Chapter 7 — Aldehydes, Ketones and Carboxylic Acids

Introduction to the carbonyl group
The carbonyl group (C=O) is one of the most important functional units in organic chemistry. It consists of a carbon atom double-bonded to an oxygen atom. Because oxygen is more electronegative than carbon, the π-electrons are drawn toward oxygen producing a strong dipole: oxygen becomes partially negative (δ−) and the carbon partially positive (δ+). This polarity makes the carbonyl carbon electrophilic and a natural target for nucleophiles.

Hybridisation and geometry
The carbonyl carbon is sp2 hybridised. One s and two p orbitals mix to form three sp2 orbitals arranged in a trigonal planar geometry around the carbonyl carbon with bond angles close to 120°. The remaining unhybridised p orbital on carbon overlaps sideways with a p orbital on oxygen to form the π-bond. The oxygen atom has two lone pairs: one occupies an sp2 orbital and the other resides in a p orbital involved in π-bonding.

Resonance and bond character
The carbonyl group can be represented by resonance forms: the major form is R–C(=O)–R', while a minor ionic resonance form places a negative charge on oxygen and a positive charge on carbon (R–C(–O−)=R'+). The true structure is a hybrid, and although the ionic form contributes less, it explains why the carbonyl carbon carries significant electrophilic character and why oxygen is nucleophilic. Resonance also explains observed bond lengths and partial double bond character in adjacent bonds when conjugation or electron-withdrawing/donating groups are present.

Electronic effects and reactivity
Substituents attached to the carbonyl carbon influence reactivity. Electron-withdrawing groups (–NO2, –Cl, carbonyl conjugation) increase the positive character at carbon and make nucleophilic attack easier. Electron-donating groups (alkyl groups, resonance donors) decrease electrophilicity. Steric hindrance by bulky substituents can slow reaction rates by physically blocking nucleophiles.

Comparison across carbonyl types
Aldehydes have at least one hydrogen on the carbonyl carbon (R–CHO) whereas ketones have two carbon groups (R–CO–R'). Aldehydes are generally more reactive toward nucleophiles: the hydrogen offers less electron donation and less steric hindrance than a carbon substituent. Carboxylic acid derivatives introduce leaving groups (e.g., Cl, OR, NR2) and their reactivity is governed by both electronic factors and the stability of the leaving group. Understanding these bonding and electronic features is foundational for predicting mechanisms and choosing reagents in synthesis.

Principles of IUPAC naming
Always identify the longest carbon chain that contains the functional group of interest. For aldehydes and carboxylic acids the carbonyl carbon is at the end of the chain and is given the lowest possible number; for ketones the carbonyl may be internal and must be located by a position number. Replace the suffix –e of the parent alkane by the appropriate ending: –al for aldehydes, –one for ketones and –oic acid for carboxylic acids. When multiple functional groups are present, priority rules determine which gets the principal suffix (carboxylic acids usually have higher priority than aldehydes and ketones).

Naming aldehydes
For open-chain aldehydes, use the suffix –al after numbering from the end nearest the aldehyde group so the carbonyl carbon becomes C1. Examples: CH3CHO is ethanal; CH3CH2CHO is propanal. For aldehydes attached to aromatic rings, common names like benzaldehyde are accepted, but you can use benzene carbaldehyde when specificity is required.

Naming ketones
Ketones are named by replacing –e with –one and giving the position number of the carbonyl carbon. For example, CH3COCH3 is propanone (commonly called acetone) and CH3CH2COCH3 is butan-2-one (or 2-butanone). In cyclic ketones the prefix cyclo- is used and numbering gives the carbonyl carbon position (cyclohexanone, where the carbonyl is C1 by default).

Naming carboxylic acids
Carboxylic acids use the suffix –oic acid. Numbering begins at the carboxyl carbon as C1. Ethanoic acid (common acetic acid) is CH3COOH. Substituted benzoic acids are named by indicating ring positions (e.g., 4-nitrobenzoic acid). If other functional groups are present, use prefixes (e.g., hydroxy-, nitro-) with numbering reflecting their positions.

Common names and trivial names
Many simple carbonyls and acids have common names widely used in industry and exam contexts: acetaldehyde (ethanal), acetone (propanone), acetic acid (ethanoic acid). Know both systematic and common names. For exam questions, provide the IUPAC name and the common name if familiar, and ensure numbering is clear and unambiguous.

Practice naming rules
Remember to indicate stereochemistry where relevant and to use locants for substituents. For multifunctional molecules, follow priority order (carboxylic acids > aldehydes > ketones > alcohols > amines) when assigning the main suffix. Keep practice simple chains and aromatic examples to gain confidence in applying the rules quickly.

Oxidation of alcohols as the fundamental route
One of the simplest laboratory routes to aldehydes and ketones is oxidation of alcohols. Primary alcohols oxidise first to aldehydes and then further to carboxylic acids if stronger oxidants or prolonged conditions are used. Secondary alcohols oxidise to ketones. Tertiary alcohols do not oxidise easily because there is no hydrogen on the carbon bearing the OH; strong conditions cause C–C bond cleavage instead.

Choice of oxidising agent for selective preparation
Selectivity matters: strong oxidants like potassium permanganate (KMnO4) or chromic acid will carry a primary alcohol all the way to the acid. To stop at the aldehyde, milder reagents or conditions that prevent over-oxidation are needed. In the laboratory, reagents such as pyridinium chlorochromate (PCC) or Swern oxidation (DMSO activated by oxalyl chloride or other activators) are commonly used to oxidise primary alcohols to aldehydes without further oxidation. PCC is used in anhydrous solvents and gentle conditions, while Swern proceeds at low temperature and gives good yields for sensitive substrates.

Catalytic dehydrogenation
Another practical method is catalytic dehydrogenation: heating alcohols over catalysts such as copper, nickel or palladium causes removal of hydrogen to form carbonyl compounds and molecular hydrogen. This method is employed industrially for large-scale conversions and when hydrogen gas is a useful by-product. Reaction conditions (temperature, catalyst) determine selectivity between aldehyde and acid formation.

Oxidative cleavage of alkenes and ozonolysis
Alkenes can be converted to carbonyl compounds by oxidative cleavage. Ozonolysis followed by reductive work-up (using Zn or (CH3)2S) gives aldehydes or ketones depending on substitution pattern. This method is especially useful to cleave complex olefins into simpler carbonyl fragments in synthesis planning. Alkene ozonolysis should be controlled because oxidative work-up gives acids or further oxidised products.

Other oxidation routes
Selective oxidation of methyl groups adjacent to aromatic rings (benzylic oxidation) using KMnO4 or other oxidants converts alkylbenzenes to aromatic carboxylic acids; however, milder oxidation conditions can produce aldehydes when controlled. Hydration of alkynes (acid-catalysed, mercury(II)-catalysed) yields ketones via tautomerisation, while hydroboration–oxidation of terminal alkynes followed by oxidation can give aldehydes if special reagents or conditions are used.

Laboratory cautions and practical tips
For obtaining aldehydes, distillation during oxidation (to remove the aldehyde as it forms) is a classical practical technique. Protect sensitive groups from strong oxidants and plan work-up steps to isolate and purify the desired carbonyl. Understanding reagent strength and reaction conditions allows predictable outcomes and higher yields.

Partial reduction of esters and acid chlorides
Carboxylic acid derivatives can be transformed into aldehydes when reduction is carefully controlled. Reagents like DIBAL-H (diisobutylaluminium hydride) at low temperature (-78 °C) deliver one hydride equivalent and convert esters to aldehydes after controlled work-up. Acid chlorides can be reduced to aldehydes using bulky hydride reagents such as lithium tri-tert-butoxyaluminium hydride which are sterically hindered and less reactive; appropriate quench and work-up are needed to avoid over-reduction to alcohols.

Hydroformylation
Industrial synthesis of aldehydes often uses hydroformylation: addition of synthesis gas (CO and H2) across an alkene using Rh- or Co-based catalysts gives aldehydes (alkyl aldehydes) by chain extension. Hydroformylation is a major route for producing linear aldehydes from alkenes on a large scale; catalyst choice and ligands determine regioselectivity (linear vs branched aldehydes).

From nitriles and imines
Nitriles can be partially reduced to imines and then hydrolysed to aldehydes under controlled conditions. DIBAL-H can reduce nitriles to imines which on hydrolysis give aldehydes. Alternately, using catalytic hydrogenation under modified conditions can lead to selective transformations. Imines themselves (Schiff bases) formed from aldehydes and amines can be hydrolysed back to aldehydes under controlled conditions, useful in protective group strategies or in synthetic sequences.

From alkenes and alkynes
Hydroboration–oxidation of terminal alkenes normally gives alcohols, but specific sequences and protection/deprotection steps permit conversion to aldehydes. Hydration of terminal alkynes under anti-Markovnikov conditions (via hydroboration) followed by oxidation yields aldehydes. In contrast, Markovnikov hydration of alkynes normally produces ketones.

Friedel–Crafts acylation and other aromatic acylations
Friedel–Crafts acylation of aromatic rings with acyl chlorides and AlCl3 provides aromatic ketones directly (e.g., benzene + RCOCl → ArCOR). This route is fundamental for making aromatic ketones like acetophenone and is widely applied in synthesis. Careful control of reaction conditions avoids polyacylation and rearrangements.

Choosing the right method
The selection of method depends on substrate sensitivity, presence of other functional groups, desired selectivity, scale and availability of reagents. For laboratory-scale synthesis, DIBAL-H and hydride strategies, Friedel–Crafts acylation, and ozonolysis are common reliable approaches. Consider purification and safety when planning the route.

Polarity and boiling points
Carbonyl compounds are polar because of the C=O dipole. This polarity produces dipole–dipole attractions between molecules which raise their boiling points compared with non-polar hydrocarbons of similar molecular mass. However, they lack O–H bonds and so do not form the strong hydrogen bonds that alcohols do, which is why alcohols often have higher boiling points than corresponding aldehydes/ketones.

Solubility in water
Small aldehydes and ketones such as formaldehyde, acetaldehyde and acetone are miscible or highly soluble in water. This is because their oxygen can act as a hydrogen-bond acceptor with water molecules. As alkyl chain length increases, hydrophobic character dominates and solubility in water decreases. Branching also reduces solubility less strongly than linear chains of the same molecular weight.

Odour and volatility
Many low molecular weight aldehydes and ketones are volatile and possess characteristic odours used in fragrances and flavourings: benzaldehyde smells of almonds, vanillin has a vanilla aroma. Some carbonyls can be irritating to the eyes and mucous membranes; low boiling carbonyls are flammable and should be handled with care.

Comparing aldehydes and ketones
Aldehydes are generally more reactive toward nucleophiles than ketones due to less steric hindrance and fewer electron-donating groups. Aldehydes are also more easily oxidised to acids, whereas ketones are resistant to further oxidation except under strong conditions. These chemical differences reflect in some physical trends such as reactivity-dependent stability during storage and distillation.

Spectroscopic signatures
In IR spectroscopy, the carbonyl stretch is strong and sharp, typically observed near 1700 cm−1. Conjugation with double bonds or aromatic rings lowers the stretch frequency (to around 1680 cm−1) while ring strain or electron-withdrawing substituents can shift it higher. Aldehydes show weak C–H stretches around 2700–2850 cm−1. In 1H NMR, aldehydic protons appear at δ 9–10 ppm; α-protons next to carbonyls are deshielded and often appear between δ 2–3 ppm. 13C NMR places carbonyl carbons downfield: ~190–220 ppm for aldehydes and ketones.

Practical implications
These physical properties affect purification and handling. Distillation is commonly used for isolation; low boiling aldehydes should be distilled as formed to avoid oxidation. Solvent choice for reactions is guided by polarity: polar aprotic solvents are often chosen for nucleophilic additions, while protic solvents can participate in hydrogen bonding and affect reaction rates.

Overview of nucleophilic addition
Because the carbonyl carbon is electron-poor due to the polar C=O bond and resonance, it is susceptible to attack by nucleophiles. The general mechanism involves addition of a nucleophile to the carbonyl carbon forming a tetrahedral alkoxide intermediate. Subsequent protonation of the alkoxide provides the neutral addition product. Variations occur depending on whether the medium is acidic or basic, whether the nucleophile is neutral or anionic, and whether the product can revert to starting material.

Step-by-step in base
In basic conditions a nucleophile (Nu−) directly attacks the carbonyl carbon displacing electron density into oxygen and forming an alkoxide intermediate (R–C(−O−)(Nu)–R'). Protonation of the alkoxide during aqueous work-up gives the addition product, for example an alcohol if Nu− is H− or an organometallic fragment. Nucleophiles include hydride (from NaBH4, LiAlH4), cyanide (CN−), alkoxide (OR−), and organometallic reagents (R− from Grignard reagents).

Acid-catalysed additions
Under acidic conditions, the oxygen of the carbonyl may be protonated first, increasing the electrophilicity of the carbon and making nucleophilic attack easier even for neutral nucleophiles like water or alcohols. The protonation step is reversible and care must be taken to avoid conditions that lead to side reactions such as dehydration or polymerisation. Acid catalysis is commonly used in hydrate and acetal/hemiacetal formation.

Reversibility and equilibrium
Some nucleophilic additions are reversible and exist in equilibrium (e.g., hydration to form geminal diols, hemiacetal formation). The position of equilibrium depends on the stability of the product and the reaction conditions; removal of a product (e.g., dehydration to form an α,β-unsaturated carbonyl) can drive the reaction to completion.

Factors affecting rate and selectivity
Electronic and steric effects are important. Aldehydes are generally faster to react than ketones because of less steric hindrance and fewer electron-donating alkyl groups. Electron-withdrawing substituents on the carbonyl increase rate by stabilising the negative charge on oxygen in the intermediate; bulky substituents slow nucleophile approach and reduce rate. Solvent polarity affects reaction energetics by stabilising charged intermediates.

Practical examples and implications
Formation of cyanohydrins (addition of HCN) is a reversible reaction widely used to install a C–C bond and create useful synthetic intermediates. Addition of alcohols to aldehydes produces hemiacetals and acetals (protecting groups) used in multi-step syntheses. Understanding mechanism helps predict which reagents and conditions give the desired product and avoid side reactions such as self-condensation or over-reduction.

Hydride reductions: general principles
Reduction of carbonyl compounds to alcohols is commonly achieved using hydride-donating reagents. Sodium borohydride (NaBH4) is a relatively mild and selective hydride source that reduces aldehydes and ketones to primary and secondary alcohols respectively, and generally leaves esters, acids and amides untouched. Lithium aluminium hydride (LiAlH4) is much stronger and reduces nearly all carbonyl-containing functional groups including esters, acids and amides to alcohols or amines (after further work-up).

Mechanism of hydride addition
The active species is a hydride ion equivalent (H−) which attacks the electrophilic carbonyl carbon to form an alkoxide intermediate. This alkoxide is then protonated during aqueous or acidic work-up to give the corresponding alcohol. The course of addition can be influenced by sterics and electronics so that approach of hydride occurs preferentially from the less hindered face of the planar carbonyl carbon.

Selective reductions
Selectivity can be achieved through reagent choice, solvent and temperature. NaBH4 in alcoholic solvents at 0–25 °C typically reduces aldehydes and ketones selectively. DIBAL-H at low temperature can partially reduce esters or nitriles to aldehydes if controlled. Catalytic hydrogenation (H2 with Pd/C or Pt) can also reduce carbonyls, but it frequently reduces C=C bonds as well; thus catalysts and conditions are chosen carefully to target carbonyl reduction when required.

Stereochemistry of reductions
When reduction creates a chiral centre at the former carbonyl carbon (a prochiral centre), the addition of hydride can produce enantiomers. In the absence of chiral reagents or catalysts, this gives racemic mixtures. Stereoselective reductions can be achieved using chiral hydride reagents, chiral catalysts (asymmetric hydrogenation), or bulky reagents that favour one face of attack over the other.

Special reductions to remove carbonyl functionality
When removal of the carbonyl oxygen is required, special methods are used: Wolff–Kishner reduction (hydrazine and strong base, high temperature) reduces C=O to CH2 by converting to hydrazone and eliminating N2. Clemmensen reduction (Zn/Hg amalgam in HCl) does the same under strongly acidic conditions. Choice depends on substrate sensitivity to acid or base and presence of other reducible groups.

Laboratory handling
LiAlH4 is pyrophoric and reacts violently with water; all manipulations must be anhydrous and followed by careful quenching procedures. NaBH4 is safer but still reacts with water and protic solvents releasing hydrogen; measure stoichiometry carefully to avoid side reactions and ensure complete quench before work-up.

General mechanism of nucleophilic acyl substitution
Acyl compounds (acid derivatives such as acid chlorides, anhydrides, esters, amides) react by nucleophilic acyl substitution. The mechanism begins with nucleophilic attack at the electrophilic carbonyl carbon to form a tetrahedral intermediate. Collapse of this intermediate leads to expulsion of a leaving group and reformation of the carbonyl. The ease of the substitution depends on how good the leaving group is and how stable the intermediate is.

Order of reactivity
The reactivity of acyl derivatives towards nucleophilic attack decreases in the order: acid chloride > anhydride > ester > amide > carboxylate. Acid chlorides and anhydrides have good leaving groups (Cl− and carboxylate) and react rapidly with alcohols, amines and water. Esters are less reactive because the alkoxide leaving group is less stable than chloride. Amides are least reactive due to strong resonance stabilisation between the nitrogen lone pair and the carbonyl, and because amide nitrogen is a poor leaving group.

Typical transformations
Acid chloride + alcohol → ester (often performed in presence of pyridine to capture HCl); acid chloride + ammonia/amine → amide; ester + base → carboxylate (saponification); amide hydrolysis requires high temperature or acid/base catalysis and often prolonged reaction times. Anhydrides can acylate alcohols or amines giving esters and amides respectively. Transesterification exchanges ester alkyl groups under acid or base catalysis.

Mechanistic details under acidic vs basic conditions
Under basic conditions, nucleophiles are usually stronger (e.g., RO−, OH−) and attack occurs directly; products are often deprotonated and require work-up to isolate neutral products. Under acidic conditions, protonation of the carbonyl increases electrophilicity and neutral nucleophiles (alcohols, amines) can attack; proton transfers facilitate departure of leaving groups. Understanding these pathways helps in choosing reagents and protecting functional groups during multi-step synthesis.

Activation strategies and coupling reagents
In peptide synthesis and complex molecule assembly, coupling reagents (DCC, EDC, HATU) convert carboxylic acids into activated species that react with amines to form amides under mild conditions. Such methods avoid harsh reagents like SOCl2 and allow selective formation of amide bonds with minimal racemisation. In small-scale organic synthesis, acid chlorides prepared using SOCl2 are commonly used to create esters and amides with good yields.

Synthetic planning
Choosing which derivative to prepare depends on subsequent steps: reactive derivatives (acid chlorides) are used for difficult acylations; esters are stable storage forms; amides are desired end products (proteins, pharmaceuticals) and require activation for formation. Consider reagent compatibility and downstream transformations while planning sequences.

Enol and enolate chemistry as the foundation
Carbonyl compounds with α-hydrogen atoms can form enols (tautomeric isomers) or enolates (deprotonated forms in basic conditions). Enolates are nucleophilic at the α-carbon and can attack electrophilic carbonyl carbons of another molecule. This behaviour underlies the aldol reaction, a cornerstone of carbon–carbon bond formation in organic synthesis.

The aldol addition mechanism
In basic media, a base (OH− or an alkoxide) deprotonates the α-carbon to form the enolate. The enolate then attacks a second molecule's carbonyl carbon, forming a new C–C bond and giving a β-hydroxy carbonyl (an aldol). Under acidic conditions, enol formation is catalysed first and the enol attacks a protonated carbonyl; the mechanistic details differ but the end product is similar.

Dehydration to α,β-unsaturated carbonyls
On heating or under dehydrating conditions, β-hydroxy carbonyls often lose water to form α,β-unsaturated carbonyl compounds (conjugated enones or enals). This step is driven by the stability gained through conjugation between the carbonyl and the C=C double bond. The resulting α,β-unsaturated systems are synthetically useful in further conjugate additions and Michael reactions.

Self-condensation vs crossed aldol reactions
When two identical carbonyl molecules react, self-aldol condensation occurs. Crossed aldol condensations between two different carbonyl compounds can give mixtures unless one partner lacks α-hydrogens (so it cannot form an enolate) or conditions favour formation of a single enolate (using a strong, non-nucleophilic base like LDA to form the enolate selectively). For high selectivity, pre-form the enolate at low temperature and then add the electrophilic partner.

Intramolecular aldols and ring formation
Intramolecular aldol reactions are especially powerful to form five- and six-membered rings because the entropic advantage and ring stability favour cyclisation. A dialdehyde or a ketoaldehyde can cyclise via intramolecular enolate attack to give cyclic β-hydroxy carbonyls which often dehydrate to give conjugated cyclic enones.

Synthetic importance and control
Control over stereochemistry and regiochemistry is important in complex syntheses. Conditions such as solvent, base strength, temperature, and the use of chiral auxiliaries or catalysts can direct outcomes. Aldol chemistry builds complex carbon skeletons and forms the backbone of many natural product and pharmaceutical syntheses.

When enolisation is not possible: Cannizzaro reaction
The Cannizzaro reaction occurs with aldehydes that lack α-hydrogen atoms (non-enolizable aldehydes), such as benzaldehyde. In the presence of concentrated base (e.g., NaOH), two molecules of the aldehyde undergo disproportionation in which one molecule is oxidised to the carboxylate and the other is reduced to the alcohol. This redox process happens by hydride transfer between an alkoxide-like intermediate and another aldehyde molecule.

Mechanism outline
Base first adds to the carbonyl to form a tetrahedral alkoxide intermediate. This intermediate is capable of transferring a hydride ion to another aldehyde molecule, reducing it to an alkoxide which on protonation gives the alcohol. The donor aldehyde is oxidised to a carboxylate. The overall stoichiometry is: 2 RCHO + OH− → RCH2OH + RCOO−.

Scope and limitations
Cannizzaro only occurs for aldehydes without α-hydrogens because enolate formation would be more favourable and lead to aldol pathways. Electron-withdrawing substituents and conjugation can influence the ease of hydride transfer. Reaction conditions are strongly basic and often aqueous.

Tishchenko reaction: ester formation by disproportionation
Tishchenko reaction is a related process but results in an ester rather than an alcohol plus acid. It is catalysed by alkoxides or aluminium alkoxides and proceeds through hydride transfer within a di-alkoxide complex leading to an ester (RCO2CH2R). For example, two molecules of benzaldehyde can combine to form benzyl benzoate under Tishchenko conditions with aluminium alkoxide catalysts.

Comparison and synthetic use
Both reactions demonstrate hydride transfer pathways without external redox agents. Cannizzaro is useful to obtain both alcohols and acids from non-enolizable aldehydes. Tishchenko offers a mild way to prepare esters from aldehydes when direct esterification of corresponding acids is impractical. Choice between these reactions depends on catalyst, solvent and desired product.

Practical notes
In the laboratory, careful control of concentration, temperature and choice of catalyst directs whether disproportionation gives alcohol+acid (Cannizzaro) or ester (Tishchenko). Knowing whether an aldehyde has α-hydrogens is the first step in predicting which pathway will be observed under basic conditions.

Haloform reaction as a diagnostic and synthetic process
The haloform reaction selectively transforms methyl ketones (RCOCH3), ethanol and certain secondary alcohols with a methyl group adjacent to the OH into a carboxylate and haloform (CHX3, where X = Cl, Br, I) using halogen in base. The reaction is useful as an identification test (iodoform test using I2/NaOH gives yellow CHI3 precipitate) and has practical synthetic applications for cleavage of methyl ketones.

Mechanism in brief
Under basic conditions, successive α-halogenation occurs replacing the three α-hydrogens by halogens to form a trihalomethyl ketone. The trihalomethyl group is a good leaving group as CX3−; base then causes cleavage of the C–C bond adjacent to the carbonyl, giving a carboxylate and CX3−. Protonation of CX3− yields the haloform (CHX3) which often precipitates (iodoform is yellow and characteristic).

Scope and selectivity
Methyl ketones give positive iodoform test; secondary alcohols with a methyl group on the carbon bearing OH (e.g., isopropanol after oxidation) also produce a positive test. Aldehydes like ethanal also give iodoform. Compounds lacking the CH3CO– group do not undergo this reaction. Reaction conditions involve halogen (I2 preferred for easy detection) and strong base (NaOH).

Other oxidation reactions
Oxidation of aldehydes to carboxylic acids is straightforward with oxidising agents like KMnO4, Ag2O (Tollens'), or mild oxidants depending on sensitivity. Strong oxidants can cleave C–C bonds adjacent to carbonyls. Ozonolysis cleaves alkenes to carbonyl fragments; oxidative ozonolysis or permanganate cleavage can yield carboxylic acids from alkenes with suitable substitutions.

Practical considerations
Tollens' and Fehling's tests provide chemical evidence of aldehydes through reduction of metal ions to metallic silver or copper(I) oxide respectively. Careful control of oxidation conditions is required to obtain desired products without over-oxidation. For haloform, iodine is preferred for visual detection whereas chlorine or bromine are used in specific synthetic contexts. Disposal of haloforms and halogenated by-products must follow safety and environmental protocols due to toxicity concerns.

Applications and synthetic use
Haloform reaction is a useful laboratory test and cleavage method; oxidation reactions are fundamental tools in converting between oxidation states of carbon (alcohol ↔ aldehyde ↔ acid) and in breaking complex molecules into simpler units for analysis or further transformation.

Purpose and overview
The Wittig reaction is a fundamental method to convert aldehydes or ketones into alkenes using phosphonium ylides (phosphoranes) of the general form Ph3P=CR2. It is widely used to construct carbon–carbon double bonds in organic synthesis because it allows the combination of two fragments—an aldehyde/ketone and an ylide-derived carbon fragment—under mild conditions.

Formation of the ylide
Phosphonium ylides are prepared by first forming a phosphonium salt: alkyl halide reacts with triphenylphosphine (Ph3P) to give R–PPh3+ X−. Deprotonation of the phosphonium salt with a strong base (e.g., n-BuLi, NaHMDS) forms the ylide Ph3P=CR2 which contains a carbanionic carbon next to positively charged phosphorus. The ylide has resonance forms that describe its reactivity: a neutral ylide form and a carbanion–phosphonium ionic form.

Mechanistic sequence
The ylide attacks the carbonyl carbon to form a four-membered cyclic oxaphosphetane intermediate (or a betaine which collapses to that intermediate). This intermediate fragments to give the desired alkene and triphenylphosphine oxide (Ph3P=O). The overall reaction is atom-economical with triphenylphosphine oxide as a common by-product which must be removed during purification.

Stereochemical control
The stereochemistry (E or Z) of the produced alkene depends on the nature of the ylide: unstabilised ylides (those lacking electron-withdrawing substituents) typically give Z-alkenes, while stabilised ylides (bearing electron-withdrawing groups adjacent to the carbanion) favour E-alkenes because of thermodynamic control and the nature of the transition states. Reaction conditions (temperature, solvent, counterion) can influence stereochemical outcomes.

Comparisons and alternatives
The Horner–Wadsworth–Emmons (HWE) reaction uses phosphonate carbanions and often provides higher selectivity for E-alkenes. The Julia olefination is another alternative that offers different stereochemical and functional group compatibilities. The choice depends on desired alkene stereochemistry and functional group tolerance.

Applications and practical notes
Wittig reaction is very useful in total synthesis to install alkenes at defined positions. Reagent handling requires dry, inert conditions when preparing ylides with strong bases. The by-product Ph3P=O is sometimes difficult to remove but can be separated by chromatography or extraction with appropriate solvents. Knowledge of stabilised vs unstabilised ylides helps predict products and design synthetic sequences.

Structure and resonance stabilisation
Carboxylic acids contain the –COOH functional group where one oxygen is double-bonded to carbon and the other bears a hydroxyl. When deprotonated, the carboxylate anion (RCOO−) is resonance-stabilised: the negative charge is delocalised equally over the two oxygen atoms, making the anion particularly stable. This resonance stabilisation accounts for the increased acidity of carboxylic acids compared with alcohols.

Factors affecting acidity
The acidity of carboxylic acids (measured by pKa) is influenced by substituents near the carboxyl group. Electron-withdrawing groups (e.g., –NO2, –Cl, –CF3) stabilise the carboxylate and lower pKa, increasing acidity. Electron-donating groups (alkyl, –OCH3) destabilise the anion, raising pKa and lowering acidity. Conjugation with aromatic rings and resonance also alters acidity: benzoic acid is slightly more acidic than aliphatic acids due to resonance delocalisation into the ring system.

Hydrogen bonding and physical properties
Carboxylic acids form strong hydrogen-bonded dimers in nonpolar solvents and in the gas phase. Two molecules associate through a pair of O–H···O hydrogen bonds forming a cyclic dimer, which significantly increases boiling points relative to molecules of similar molar mass. In polar solvents like water, acids can solvate and ionise; lower molecular weight acids (formic and acetic acid) are soluble due to hydrogen bonding with water, but solubility decreases as hydrocarbon chain length increases.

Acid strength trends and pKa values
Typical pKa values: formic acid ≈ 3.75, acetic acid ≈ 4.76, benzoic acid ≈ 4.20. Alcohols have much higher pKa values (~16), showing how much more acidic carboxylic acids are due to resonance stabilisation of the conjugate base. Inductive effects act over multiple bonds but decrease with increasing distance from the carboxyl group; a substituent at the α-position affects acidity more strongly than one at the γ-position.

Derivatisation and reactivity related to acidity
The acidity allows conversion to carboxylate salts using bases, and activation as acid chlorides or esters for further synthetic steps. The carboxylate is a better leaving group after activation (for example conversion to an acid chloride using SOCl2) and the acidity also permits formation of esters via Fischer esterification under acidic catalysis. Understanding acidity and hydrogen bonding guides choices for extraction, purification and reaction conditions in the laboratory.

Practical implications
Choosing bases and solvents appropriately is essential when performing reactions with carboxylic acids. For example, deprotonation with strong base converts acids to soluble salts useful in extraction and work-up. Knowledge of hydrogen bonding and dimer formation explains why carboxylic acids often have higher boiling points and why their vapours can be pungent and irritating.

Oxidation routes
Carboxylic acids are commonly prepared by oxidation of primary alcohols and aldehydes. Strong oxidants such as potassium permanganate (KMnO4) or chromic acid convert primary alcohols or aldehydes to carboxylic acids under aqueous conditions. This is a straightforward laboratory route: primary alcohols are oxidised via the aldehyde intermediate unless conditions and reagents are chosen to arrest the process.

Benzylic oxidation
Aromatic side-chains at the benzylic position bearing at least one benzylic hydrogen are easily oxidised to carboxylic acids with KMnO4 or hot acidic conditions. For example, toluene is oxidised to benzoic acid. The reaction is useful in functional group interconversion and in structural determination because any alkyl side chain attached to a benzene ring with benzylic hydrogens can be fully oxidised to the benzoic acid, revealing substitution patterns.

Hydrolysis of nitriles and esters
Nitriles (R–C≡N) undergo hydrolysis under acidic or basic conditions to give amides which further hydrolyse to carboxylic acids; final acidification yields the free acid. Esters can be hydrolysed by acid-catalysed esterification reversal or by base-promoted saponification (which gives carboxylate salts that must be acidified to obtain the acid). Saponification is convenient and often quantitative for converting esters to carboxylic acids and alcohols.

Carboxylation of organometallics (Grignard reagents)
Grignard reagents react with carbon dioxide to form carboxylate salts which on acid work-up yield carboxylic acids. This method allows the introduction of a carboxyl group at a chosen position: R–MgX + CO2 → RCO2MgX → RCO2H (after H3O+). It is a versatile synthetic method that can build acids from alkyl or aryl magnesium reagents, but requires anhydrous conditions and exclusion of protic functional groups.

Other methods and selectivity
Oxidative cleavage of alkenes or ozonolysis can yield carboxylic acids depending on the substitution pattern and work-up conditions. Certain catalytic oxidations used industrially can produce acids from hydrocarbons. Choosing a method depends on functional group tolerance, yield considerations and practicality in a laboratory or industrial setting.

Practical considerations
Carboxylic acids are often isolated by acidification of aqueous carboxylate solutions followed by extraction, drying and purification. When preparing acids by oxidation, control of temperature and reagent stoichiometry prevents over-oxidation of sensitive groups. For Grignard carboxylation, ensure absence of water and oxygen to prevent quenching of the organometallic reagent.

Activation of carboxylic acids
Carboxylic acids are versatile precursors to many derivatives: acid chlorides, esters, amides and anhydrides. Because the –OH of the carboxyl group is a poor leaving group, activation steps are commonly used to convert the acid into a better leaving group. Thionyl chloride (SOCl2) converts a carboxylic acid to its acid chloride (RCOCl) releasing SO2 and HCl; this is a common laboratory procedure to prepare reactive acylating agents.

Formation of esters (Fischer esterification and transesterification)
Fischer esterification is the reversible acid-catalysed reaction between a carboxylic acid and an alcohol to give an ester and water (RCOOH + R'OH ⇌ RCOOR' + H2O). Removing water or using excess alcohol shifts equilibrium toward ester formation. Transesterification exchanges the alkyl group of an ester in presence of acid or base catalyst and is widely used in biodiesel production and protecting group exchange.

Amide formation and peptide coupling
Direct conversion of carboxylic acids to amides by reaction with ammonia or amines is slow because OH is a poor leaving group; activation as an acid chloride or use of coupling agents (DCC, EDC, HATU) is standard in peptide synthesis. Activated esters and mixed anhydrides are also used to form amide bonds under mild conditions, minimising racemisation of chiral centres.

Anhydrides and acyl transfer reactions
Anhydrides ((RCO)2O) form via dehydration of carboxylic acids or reaction of acid chlorides with carboxylates. They are good acylating agents for making esters and amides because the leaving group is a carboxylate which is relatively stable. Symmetric and mixed anhydrides are employed in acylation reactions where moderate reactivity is required.

Hydrolysis and saponification
Esters hydrolyse under acidic or basic conditions. Base-catalysed hydrolysis (saponification) proceeds irreversibly yielding carboxylate salts and alcohols and is commonly used to cleave esters quantitatively. Amides require stronger conditions for hydrolysis due to resonance stabilisation of the amide bond; acidic hydrolysis gives carboxylic acids and ammonium salts, while basic hydrolysis gives carboxylate and amine upon work-up.

Synthetic planning and selectivity
Choice of derivative depends on reactivity required and functional group tolerance. Acid chlorides are highly reactive and used when rapid acylation is required; esters are stable storage forms; amides are end-products in many syntheses. Protecting groups, choice of solvents and bases, and activation strategies are key to multi-step syntheses involving carboxylic acids and derivatives.

IR spectroscopy of carbonyls
Infrared spectroscopy is a quick method to identify the presence of carbonyl groups. The C=O stretching vibration appears as a strong, sharp band typically between 1650 and 1750 cm−1. Conjugation with a double bond or aromatic ring lowers the frequency (around 1680 cm−1) because conjugation reduces double-bond character; electron-withdrawing substituents raise the frequency. Carboxylic acids display a broad O–H stretching band between 2500 and 3300 cm−1 overlapping other signals and a strong C=O band near 1700 cm−1. Aldehydes show weak bands at 2700–2850 cm−1 due to the aldehydic C–H stretch.

1H NMR signatures
Proton NMR is diagnostic for certain functional groups. Aldehydic protons resonate downfield near δ 9–10 ppm and often appear as a singlet if there are no neighbouring protons. Protons α to carbonyl groups are deshielded and generally appear between δ 2–3 ppm for ketones and slightly shifted depending on substituents and conjugation. Carboxylic acid protons appear broad and very downfield (δ 10–13 ppm) and exchange with D2O, disappearing on deuteration. Aromatic protons appear in the δ 7–8 ppm region and coupling patterns give substitution information.

13C NMR and carbonyl carbon shifts
Carbon-13 NMR places carbonyl carbons far downfield: aldehyde and ketone carbonyl carbons typically resonate around δ 190–220 ppm, whereas carboxylic acids and esters appear a little upfield around δ 160–185 ppm. The distinct chemical shift of the carbonyl carbon makes 13C NMR a powerful tool for confirming the presence and type of carbonyl-containing functional groups.

Combining techniques for structure elucidation
Use IR to confirm the presence of C=O and O–H stretches, 1H NMR to find aldehydic protons and proton environments, and 13C NMR to confirm the presence of carbonyl carbons. Mass spectrometry and elemental analysis can provide molecular weight and formula. For complete structural assignment, combine all spectroscopic data with chemical tests (Tollens', iodoform) to confirm functional groups.

Practical examples and interpretation tips
If IR shows a strong band at 1715 cm−1 and a broad band at 2500–3300 cm−1, suspect a carboxylic acid. A sharp band at 1720 cm−1 with a 1H NMR singlet at δ 9.8 ppm suggests an aldehyde. A 13C NMR peak near 205 ppm indicates a ketone carbonyl. Learning typical ranges and how substituents shift peaks is essential for confident spectroscopic identification.

Conjugation with aromatic rings
Aromatic carbonyl compounds such as benzaldehyde (C6H5CHO) and acetophenone (C6H5COCH3) have the carbonyl group conjugated with the aromatic ring. This conjugation delocalises electron density between the ring and the carbonyl, affecting reactivity and spectroscopic properties. Conjugation lowers the carbonyl stretching frequency in IR and can stabilise certain intermediates in reactions.

Reactivity differences
Benzaldehyde lacks α-hydrogens and therefore cannot form enolates; it does not undergo aldol self-condensation. Instead, in strong base benzaldehyde undergoes the Cannizzaro reaction giving benzyl alcohol and benzoate. Acetophenone, having α-hydrogens, participates in enolate chemistry: it undergoes α-halogenation, aldol reactions and may give positive iodoform tests because it is a methyl ketone. The aromatic ring can either donate or withdraw electrons through resonance and inductive effects, influencing carbonyl electrophilicity.

Electrophilic aromatic substitution and competing reactions
The aromatic ring still undergoes electrophilic substitution reactions typical of benzene derivatives but the electron-withdrawing effect of the carbonyl group deactivates the ring and directs incoming electrophiles to meta positions relative to the carbonyl. If strong activating substituents are present on the ring, they can override this directing effect. When planning reactions, consider both ring chemistry and carbonyl chemistry as potential pathways.

Reduction and oxidation pathways
Benzaldehyde is easily oxidised to benzoic acid using mild oxidants. It can be reduced to benzyl alcohol by NaBH4. Acetophenone is more resistant to oxidation but can be α-halogenated and further transformed. Both compounds serve as useful building blocks in synthesis of fragrances, dyes and pharmaceuticals. Functional group interconversions on the carbonyl or ring enable access to many derivatives.

Synthetic applications
Benzaldehyde is a precursor to cinnamaldehyde via crossed aldol condensation with acetaldehyde. Acetophenone is used to prepare α-bromo ketones, enones via aldol condensations, and in Friedel–Crafts acylations when extended to larger aromatic systems. Both are common laboratory reagents and illustrate how aromatic conjugation influences the reactivity of carbonyl compounds.

Practical and spectroscopic notes
IR of benzaldehyde shows a C=O stretch around 1700 cm−1 slightly shifted by conjugation; 1H NMR for benzaldehyde includes an aldehydic proton at about δ 9.8–10 ppm. For acetophenone, the methyl group attached to the carbonyl shows a singlet near δ 2.5 ppm while aromatic protons appear between δ 7–8 ppm. Recognising these signatures aids identification in the lab.

Carbonyls in biochemistry
Carbonyl-containing compounds are ubiquitous in biology. Aldehydes and ketones appear in sugars (aldoses and ketoses) and intermediates in metabolism. Carboxylic acids form the backbone of amino acids and fatty acids; their ionised forms (carboxylates) participate in salt formation and enzyme recognition. Enzymes catalyse carbonyl chemistry elegantly, using nucleophilic addition, redox, and decarboxylation reactions under mild conditions with exceptional selectivity.

Industrial applications
Many carbonyl compounds are produced industrially at large scale. Formaldehyde is used to make resins and plastics (urea–formaldehyde, phenol–formaldehyde), acetone is an important solvent and intermediate, and acetic acid is a major feedstock for ester production and vinyl acetate synthesis. Benzaldehyde, vanillin and other aromatic aldehydes are important in fragrances and flavour industries. The ability to interconvert carbonyls and build carbon–carbon bonds makes them central to fine chemical and pharmaceutical manufacture.

Synthetic building blocks
Named reactions involving carbonyls (aldol condensation, Wittig, Cannizzaro, Tishchenko) are used to assemble complex molecules. For example, aldol condensations create carbon frameworks in steroid and terpene syntheses, Wittig reactions build conjugated alkenes important in drug development, and selective reductions allow functional group manipulations essential in multistep routes. Carbonyl chemistry therefore underpins both discovery and process chemistry.

Environmental and safety considerations
Some carbonyl compounds are hazardous: formaldehyde is toxic and potentially carcinogenic; many volatile carbonyls are flammable and irritant. Industrial processes strive to minimise emissions and use safer alternatives where possible. Waste streams containing halogenated by-products (from haloform or other halogenation steps) require proper treatment to avoid environmental contamination.

Green chemistry and modern developments
Modern industrial and laboratory practices aim for greener methods: catalytic processes, atom-economical reactions (e.g., transfer hydrogenation), and avoiding stoichiometric toxic reagents. Biocatalysis uses enzymes for enantioselective reductions and oxidations of carbonyls. Continuous flow processes improve safety for hazardous steps such as ozonolysis, and research on selective catalysts continues to expand sustainable routes to carbonyl-derived products.

Examples illustrating relevance
Glucose (an aldose) interconverts between cyclic hemiacetal forms relevant to metabolism; acetic acid production via methanol carbonylation supplies feedstock for many industries; vanillin synthesis from lignin-derived precursors exemplifies renewable feedstock use in flavour chemistry.

Qualitative tests for functional groups
Chemical tests help confirm presence of aldehydes, ketones and carboxylic acids. Tollens' reagent (ammoniacal silver nitrate) oxidises many aldehydes producing a silver mirror or black silver deposit; it does not oxidise most ketones. Fehling's or Benedict's solution gives a red precipitate (Cu2O) with reducing aldehydes (especially aliphatic ones). The iodoform test (I2/NaOH) gives a characteristic yellow precipitate (CHI3) for methyl ketones and ethanol. Carboxylic acids produce effervescence with carbonates and give a broad O–H stretch in IR.

Purification techniques
Simple and fractional distillation separate carbonyl compounds based on volatility. Aldehydes prone to oxidation are often distilled as soon as formed to prevent conversion to acids. Vacuum distillation is used for high-boiling or thermally sensitive compounds. Recrystallisation and chromatography are applied for purification of solids and complex mixtures, while extraction and acid–base work-up separate acids from neutral or basic components.

Handling reactive reagents
Some reagents used in carbonyl chemistry require careful handling: LiAlH4 reacts violently with water and must be handled under anhydrous conditions with controlled quenching; acid chlorides react with moisture to release HCl and must be handled under dry conditions and often in presence of base to sequester HCl. DIBAL-H reductions require low temperatures and careful quench to avoid over-reduction.

Identification protocol in the lab
A typical identification workflow: obtain an IR spectrum to check for C=O and O–H stretches; record 1H NMR to locate aldehydic protons or characteristic signals; perform Tollens' or iodoform tests to confirm aldehyde or methyl ketone functionality. Combine these results with boiling point and chromatography to reach a confident identification.

Safety, disposal and good practice
Work in a fume hood when handling volatile or toxic carbonyls; wear gloves and eye protection. Dispose of halogenated waste and heavy-metal containing reagents (e.g., Ag from Tollens') according to institutional and environmental regulations. Keep clear records of observations, reagent grades, and reaction conditions to allow reproducibility.

Examples of common lab set-ups
Use a simple distillation apparatus with thermometer, condenser and receiving flask for purification of low-boiling aldehydes. For DIBAL-H reductions, use dry ether solvents and a low-temperature bath (dry-ice/acetone) and slowly add reagent to substrate under inert atmosphere. For Tollens' test, prepare reagent freshly and rinse glassware thoroughly afterwards to avoid metal deposition.

Retrosynthetic thinking and functional group interconversion
Synthetic strategy often begins by analysing the target molecule and identifying key bonds to form or functional groups to introduce. Retrosynthetic disconnections reverse synthetic steps conceptually: for an α,β-unsaturated carbonyl, consider whether an aldol condensation or a Wittig olefination is the best forward step. For a carboxylic acid, plan oxidation of an alcohol, hydrolysis of an ester or carboxylation of a Grignard reagent as possible routes.

Protecting groups and selectivity
When a molecule contains several reactive functional groups, protecting groups enable selective reactions at one site while others remain inert. Aldehydes and ketones can be protected as acetals (RCH(OR')2) under acid catalysis; acetals are stable to base and can be removed by aqueous acid to regenerate the carbonyl. Choose protecting groups that are stable to subsequent steps and remove them under conditions that do not damage the rest of the molecule.

Sequence planning and reagent choice
Plan sequences to avoid unnecessary functional group transformations and to keep yields high. For example, form an amide via activation of the carboxylic acid (acid chloride or coupling reagent) rather than trying direct condensation of acid and amine which is often slow. Order oxidation and reduction steps to prevent undesired over-reduction or oxidation of sensitive moieties. Use chemoselective reagents (NaBH4 vs LiAlH4) to reduce only the desired functional group.

Use of convergent synthesis
Convergent strategies build fragments separately and join them late in the sequence, improving overall yield and simplifying purification. For example, prepare an aldehyde fragment and an ylide fragment separately and join them using a Wittig reaction to form a complex alkene. Convergent approaches are especially useful in complex molecule synthesis such as peptides and natural products.

Examples of multi-step routes
Design a route from benzene to benzylamine: Friedel–Crafts acylation gives acetophenone or benzoyl derivative, side-chain oxidation or reduction as needed, convert to acid chloride then to amide, and finally reduce amide to amine using LiAlH4. Each step requires attention to protecting groups, reaction conditions and purification to avoid racemisation and side reactions.

Practical tips and yield considerations
Minimise number of steps to improve overall yield (each step multiplies losses). Use high-yielding, reliable reactions and plan for easy purification (crystallisation, distillation, extraction). Keep records of stoichiometry, temperatures and times; small changes can affect selectivity and yield. Think about scalability if moving from lab to pilot scale: choose reagents and solvents that are commercially viable and have acceptable safety profiles.