acids show broad O–H 2500–3300 cm−1 plus C=O ≈ 1700 cm−1; esters ~1735–1750 cm−1.
• 1H NMR: aldehydic proton ~9–10 ppm; loss of this peak on oxidation to acid or reduction to alcohol confirms transformation.

Strategic tips

• Sequence controlling oxidation state: plan from most oxidized or most reduced end; use protecting groups to prevent unwanted reactions.
• When converting carboxylic acids to aldehydes, convert first to acid chloride then use Rosenmund or use specialized reducing agents (LiAlH(OtBu)3) to avoid over-reduction.
• To form carbon–carbon bonds, use aldol, Claisen, Wittig, or Grignard depending on desired connectivity and tolerance to functional groups.

Fig 17 — Educational Diagram: Named Reactions and Important Reagents

Overview
This topic covers characteristic named reactions of aldehydes, ketones and carboxylic acids and the important reagents/tests used to identify or transform these functional groups. Emphasis is on conditions, general equations, mechanistic idea and why each reaction is useful.

Key named reactions (what, when, why)

• Aldol Condensation
  Two carbonyl compounds (usually aldehydes or ketones with α-hydrogen) combine under base (or acid) catalysis to give a β-hydroxy carbonyl (aldol addition); dehydration gives an α,β-unsaturated carbonyl (aldol condensation). Important for C–C bond formation and synthesis of conjugated systems.
• Cannizzaro Reaction
  Non-enolisable aldehydes (no α‑H, e.g., benzaldehyde) react under strong base to disproportionate: one molecule is reduced to the alcohol and another is oxidized to the carboxylate. Useful when direct oxidation or reduction is not applicable.
• Haloform Reaction
  Methyl ketones (or ethanol) undergoing exhaustive α-halogenation in presence of halogen + base give a carboxylate and haloform (CHX3; e.g., chloroform or iodoform). This is a qualitative test for methyl ketones (positive iodoform test gives yellow precipitate of CHI3).
• Benzoin Condensation
  Two aromatic aldehydes (e.g., benzaldehyde) couple in the presence of cyanide catalyst to give benzoin (α‑hydroxy ketone). Useful in building α‑hydroxyketone motifs.
• Kolbe Electrolytic Decarboxylation (Kolbe Reaction)
  Electrochemical decarboxylation of carboxylate salts gives radicals that dimerize to symmetrical hydrocarbons. Historically important for preparing hydrocarbons from carboxylic acids.
• Hell–Volhard–Zelinsky (HVZ) Reaction
  α‑Halogenation of carboxylic acids: treat acid with P and Br2 (or Cl2) to get α‑halo acid (via acid chloride intermediate). Useful to introduce an α‑halogen for further substitution/elimination.

Important reagents & tests (what they indicate and typical equations)

• Tollens' reagent (ammoniacal silver nitrate, [Ag(NH3)2]+): oxidises aldehydes producing a silver mirror (Agº). Quick qualitative test for aldehydes (except some hindered ones).
• Fehling's reagent (Cu2+ complex): aldehydes reduce blue Cu2+ to red Cu2O precipitate; ketones (except α‑hydroxyketones) are generally negative. Used as a test for reducing sugars and aldehydes.
• 2,4‑Dinitrophenylhydrazine (2,4‑DNP): reacts with carbonyls (aldehydes & ketones) to form yellow/orange/red hydrazone precipitates — a positive test for presence of C=O.
• Schiff's reagent: detects aldehydes by formation of a magenta/fuchsia color.
• NaBH4 and LiAlH4: common hydride reducing agents. NaBH4 (milder) reduces aldehydes and ketones to alcohols. LiAlH4 (stronger) reduces aldehydes, ketones, esters, carboxylic acids and amides to alcohols/amines.
• Oxidising agents: KMnO4, K2Cr2O7 / H2SO4 (Jones reagent): oxidise primary alcohols to carboxylic acids and aldehydes to carboxylic acids; secondary alcohols to ketones. Strong oxidisers can cleave/oxidise side chains on aromatics.
• PCC (pyridinium chlorochromate): a milder oxidant — oxidises primary alcohols to aldehydes (without overoxidation to acids) in non‑aqueous conditions.
• SOCl2 (Thionyl chloride): converts carboxylic acids to acyl chlorides (RCOCl) with retention of the carbon skeleton and release of SO2 and HCl gas.
• Grignard reagent (RMgX): powerful C–C bond forming nucleophile; reacts with carbonyls to give alcohols after hydrolysis (formaldehyde → primary alcohol, aldehyde → secondary alcohol, ketone → tertiary alcohol).

Mechanistic ideas (brief)

• Aldol: base abstracts α‑H → enolate → nucleophilic attack on carbonyl carbon of another molecule → β‑hydroxy product → dehydration (E1cB or acid‑catalysed) to give conjugated enone.
• Cannizzaro: strong base forms hydride‑transfer step between two aldehyde molecules (no enolizable H), giving carboxylate + alkoxide (then protonation to alcohol).
• Haloform: repeated α‑halogenation at methyl group next to carbonyl → formation of trihalomethyl ketone → base‑induced cleavage to carboxylate + CX3− which forms haloform (CHX3) on protonation.

Why these matter for Class 12
These named reactions and reagents are used for structure identification (tests), laboratory transformations (reductions, oxidations, functional group interconversions) and C–C bond formation — foundational skills for organic synthesis and analysis.

Fig 18 — Educational Diagram: Comparative Reactivity and Mechanistic Concepts

Overview: Carbonyl-containing compounds (aldehydes, ketones, carboxylic acids and their derivatives) show characteristic reactivity because the C=O group is polarized (Oδ− and Cδ+). Reactivity is governed by electronic factors (inductive and resonance effects), steric hindrance, leaving-group ability (for acyl derivatives) and catalysis (acid/base).

Key mechanistic concepts:

1. Carbonyl polarization and electrophilicity: The carbonyl carbon is electrophilic due to the O atom withdrawing electron density. The stronger the electron withdrawal (or the fewer electron-donating groups), the more electrophilic and reactive the carbonyl C is to nucleophiles.

2. Nucleophilic addition to aldehydes and ketones (general): A nucleophile attacks the carbonyl C forming a tetrahedral alkoxide intermediate which is then protonated.

Steps: Nu: attack → tetrahedral alkoxide intermediate → protonation → addition product (e.g., alcohols, cyanohydrins, hemiacetals).

3. Why aldehydes are more reactive than ketones: Aldehydes have only one alkyl group (less electron-donation by +I) and less steric hindrance than ketones, so they are both more electrophilic and more accessible to nucleophiles. Thus, rate: aldehyde > ketone (for nucleophilic addition).

4. Conjugation and resonance: Conjugation of the carbonyl with a C=C or aromatic ring (e.g., enone or benzaldehyde) lowers electrophilicity because resonance delocalizes the positive character on C, reducing reactivity toward nucleophilic addition.

5. Nucleophilic acyl substitution (for carboxylic acids and derivatives): Mechanism is addition–elimination. Nu: attacks C=O → tetrahedral intermediate → collapse with expulsion of a leaving group (L−). The leaving-group ability strongly controls reactivity: better leaving group = more reactive derivative.

6. Reactivity order of acyl derivatives: acid chloride > acid anhydride > ester > amide. Reason: acid chlorides have a very good leaving group (Cl−) and little resonance stabilization; amides are least reactive because the nitrogen lone pair delocalizes into the carbonyl, stabilizing it and making the C less electrophilic.

7. Effect of acid/base catalysis: Acid catalysis increases electrophilicity of the carbonyl (protonation of O) and stabilizes leaving groups; base catalysis increases nucleophilicity (generation of stronger nucleophiles) and often proceeds by deprotonation steps (e.g., enolate formation for alpha reactions).

8. Alpha-hydrogen acidity and enolate chemistry: Alpha H's next to C=O are acidic because the resulting enolate is resonance-stabilized. Acidity depends on the carbonyl type: ketones (pKa ≈ 19) are less acidic than aldehydes (pKa ≈ 17 for simple aldehydes), and esters have less acidic α-H than ketones. Enolates undergo nucleophilic substitution (alkylation), aldol condensation, Michael addition, etc.

9. Special stability notes: Amides have strong resonance stabilization (partial C–N double-bond character), making them resistant to hydrolysis without activation. Carboxylate anions are stabilized by resonance; hence carboxylic acids are much more acidic than alcohols.

Practical consequences: Choose reagents based on reactivity: to acylate an alcohol or amine, use an acid chloride for fast reaction; for milder conditions use activated esters or coupling reagents. To reduce a carbonyl selectively, choose reagents accordingly (e.g., NaBH4 reduces aldehydes and ketones but not esters readily; LiAlH4 reduces esters and carboxylic acids).

Fig 19 — Educational Diagram: Applications and Important Examples

This section summarises the principal applications and typical examples of aldehydes, ketones and carboxylic acids, emphasising how their structure and reactivity determine real-world uses. Key features are the polar C=O group, which undergoes nucleophilic addition (aldehydes and ketones) and acid–base/derivative chemistry (carboxylic acids). The presence or absence of α‑hydrogens, resonance stabilization and hydrogen bonding control reactivity, acidity and physical properties.

• Aldehydes: More reactive than ketones toward nucleophiles because the carbonyl carbon is less sterically hindered and less electron‑donating. Common applications arise from reactivity and volatility: disinfectants (formaldehyde), fragrances (benzaldehyde), intermediates in synthesis (benzaldehyde, acetaldehyde).
• Ketones: Good solvents (acetone) and intermediates in organic synthesis; less reactive than aldehydes so often used where controlled reactivity is needed (e.g., acetone, methyl ethyl ketone).
• Carboxylic acids: Acidity and ability to form esters and amides make them central to biochemistry (fatty acids, citric acid) and industry (acetic acid, benzoic acid). Carboxylic acids participate in esterification, formation of acid chlorides and polymer formation (polyesters, polyamides when combined with diols/diamines).

Industrial and laboratory transformations used in applications include oxidation of alcohols to aldehydes/acids, reduction of carbonyls to alcohols, formation of derivatives (oximes, hydrazones, 2,4‑DNP adducts), esterification (for flavors, fragrances, pharmaceuticals), acylation (Friedel–Crafts), and polymerisation (polyesters from dicarboxylic acids and diols). Biological relevance includes reducing sugars (aldoses) which give silver mirror (Tollens') test and fatty acids (carboxylic acids) involved in metabolism.

Practical considerations derived from properties:

• Boiling points: carboxylic acids > alcohols > aldehydes/ketones of similar molar mass (due to strong hydrogen bonding).
• Acidity: pKa of carboxylic acids ≈ 4–5; resonance stabilization of RCOO– is responsible.
• IR/Instrumental identification: strong C=O stretch near 1700 cm⁻¹; aldehydic C–H stretches around 2720–2820 cm⁻¹.

Fig 20 — Educational Diagram: Practical Techniques and Laboratory Preparations

Overview
This topic covers laboratory techniques used in the preparation, isolation and purification of aldehydes, ketones and carboxylic acids. Emphasis is on common preparative routes, reagents and practical steps (reaction setup, workup, purification and identification).

Common laboratory techniques (what and why)

• Reflux: Heating a reaction mixture to boiling while condensing and returning vapors (condenser) — allows reactions to proceed at solvent boiling point without loss of reagents or solvent.
• Distillation: Separation of liquids by boiling point. Simple distillation for large bp differences; fractional distillation (fractionating column) for closer bps; vacuum distillation for heat-sensitive, high-boiling compounds.
• Steam distillation: For isolating high-boiling or steam-volatile organic compounds (e.g., some aromatic compounds, essential oils) at temperatures below decomposition.
• Extraction (liquid–liquid): Use of separatory funnel to separate organic and aqueous layers; washing (acid/base/brine) to remove impurities; drying organic layer over anhydrous salt (MgSO4, CaCl2) before evaporation.
• Filtration: Gravity filtration for solids; vacuum (Büchner) filtration for rapid drying of crystalline solids.
• Recrystallization: Purification of solid acids/derivatives by dissolving in hot solvent and slowly cooling to give pure crystals (e.g., semicarbazones).
• Thin-Layer Chromatography (TLC): Monitoring reaction progress and checking purity (Rf values, single spot indicates purity).

Typical workup steps

• Quench the reaction if needed (e.g., remove reducing agent oxidizer carefully).
• Transfer to separatory funnel; perform washes (water, dilute acid to remove bases, dilute base to remove acids, brine).
• Dry organic layer (MgSO4 or anhydrous CaCl2), filter and concentrate by rotary evaporation.
• Purify by distillation or recrystallization as appropriate; confirm identity by melting/boiling point, IR, NMR, TLC.

Representative laboratory preparations

• Aldehydes
  • Partial oxidation of primary alcohols under controlled conditions (use a mild oxidant like PCC or Dess–Martin): RCH2OH + [O] → RCHO + H2O.
  • Rosenmund reduction: acid chloride → aldehyde by catalytic hydrogenation over Pd/BaSO4 (poisoned Pd): RCOCl + H2 (Pd/BaSO4) → RCHO.
  • Oxidation of benzyl alcohol to benzaldehyde using MnO2 (selective for benzylic alcohols).
• Ketones
  • Oxidation of secondary alcohols: R1CHOH R2 + [O] → R1COR2 + H2O (e.g., 2-propanol → acetone with K2Cr2O7 or PCC).
  • Friedel–Crafts acylation (aromatic ketones): Ar–H + RCOCl (AlCl3) → Ar–CO–R.
  • Hydration of internal alkynes (acid/Hg2+ catalysis) — gives Markovnikov ketone after tautomerization.
• Carboxylic acids
  • Oxidation of primary alcohols or aldehydes using strong oxidants (KMnO4, K2Cr2O7/H2SO4): RCH2OH → RCOOH.
  • Oxidation of alkyl benzenes (side-chain oxidation with KMnO4) to benzoic acid (e.g., toluene → benzoic acid).
  • Hydrolysis of nitriles: R–CN + 2 H2O (acidic or basic hydrolysis) → RCOOH + NH3 (after acidification for basic hydrolysis).
  • Carboxylation of Grignard reagents: R–MgX + CO2 → RCOOMgX → RCOOH (after acid workup).

Derivatives (for purification/characterisation)
Conversion of carbonyl compounds to solid derivatives for purification and identification: 2,4‑DNP derivatives, semicarbazones, oximes, hydrazones — prepared by reaction with the appropriate reagent and purified by recrystallization.

Laboratory examples (short procedures)

• Preparation of benzoic acid from toluene: Reflux toluene with KMnO4 (aqueous, basic) until oxidation complete, acidify to precipitate benzoic acid, filter and recrystallize.
• Preparation of benzaldehyde from benzyl alcohol: Stir benzyl alcohol with MnO2 in inert solvent (e.g., CH2Cl2) at room temperature; filter off MnO2 and distill product if needed.
• Preparation of acetone by oxidation of isopropyl alcohol: Controlled oxidation with dichromate under reflux; distil to collect acetone (or use commercial methods).

Safety and practical tips

• Many oxidizing agents (KMnO4, K2Cr2O7) are corrosive/oxidizing — use gloves and eye protection; chromium(VI) is toxic (dispose properly).
• Use anhydrous conditions for Grignard reactions; glassware must be dry and oxygen-free.
• Monitor reactions by TLC; stop oxidations promptly to avoid over-oxidation (aldehyde → acid).
• Drying agents: MgSO4 is fast and fine-grained; CaCl2 is good for non-polar solvents; remove drying agent by filtration before concentrating.

Identification hints
IR: sharp strong C=O stretch ~1700 cm-1 (aldehyde ~1725–1740, conjugation lowers it). 1H NMR: aldehyde proton 9–10 ppm (singlet), carboxylic acid proton broad 10–12 ppm; ketones lack aldehydic proton.