Chapter 12 — Amines

Fig 1 — Educational Diagram: Classification

Definition: Amines are derivatives of ammonia (NH3) in which one or more hydrogen atoms are replaced by alkyl or aryl groups. They act as bases and nucleophiles because of the lone pair on nitrogen.

Classification by number of organic groups on nitrogen:

• Primary amine (1°): One organic group attached to N — general formula RNH2. Example: methylamine (CH3NH2).
• Secondary amine (2°): Two organic groups attached — R2NH. Example: dimethylamine ((CH3)2NH).
• Tertiary amine (3°): Three organic groups attached — R3N. Example: trimethylamine ((CH3)3N).
• Quaternary ammonium salt (not a neutral amine): Four organic groups give a positively charged nitrogen R4N+ X-. Example: tetramethylammonium chloride ((CH3)4N+ Cl-).

Classification by nature of organic groups:

• Aliphatic amines: organic groups are alkyl chains (e.g., methylamine).
• Aromatic amines: nitrogen attached directly to an aromatic ring (aryl) — e.g., aniline (C6H5NH2).
• Mixed: combinations of alkyl and aryl groups (e.g., N-methylaniline).
• Cyclic and heterocyclic amines: nitrogen incorporated in a ring (e.g., pyrrole, pyridine, piperidine).

Electronic structure and consequences:

• In aliphatic amines nitrogen is approximately sp3-hybridized; the lone pair is localized on N and is available for protonation (good base).
• In aromatic amines (aniline) the lone pair is delocalized into the benzene ring by resonance, reducing basicity.
• Quaternary ammonium species have no lone pair to act as a base and are permanently charged salts.

Basicity trends (important points):

• Alkyl groups are electron donating (inductive effect) and stabilize the conjugate acid; they generally increase basicity relative to ammonia in the gas phase.
• In aqueous solution, solvation matters: secondary > primary > tertiary for simple alkylamines (because tertiary amine cations are sterically hindered and less well solvatable).
• Aromatic amines (e.g., aniline) are much less basic due to resonance delocalization of the lone pair into the ring. Electron-withdrawing substituents (–NO2) further reduce basicity; electron-donating groups (–OCH3, –CH3) increase it.
• Conjugate acid formation (protonation) is: RNH2 + H+ → RNH3+. The equilibrium is governed by Kb (or pKb) in water.

Summary diagram suggestion: A simple tree starting with "Amines" splitting into "By substitution: primary / secondary / tertiary / quaternary" and "By nature: aliphatic / aromatic / heterocyclic" helps visual classification.

Fig 2 — Educational Diagram: Nomenclature

What is being named? Amines are organic compounds containing one or more –NH (amino) groups. Nomenclature gives a systematic (IUPAC) and common name to each amine.

Basic rules (IUPAC):

• Identify the longest carbon chain attached to the nitrogen. Use the suffix -amine on that parent hydrocarbon: e.g., CH3NH2 = methanamine (commonly methylamine).
• Number the parent chain so that the carbon bearing the –NH2 group gets the lowest possible locant; indicate the position before the parent name: e.g., CH3CH(NH2)CH3 = propan-2-amine (commonly isopropylamine).
• For substituted amines (secondary, tertiary) use N‑ to show substituents on nitrogen. Name the N‑substituents alphabetically and then the parent amine: e.g., CH3–NH–CH2CH3 = N‑methyl­ethanamine.
• If the –NH2 group is not the principal functional group (a higher‑priority group such as carboxylic acid is present), name the –NH2 as the prefix amino‑ (with locant): e.g., 4‑aminobenzoic acid.
• Aromatic amines: the simplest aromatic amine is aniline (common) = benzenamine (IUPAC). Substituents on the ring are numbered with the –NH2 group as position 1: e.g., p‑phenylenediamine = 1,4‑diaminobenzene.
• Cyclic amines: if the ring is the parent, use the ring name + -amine (cyclohexylamine) or special heterocyclic names where established (e.g., piperidine, pyrrolidine, aziridine for three‑membered N‑ring systems when common names are accepted).

Primary, secondary, tertiary (naming consequences):

• Primary amine: R–NH2. Parent chain contains the N; use suffix -amine (e.g., ethanamine).
• Secondary amine: R1–NH–R2. Name as N‑substituted derivative of the parent amine (e.g., N‑ethylmethanamine or more simply N‑ethyl­methylamine).
• Tertiary amine: R1–N(R2)–R3. Use N,N‑ to show two substituents on N (e.g., N,N‑dimethylmethanamine = trimethylamine when parent chosen as methane).)

Common vs IUPAC names: Many amines have well‑known common names (methylamine, aniline). IUPAC often uses systematic names (methanamine, benzenamine), but both are widely used in school chemistry; teachers usually accept common names with correct structures.

Quick naming steps (flow): 1) Find longest chain attached to N → 2) Decide if –NH2 is principal functional group or substituent → 3) Number chain for lowest locant for amine → 4) Use suffix -amine or prefix amino‑ as appropriate → 5) For N‑substituents use N‑ notation.

Important notes:

• Use hyphens and locants: propan‑2‑amine (IUPAC) instead of isopropylamine for clarity.
• When multiple amine groups are present use prefixes di‑, tri‑ and give locants: e.g., 1,2‑diaminoethane (ethylenediamine).
• Show N‑substitution explicitly for secondary/tertiary: N‑methyl‑N‑phenylpropan‑2‑amine.

Fig 3 — Educational Diagram: Structure and Bonding

Overview: Amines are derivatives of ammonia (NH3) in which one or more H atoms are replaced by alkyl or aryl groups. General formulas: primary RNH2, secondary R2NH, tertiary R3N. They may be aliphatic (alkyl) or aromatic (aryl).

Geometry & Hybridization:

• N in simple amines is essentially sp3 hybridized: three sigma bonds (to C or H) + one lone pair occupy four sp3 orbitals.
• Shape: trigonal pyramidal (similar to NH3). Typical H–N–C or C–N–C bond angle ≈ 107° (slightly less than tetrahedral 109.5° due to lone-pair repulsion).
• Typical bond lengths: C–N single bond ≈ 1.47 Å (147 pm); N–H ≈ 1.01 Å (101 pm).

Lone pair characteristics & bonding consequences:

• The lone pair on N is localized in an sp3 orbital in aliphatic amines and is available for protonation (gives basicity) and nucleophilic attack.
• Pyramidal inversion: the nitrogen center can invert (through a planar transition state) with a low barrier (~5–7 kcal·mol⁻¹), so amines are stereochemically not stable at nitrogen.

Aromatic (aryl) amines — resonance effects:

• In aniline (Ph–NH2) the N lone pair is partly delocalized into the benzene ring (conjugation), forming resonance structures that spread electron density into the ring. This reduces lone-pair availability for protonation, so aniline is much less basic than aliphatic amines.
• If conjugation is strong, the nitrogen becomes effectively more sp2-like and the C–N bond shortens and the geometry at N becomes more planar (bond angle ≈120°).
• Contrast: in pyridine the lone pair on N lies in an sp2 orbital orthogonal to the aromatic π system and is hence available for protonation — pyridine behaves as a base. In pyrrole the lone pair is part of the aromatic sextet and is not basic.

Factors affecting basicity (availability of the lone pair):

• Inductive (+I) effect of alkyl groups increases electron density on N → tends to increase basicity (gas phase).
• Resonance/delocalization (as in aniline) decreases basicity by withdrawing lone-pair density into a π-system.
• Solvation & steric effects: in aqueous solution solvation of the conjugate acid (R3NH+) stabilizes it. Steric hindrance (especially in tertiary amines) can reduce solvation and thus lower observed basicity in water. Consequently, in water typical order (simple alkylamines) is: secondary > primary > tertiary. In the gas phase, basicity often follows tertiary > secondary > primary due to intrinsic electron-donating effects.
• Electron-withdrawing substituents (–NO2, –CN) decrease basicity; electron-donating groups (–OH, –OR, –NH2) increase basicity on the ring if they donate by induction but may have complex effects if they also engage in resonance.

Key bonding illustrations:

• For aliphatic amines: localized sp3 lone pair on N interacting as a Lewis base (R3N + H+ → R3NH+).
• For aniline: resonance structures showing N lone pair delocalized into the ring (reduce basicity).
• For pyridine vs pyrrole: show lone pair orientation — orthogonal and available vs part of aromatic sextet and unavailable.

Typical energetic/structural numbers:

• C–N single bond length ≈ 1.47 Å; N–H ≈ 1.01 Å.
• Bond angle at N in sp3 amines ≈107°; when lone pair is delocalized toward sp2 character, angle approaches ≈120°.
• Pyramidal inversion barrier ≈ 5–7 kcal·mol⁻¹ (fast inversion at room temperature).

Summary: Structure and bonding in amines are governed by the hybridization of nitrogen and the localization or delocalization of its lone pair. These structural features control chemical behaviour such as basicity, nucleophilicity, and participation in resonance — central to understanding reactivity in aliphatic vs aromatic amines.

Fig 4 — Educational Diagram: Methods of Preparation

Overview
Amines (RNH2, R2NH, R3N, ArNH2) are prepared by methods that either introduce an –NH2 (or substituted –NRn) group into a carbon skeleton or convert an existing nitrogen‑containing group into an amine. Choice of method depends on whether the target is a primary, secondary or tertiary amine and whether the amine is aliphatic or aromatic.

Main laboratory and industrial methods (with rationale)

• Alkylation of ammonia or amines
  NH3 + R–X → RNH2 (primary); RNH2 + R'–X → R2NH (secondary) etc. (via nucleophilic substitution). Simple but gives mixtures due to over‑alkylation; useful for some secondary/tertiary amines when controlled (use excess ammonia to favor primary amine).
• Reductive amination (most versatile for primary/secondary/tertiary)
  Carbonyl (aldehyde/ketone) + amine (NH3 or RNH2) → imine (Schiff base) → reduced (H2/Pd, NaBH3CN, NaBH(OAc)3) → amine. This installs an amine at a carbonyl carbon with high selectivity and is widely used in drug synthesis.
• Reduction of nitriles
  R–C≡N + 4[H] → R–CH2NH2 (LiAlH4 or catalytic H2/Ni). Good for converting R–C≡N (made from R–X + CN–) into primary amines.
• Reduction of nitro compounds
  Ar–NO2 or R–NO2 + 6[H] → Ar–NH2 or R–NH2. Reagents: H2/Pd (catalytic hydrogenation), or Fe/HCl, Sn/HCl, Zn/HCl (laboratory). Major route to aromatic amines (e.g., aniline from nitrobenzene).
• Reduction of amides
  R–CONH2 + 4[H] → R–CH2NH2 (LiAlH4). Converts amide to amine without changing the carbon skeleton; often used for aliphatic primary amines.
• Gabriel phthalimide synthesis (for primary amines)
  Potassium phthalimide reacts with R–X (SN2) to give N–alkylphthalimide; hydrolysis or hydrazinolysis then liberates the primary amine RNH2. Advantage: avoids over‑alkylation and gives primary amines selectively.
• Hofmann bromamide rearrangement (for primary amines from amides)
  R–CONH2 + Br2 + 4NaOH → R–NH2 + Na2CO3 + other products. Mechanism: N‑bromination, base‑promoted rearrangement to isocyanate, hydrolysis to primary amine (one C atom retained; no addition of new C).
• Conversion from azides / Curtius rearrangement / Staudinger
  Alkyl/aryl azides (R–N3) reduced (PPh3/H2O or catalytic H2) → RNH2. Curtius rearrangement (acyl azide → isocyanate → RNH2 after hydrolysis) is used to convert carboxylic acids to amines (useful in synthesis).
• Aromatic amines (special considerations)
  Direct alkylation of aniline is difficult (aniline is less nucleophilic and ring can be alkylated). The usual route is nitration of aromatic ring to give Ar–NO2 followed by selective reduction to Ar–NH2. Industrially, catalytic hydrogenation of nitroarenes is common.

Key mechanistic highlights

• Gabriel synthesis: K+ phthalimide (nucleophilic N) performs SN2 on R–X → N‑alkylphthalimide. Hydrolysis (acidic/alkaline) or hydrazinolysis gives RNH2 and phthalic hydrazide or phthalic acid byproducts.
• Hofmann rearrangement: Amide → N‑bromamide → rearrangement (migration of R from C to N) → isocyanate → hydrolysis to primary amine (RNH2) + CO2.
• Reductive amination: Aldehyde/ketone + NH3 (or RNH2) → imine/enamine → chemoselective reduction (hydride donors or H2 catalysts) → amine; selectivity controlled by stoichiometry & reducing agent strength.

Limitations & practical tips

• Simple alkylation of NH3 often gives mixtures (primary → secondary → tertiary → quaternary ammonium). Use excess NH3 to favour primary amine or use Gabriel/Hofmann to get primary amines cleanly.
• Reduction methods must be chosen to avoid reducing other sensitive groups (use chemoselective reagents like NaBH3CN for reductive amination in presence of reducible functionalities).
• Aromatic amines are best prepared via nitro reduction; direct substitution methods are limited by the low nucleophilicity of anilines and side reactions (e.g., acylation or sulfonation of ring).

Industrial examples
Aniline (C6H5NH2) — industrially from nitrobenzene hydrogenation (H2/Pd or Fe/HCl historically) — used for dyes, polyurethane precursors. Methylamine — large scale production often by reaction of methanol with ammonia over catalysts (synthesis gas/amine industry routes).

Fig 5 — Educational Diagram: Physical Properties

Overview: Amines (RNH2, R2NH, R3N) are derivatives of ammonia in which one or more H atoms are replaced by alkyl or aryl groups. Their physical properties are governed by molecular size, hydrogen bonding ability (presence of N–H), polarity and resonance (for aryl amines).

State, colour and odour: Low molecular weight aliphatic amines are gases or volatile liquids with a characteristic fishy, ammoniacal odour. Higher members are colourless liquids or solids. Many low-MW amines are hygroscopic and some are very soluble in water.

Boiling points: Primary and secondary amines have N–H bonds and can form intermolecular hydrogen bonds, so they have higher boiling points than comparable hydrocarbons. Tertiary amines lack N–H and cannot form N–H···N hydrogen bonds with each other, so they usually have lower boiling points than primary/secondary amines of similar molar mass. However, boiling point also increases with molecular weight and surface area, so bulky tertiary amines can have high b.p. Comparison with alcohols: for the same carbon skeleton, alcohols (O–H) have stronger hydrogen bonding than amines and therefore higher boiling points.

Solubility in water: Amines form hydrogen bonds with water (via N–H···O or lone pair···H–O), so small aliphatic amines (typically up to C3–C4) are completely miscible in water. Solubility decreases as the nonpolar hydrocarbon portion grows. Aromatic amines (anilines) are less soluble due to the nonpolar benzene ring and delocalization of the lone pair into the ring.

Basicity (physical manifestation): Amines are basic because the nitrogen lone pair can accept a proton. In the gas phase, alkyl substitution generally increases basicity (electron-donating inductive/hyperconjugative effects). In aqueous solution, solvation of the conjugate acid (RNH3+) and steric factors matter: often secondary > primary > tertiary (alkyl) in aqueous basicity because primary/secondary ammonium ions are better solvated than bulky tertiary ones. Aromatic amines (aniline) are much less basic owing to delocalization of the lone pair into the aromatic ring (resonance).

Other properties: Density of common amines is usually lower than water for low MW compounds; many amines are flammable; some form salts (ammonium salts) which are ionic solids (water-soluble) and have much higher melting points than the parent amine.

Fig 6 — Educational Diagram: Basicity of Amines

What is basicity? Basicity of an amine is its ability to accept a proton (H+). In aqueous solution an amine (B) reacts with water:

B + H₂O ⇌ BH⁺ + OH⁻

The equilibrium constant for this reaction is the base dissociation constant K_b:

K_b = [BH⁺][OH⁻]/[B]

We usually use pK_b = −log K_b; pK_a of the conjugate acid and pK_b are related (at 25 °C):

pK_a (BH⁺) + pK_b (B) = 14

Factors that determine basicity

• Availability of lone pair: Basicity depends on how available the nitrogen lone pair is to accept H⁺.
• Inductive (+I) effect: Alkyl groups donate electron density to N, increasing basicity (stabilises B relative to BH⁺). In the gas phase alkyl substitution generally increases basicity (3° > 2° > 1° > NH₃).
• Solvation and steric effects (aqueous): In water, solvation of the protonated form BH⁺ stabilises it. Primary and secondary aminium ions are better solvated than tertiary ions; steric hindrance around N lowers solvation. Hence aqueous basicity often follows: 2° > 1° > 3° for simple aliphatic amines.
• Resonance delocalisation: If the lone pair is delocalised (e.g., aniline, where N lone pair participates in aromatic ring), basicity decreases because lone pair is less available. Substituents that withdraw electron density (–NO₂, –CF₃) further reduce basicity; electron-donating groups (+I, +R) increase it.
• Hybridisation: Greater s-character pulls electron density closer to nucleus and reduces basicity. Order: sp³ (most basic) > sp² > sp (least basic).

Aliphatic vs aromatic amines

• Aliphatic amines: Lone pair on sp³ N is localized → relatively strong bases (but see solvation/steric trends above).
• Aromatic amines (aniline): Lone pair on N is conjugated with the benzene ring → much less basic than aliphatic amines. Electron-donating substituents at para/ortho increase basicity (but steric hindrance can counteract at ortho); electron-withdrawing substituents decrease it.

Qualitative trends summary

• Gas phase: 3° > 2° > 1° > NH₃ (alkyl groups increase electron density, less solvation effect).
• Aqueous phase: 2° ≳ 1° > 3° (solvation stabilizes BH⁺ of 1° and 2° more effectively; 3° is sterically hindered).
• Aniline < aliphatic amines (due to resonance); strong –R groups (e.g., –NO₂) make amines very weak bases.

Common observations and consequences: Because basicity affects protonation, it determines formation of ammonium salts (useful for purification), solubility in water, reactivity in electrophilic substitutions (e.g., aniline needs protection), and biological activity (many drugs are amines and are protonated at physiological pH).

Practical note: To predict basicity, ask: how available is the lone pair? Are there electron-donating groups nearby? Is the lone pair delocalised? How well can the conjugate acid be solvated?

Fig 7 — Educational Diagram: General Chemical Reactions

Overview

Amines (RNH2, R2NH, R3N, ArNH2) show a set of characteristic reactions that arise from the lone pair on nitrogen and, for aromatic amines, conjugation with the ring. Key general reactions include acid–base behavior (salt formation), alkylation, acylation, formation of imines (Schiff bases), reactions with nitrous acid (diazotization for aryl amines), electrophilic aromatic substitution effects, and transformations of diazonium salts (useful in synthesis of dyes and many functional-group interconversions).

1. Acid–base behavior (basicity and salt formation)

Amines are basic (lone pair accepts H+). They form ammonium salts with strong acids:

• RNH2 + HCl → RNH3+ Cl−

Basicity depends on electronic and steric effects: aliphatic amines are generally more basic than aniline (aromatic) because of resonance delocalization of the lone pair in aniline. pKb (or pKa of conjugate acid) quantifies basic strength.

2. Alkylation and quaternization

Amines act as nucleophiles toward alkyl halides to give higher amines and, on excessive alkylation, quaternary ammonium salts (used as phase-transfer catalysts and disinfectants):

• RNH2 + R'–X → R2NH (then R3N → R4N+ X− on exhaustive alkylation)

3. Acylation (formation of amides)

Amines react with acyl chlorides/anhydrides or acid derivatives to give amides (important for peptide bonds and protective group chemistry):

• RNH2 + R'COCl → R'CONHR + HCl

4. Formation of imines (Schiff bases)

Primary amines condense with aldehydes/ketones to give imines (useful in organic synthesis and as intermediates):

• RNH2 + R'CHO ⇌ R'CH=NR + H2O

5. Reaction with nitrous acid (HNO2)

Behavior differs for aliphatic and aromatic primary amines:

• Primary aliphatic amine: RNH2 + HNO2 → ROH + N2↑ + H2O (deamination; alkyl diazonium unstable → alcohol)
• Secondary aliphatic amine: R2NH + HNO2 → R2N–N=O (N‑nitrosamine; many are carcinogenic)
• Primary aromatic amine (ArNH2): ArNH2 + HNO2 + H+ → ArN2+ X− (aromatic diazonium salt, stable at 0–5 °C)

Diazonium salts (ArN2+) are versatile intermediates: they undergo Sandmeyer reactions (replacement by Cl, Br, CN), coupling to give azo dyes, or reduction/phenol formation under suitable conditions.

6. Electrophilic aromatic substitution effects (for aryl amines)

The –NH2 group is a strong ortho/para director and activator in electrophilic aromatic substitution, but the free amine can be too reactive (oxidation, polymerization). To control reactivity, amines are often temporarily converted to amides (acetylation) which are less activating and direct ortho/para.

7. Hofmann elimination (from quaternary ammonium salts)

Exhaustive methylation to form R4N+ X− followed by heating with Ag2O/H2O or strong base gives Hofmann elimination to yield the least substituted alkene (useful synthetic method):

• R4N+ OH− (heat) → alkene + R3N + H2O

Mechanistic themes

• Nucleophilicity of N (attacks electrophiles such as R'X, acyl chlorides).
• Basicity/protonation equilibrium (RNH2 ⇌ RNH3+), affecting reactivity and solubility.
• Resonance delocalization in aromatic amines reduces basicity and alters site reactivity on the ring.
• Diazonium chemistry: formation, stability (aryl diazoniums stable cold), and diverse substitutions (Sandmeyer, coupling).

Practical notes

Many reactions are pH- and temperature-sensitive (e.g., diazotization at 0–5 °C). Safety: nitrosamines are carcinogenic; handle diazonium salts with care (they can decompose explosively if dry).

Fig 8 — Educational Diagram: Aromatic Amines and Aniline Chemistry

Definition and general features
Aromatic amines are compounds in which one or more amino groups (–NH2, –NHR, –NR2) are directly bonded to an aromatic ring. The simplest and most important example is aniline (C6H5NH2), an aromatic primary amine.

Structure and resonance
In aniline the nitrogen lone pair is conjugated with the π system of the benzene ring. This delocalization (resonance) reduces the electron density on nitrogen compared with aliphatic amines and makes the lone pair less available for bonding to H+ (weaker basicity). A resonance depiction (qualitative) is: C6H5–NH2 ⇄ several structures with partial positive charge on N and negative charge delocalized on ring.

Physical properties

• Aniline: colourless to pale yellow oily liquid, characteristic odour; bp ≈ 184 °C.
• Poorly soluble in water (but readily soluble in dilute acids as anilinium salts).
• Toxic and must be handled with care (skin absorption, harmful if swallowed).

Basicity
Because of resonance, aniline is a weaker base than aliphatic amines. Typical values: anilinium ion pKa ≈ 4.6 (so pKb ≈ 9.4). Electron donating substituents (–OH, –OCH3, –CH3) at para/ortho increase basicity (make nitrogen more available); electron withdrawing groups (–NO2, –CF3) decrease basicity markedly.

Preparations (common laboratory/manufacturing methods)

• Reduction of nitrobenzene: catalytic hydrogenation (H2/Pt or Pd) or chemical reduction like Sn/HCl, Fe/HCl followed by alkaline workup: C6H5NO2 → C6H5NH2.
• From diazonium salts: aromatic diazonium salts can be reduced to give the parent amine (or used to generate many other derivatives).
• Industrial methods: catalytic hydrogenation of nitroarenes is widely used for bulk production of anilines.

Chemical behavior and important reactions

• Electrophilic aromatic substitution (EAS): The –NH2 group is a strong activating, ortho/para-directing group because of +R (resonance donation). However, under strongly acidic conditions the aniline is protonated to the anilinium ion which is deactivating and meta-directing. Therefore nitration/bromination of aniline directly gives uncontrolled products or oxidation; hence the –NH2 is often protected as an amide (acetanilide) before nitration/bromination.
• Acylation (protection): Aniline + CH3COCl → acetanilide (C6H5NHCOCH3). Acetanilide is less activating and gives cleaner para-substitution in EAS. After substitution, hydrolysis regenerates the –NH2.
• Bromination: Aniline gives multi-substitution on direct bromination. Using acetanilide gives mainly para-bromoacetanilide, then hydrolysis → p-bromoaniline.
• Diazotization (key reaction): Primary aromatic amines react with nitrous acid (HNO2) at 0–5 °C to form arenediazonium salts, R–N2+X–. These are versatile intermediates.
• Reactions of diazonium salts (many synthetic transformations):
  • Sandmeyer reactions: replacement of –N2+ by Cl, Br, CN, etc. (using CuCl, CuBr, CuCN).
  • Hydrolysis to phenol on warming, giving phenols from anilines.
  • Azo coupling: electrophilic coupling with activated aromatics (e.g., phenols, anilines) to give azo dyes (Ar–N=N–Ar').
• Reduction: Aniline can be reduced further or used as a substrate to prepare alkylated amines (via reductive alkylation) or converted into many derivatives.

Practical importance / applications
Aniline and substituted anilines are building blocks in dyes (azo dyes, indigo precursors), pharmaceuticals (e.g., intermediates in paracetamol synthesis via p-aminophenol), agrochemicals, rubber processing chemicals, and polyurethanes (as intermediates to diisocyanates).

Synthetic strategy tips (common CBSE-level points)

• To nitrate or brominate aniline selectively, first protect the –NH2 by acetylation (make acetanilide), perform EAS, then hydrolyse to restore –NH2.
• Use diazotization of aniline to access many functional groups on the benzene ring (Sandmeyer reactions, phenol formation, azo coupling).

Safety note
Aniline and many aromatic amines are toxic and some are carcinogenic; use gloves, fume hood and avoid skin contact.

Fig 9 — Educational Diagram: Diazonium Salts and Diazonium Chemistry

Definition: Diazonium salts are salts containing the diazonium group ArN2+X- (where Ar is an aryl group and X- is usually Cl-, Br-, BF4-, etc.). They are central intermediates in aromatic substitution chemistry, especially for transforming amino groups into many other functional groups.

Preparation (Diazotization)

• Start from a primary aromatic amine (ArNH2), e.g., aniline.
• Generate nitrous acid in situ: NaNO2 + HCl -> HNO2 + NaCl.
• In acidic medium at 0-5 °C, HNO2 is converted to the nitrosonium ion NO+ which reacts with ArNH2 to give the diazonium salt:
• ArNH2 + HNO2 + HCl (0-5 C) -> ArN2+ Cl- + 2 H2O.
• Conditions: low temperature (0-5 °C) and acidic medium are essential because aryl diazonium salts are stable only at low temperature.

Why only aromatic primary amines? Aliphatic diazonium ions are unstable and decompose rapidly producing alkyl cations and nitrogen gas; aromatic diazonium salts are stabilized by resonance with the aromatic ring and can be handled at low temperatures.

Key Reactions of Aryl Diazonium Salts

• Hydrolysis to phenol (on warming): ArN2+ -> ArOH + N2. This is a convenient laboratory route to phenols from anilines.
• Sandmeyer reactions (replacement by halides or CN): using Cu(I) salts to replace N2+ by Cl, Br, or CN to give ArCl, ArBr, ArCN.
• Balz-Schiemann reaction (to give aryl fluorides): convert ArN2+ to ArN2+ BF4- (isolate as solid) and thermally decompose to ArF + N2. A principal method to prepare Ar-F bonds.
• Reduction / deamination: ArN2+ + H3PO2 (or hypophosphorous acid) -> ArH + N2. Useful to remove an amino group.
• Sandmeyer-like replacement to give nitro, thiol or other derivatives using appropriate nucleophiles and catalysts.
• Coupling reactions (azo formation): ArN2+ (electrophile) couples with activated aromatic compounds (especially phenols and arylamines) in alkaline medium to give azo compounds Ar-N=N-Ar\'. These azo compounds form brightly coloured dyes.

Mechanistic notes (brief)

• Formation step involves nitrosonium ion NO+ attack on the amine to form a diazonium intermediate with loss of water.
• Many replacement reactions proceed through loss of N2 (a very good leaving group) giving reactive intermediates (aryl cation/radical) that are trapped by nucleophiles or participate in electron-transfer processes (copper mediated).

Practical points and safety

• Carry out diazotization at 0-5 °C; many diazonium salts are explosive when dry—keep them in cold aqueous solution or convert to stable tetrafluoroborate for Balz-Schiemann.
• Work under dilute conditions and avoid heating unless a specific decomposition (hydrolysis or Balz-Schiemann) is intended.

Importance / Applications

• Azo dye industry: formation of a wide range of coloured azo compounds used as textile dyes and indicators (methyl orange, Congo red family, etc.).
• Preparation of substituted aromatics: halobenzenes, fluorobenzenes, benzonitriles and phenols from aniline derivatives.
• Useful in organic synthesis to introduce or remove functional groups selectively at the aromatic ring.

Fig 10 — Educational Diagram: Tests for Amines

Overview. Amines (RNH2, R2NH, R3N / ArNH2 etc.) are classified as primary, secondary and tertiary. Two standard qualitative tests used in Class 12 chemistry to distinguish these are the Hinsberg test and the reaction with nitrous acid. Below are the reactions, observations, mechanism rationale and important conditions.

1. Hinsberg test (using benzenesulfonyl chloride or p‑toluenesulfonyl chloride as Hinsberg reagent).

• Procedure: Add Hinsberg reagent (R'‑SO₂Cl) to the amine in a non‑aqueous solvent and then add aqueous NaOH.
• Key reactions and observations:
• Primary amine (RNH₂): sulfonamide forms which has an N–H that is slightly acidic. It reacts with NaOH to give a soluble sulfonamide salt → solution clears on addition of base.
• Equation: RNH₂ + R'‑SO₂Cl → RNH‑SO₂R' + HCl; then RNH‑SO₂R' + NaOH → RNH₂‑SO₂R' (Na salt) (soluble).
• Secondary amine (R₂NH): forms N‑sulfonyl derivative which has no acidic N–H → insoluble in NaOH (ppt remains).
• Equation: R₂NH + R'‑SO₂Cl → R₂N‑SO₂R' (insoluble).
• Tertiary amine (R₃N): no N–H, does not form sulfonamide → no ppt; tertiary amine usually remains unreacted (or gives an amine salt with HCl produced).

Why it works (mechanism rationale). Formation of sulfonamide requires a nucleophilic N–H. Primary gives RNH–SO₂R' (has N–H) which can be deprotonated to a water‑soluble anion by OH–. Secondary gives R₂N–SO₂R' (no N–H) so remains insoluble. Tertiary cannot form sulfonamide because it lacks N–H.

2. Reaction with nitrous acid (HNO₂, generated in situ from NaNO₂ + HCl).

• General generation: NaNO₂ + HCl → HNO₂ + NaCl (perform at low temperature, 0–5 °C for aromatic diazotization).
• Aliphatic primary amines (RNH₂): react with HNO₂ to produce unstable aliphatic diazonium ions, which decompose to give nitrogen gas → effervescence (positive test).
• Example equation (simplified): RNH₂ + HNO₂ + HCl → R–N₂⁺Cl⁻ (unstable) → R⁺ + N₂(g) → products (often ROH if water attacks).
• Aromatic primary amines (ArNH₂): form stable arenediazonium salts at 0–5 °C, Ar–N₂⁺Cl⁻, which do not give effervescence and are isolable at low temperature and used in azo coupling / Sandmeyer reactions.
• Equation (aromatic): ArNH₂ + HNO₂ + HCl (0–5 °C) → Ar–N₂⁺Cl⁻ + 2 H₂O.
• Secondary amines (R₂NH): undergo nitrosation to give N‑nitrosoamines (R₂N–NO) — no nitrogen gas evolution; gives characteristic nitroso products (often carcinogenic compounds in food chemistry context).
• Equation (general): R₂NH + HNO₂ → R₂N–NO + H₂O.
• Tertiary amines (R₃N): normally do not form diazonium salts; at low temperature there is no diazotization and no effervescence. Some tertiary amines may be nitrosated at nitrogen or at α‑carbon under strong conditions, but this is not used as a routine class 12 test.

Important conditions & observations to remember

• Hinsberg: perform in a solvent (e.g., ether) and test solubility of product in aqueous NaOH.
• Nitrous acid: always generated in situ (NaNO₂ + HCl), keep temperature 0–5 °C for aromatic diazonium stability.
• Observation summary: aliphatic primary → effervescence (N₂); aromatic primary → formation of diazonium salt (no effervescence; cold conditions); secondary → formation of nitrosamines (no gas); tertiary → no typical diazotization result.

Safety note (real‑life relevance). Nitrosamines formed from secondary amines + nitrites are potent carcinogens and are a health concern in preserved foods (nitrites + amines in stomach can form nitrosamines). Diazotization reactions should be carried out in cold conditions with care — diazonium salts can be explosive when dry.

Fig 11 — Educational Diagram: Separation and Purification

Basic idea: Amines are basic nitrogen-containing compounds. Their separation and purification relies mainly on their acid–base behaviour, differences in volatility and solubility, and on formation of isolable derivatives. Typical operations: acid–base extraction, fractional/steam/distillation, formation of salts or derivatives (sulfonamides, amides), drying and recrystallization.

1. Acid–base extraction (most important technique)

• Organic amines (RNH2, R2NH, R3N) are basic and react with acids to give water‑soluble ammonium salts: RNH2 + HCl → RNH3+ Cl−. In a two‑layer system (organic solvent + water) a free base stays in the organic layer while its protonated salt moves to the aqueous layer.
• Procedure to separate an amine from a neutral organic compound (e.g., separate aniline from bromobenzene):
  1. Shake the organic solution with dilute HCl: the amine is protonated and transfers to aqueous layer as RNH3+Cl−; neutral organics remain in organic layer.
  2. Separate layers. Basify the aqueous layer (e.g., with NaOH) to liberate the free amine: RNH3+ + OH− → RNH2 + H2O.
  3. Extract the liberated amine into an organic solvent, dry (e.g., anhydrous Na2SO4) and evaporate solvent to get purified amine.
• Choice of pH: whether an amine is protonated depends on pH and pKa of its conjugate acid (see formulas). Typical pKa values: aliphatic amine conjugate acids ≈ 10–11, anilinium ≈ 4.6 — aromatic amines are less basic and may not fully protonate at mildly acidic pH.

2. Differential extraction and separation of mixtures of amines

• Use differences in basicity: by carefully controlling pH you can selectively protonate stronger bases while leaving weaker bases unprotonated. Example: protonate a tertiary aliphatic amine at pH 7–8 while aromatic amine remains unprotonated if its pKa is much lower.
• Hinsberg reagent (benzenesulfonyl chloride) can be used to separate primary, secondary and tertiary amines: primary amines form sulfonamides that can be deprotonated and dissolved in alkali (soluble salts), secondary amines give insoluble sulfonamides, tertiary amines do not react. This allows physical separation followed by hydrolysis to recover original amines.

3. Purification by derivative formation

• Convert amines into crystalline, easily purified derivatives then regenerate the free amine: common conversions are formation of amides or sulfonamides.
  • Example: aniline → acetanilide (by acetylation). Acetanilide is crystalline and can be purified by recrystallization; hydrolysis then returns pure aniline.
  • R–NH2 + R'COCl → R–NH–CO–R' + HCl (acylation)

4. Distillation methods

• Simple or fractional distillation for amines with sufficiently different boiling points.
• Steam distillation for relatively volatile, water‑immiscible amines (or when decomposition occurs on simple distillation).
• Vacuum (reduced pressure) distillation for heat‑sensitive amines.

5. Drying and final purification

• Dry the organic solution of the amine with anhydrous drying agents (Na2SO4, MgSO4, CaCl2). Remove drying agent by filtration and evaporate solvent.
• Final purification: fractional distillation, recrystallization of derivatives, or column chromatography (for laboratory/pure samples).

Practical points & limitations

• Aromatic amines (e.g., aniline) are less basic and require stronger acid to protonate; also they are often more soluble in organic solvents — adjust pH and solvent accordingly.
• Some amines form emulsions during extraction; adding salt (salting out) or centrifugation helps phase separation.
• Reactive or polyfunctional molecules may form side products on derivatization; choose mild conditions.

Summary flowchart (simple): crude amine mixture → (acidify) → ammonium salt in aqueous layer → (separate, basify) → free amine → (dry, distil or recrystallize/derivatize) → pure amine.

Fig 12 — Educational Diagram: Mechanisms (Key Reaction Mechanisms)

This section summarises the key reaction mechanisms of amines taught in Class 12 CBSE and explains the steps, driving forces and typical outcomes. Focus is on mechanistic types that recur across the chapter: nucleophilic substitution (SN2 and related displacements), nucleophilic acyl substitution (acylation), diazotisation and subsequent transformations of aryl amines, Hofmann-type reactions, and the Gabriel phthalimide method. For each mechanism the central electron‑flow steps and important stereochemical or electronic requirements are given.

1. Nucleophilic substitution on alkyl halides (formation of aliphatic amines)

• Type: SN2 (bimolecular) — typical when an alkyl halide (R–X) reacts with ammonia or an amine (Nu = NH3 or R'NH2) in a polar aprotic or protic medium. Mechanistic feature: concerted backside attack, one transition state, inversion at the carbon center if it is chiral.
• Key points: rate = k[Nu][R–X]; primary halides react fastest by SN2; secondary/slower; tertiary do not undergo SN2 (elimination often competes).
• Practical issue: overalkylation — once a primary amine forms it is usually more nucleophilic than ammonia and gives secondary/tertiary/quaternary products unless selective methods (Gabriel synthesis, reductive amination) are used.

2. Gabriel phthalimide synthesis (selective primary amine formation)

• Type: nucleophilic substitution of alkyl halide by the phthalimide anion (SN2), followed by hydrolysis or hydrazinolysis to give a primary amine.
• Mechanistic outline: deprotonation of phthalimide → nucleophilic attack on R–X → N-alkylphthalimide → cleavage (hydrolysis or hydrazine) → R–NH2.
• Advantage: prevents overalkylation because phthalimide supplies only one nucleophilic N and the formed phthalimide derivative is not a stronger nucleophile.

3. Nucleophilic acyl substitution (acylation of amines; Schotten–Baumann conditions)

• Type: nucleophilic attack of the amine on an acyl chloride (or anhydride) → tetrahedral intermediate → elimination of Cl− (or carboxylate) → amide formation.
• Mechanistic feature: the amine acts as nucleophile; HCl formed is neutralised (often by base) to drive reaction forward.

4. Diazotisation of aromatic amines and follow-up reactions

• Formation of diazonium salt: Ar–NH2 + HNO2 + HCl → Ar–N≡N+ Cl− (cold, 0–5 °C). Mechanism involves nitrosylation (NO+) of the aniline nitrogen, followed by proton transfers and loss of water to give the diazonium cation.
• Transformations of diazonium salts:
• Hydrolysis to phenol (warm water): Ar–N2+ → Ar–OH + N2 (loss of N2, electrophilic capture by H2O).
• Sandmeyer reaction (replacement by Cl or Br): uses CuCl/CuBr to give Ar–Cl/Ar–Br (via single electron / radical type pathway assisted by Cu(I)).
• Schiemann reaction (fluorination): Ar–N2+ BF4− → heat → Ar–F + N2 (thermal decomposition of diazonium tetrafluoroborate).
• Coupling with activated aromatic compounds (azo dye formation): diazonium cation acts as electrophile and couples at para/ortho position of activated ring (e.g., phenol, aniline derivatives) → Ar–N=N–Ar' (azo compound).

5. Hofmann bromamide rearrangement (amide → primary amine with one fewer C)

• Reagents: R–CONH2 + Br2 + NaOH (warm) → R–NH2 + NaBr + CO2 (net loss of the carbonyl carbon as CO2).
• Mechanism outline: formation of N‑bromoamide → deprotonation → intramolecular migration of R group from C to N (rearrangement) via isocyanate intermediate → hydrolysis of isocyanate → primary amine.

6. Hofmann elimination (from quaternary ammonium hydroxides)

• Preparation: exhaustive methylation of amine → quaternary ammonium salt → conversion to hydroxide (Ag2O/H2O) → heat → elimination to give alkene.
• Mechanistic feature: E2-type elimination giving the less substituted (Hofmann) alkene preferentially due to steric hindrance at the β‑protons near the bulky +NR4 group.

7. Basicity and protonation (mechanistic consequences)

• Amines act as Brønsted bases; protonation (R3N + H+ ⇌ R3NH+) affects nucleophilicity and reactivity. Henderson–Hasselbalch relates pH and fraction protonated.
• Aromatic amines (anilines) are weaker bases because lone pair is delocalised into the ring (resonance), affecting electrophilic substitution patterns and diazotisation rates.

Mechanistic advice for students: always show electron‑flow (curved arrows), identify the nucleophile and electrophile, note any intermediates (carbocations, diazonium, isocyanates), and consider stereochemistry (SN2 inversion, E2 anti‑periplanar requirement).

Fig 13 — Educational Diagram: Factors Affecting Reactivity

Scope: For amines, 'reactivity' usually refers to basicity (ability to accept a proton) and nucleophilicity (ability to donate an electron pair to an electrophile). Both are controlled by how available the nitrogen lone pair is and how well the conjugate acid (NH+ species) is stabilized by the medium.

Main factors:

• Inductive effect (+I and −I): Electron-donating alkyl groups (+I) push electron density toward N increasing basicity (and nucleophilicity) for aliphatic amines. Electron-withdrawing groups (−I) decrease basicity. Example: p-nitroaniline < aniline < p-methylaniline (basicity increases with electron donation).
• Hyperconjugation: Alkyl groups stabilize the positive charge on the protonated amine by hyperconjugation, increasing basicity (helps explain why alkylamines are stronger bases than ammonia in gas phase).
• Resonance (delocalization): If the lone pair on N is delocalized (as in aniline, amides), it is less available to accept H+, so basicity decreases. Resonance stabilization of the unprotonated form reduces basicity. In aniline the lone pair donates into the benzene ring; in amides it is shared with the carbonyl.
• Hybridization: The greater the s-character of the orbital holding the lone pair, the closer electrons are held to nucleus and the less basic the amine. Order of basicity: sp3 (alkyl amine) > sp2 (aniline) > sp (iminic) for lone-pair availability.
• Solvation and hydrogen bonding (solvent effects): In aqueous solution, the conjugate acid (RNH3+) is stabilized by hydrogen bonding with water. Better solvation favors higher observed basicity. For example, in water: secondary amines often appear more basic than tertiary because steric hindrance in tertiary amines reduces solvation of the protonated form.
• Steric hindrance: Bulky substituents around N hinder approach of H+ or an electrophile and reduce nucleophilicity and often aqueous basicity (even if electron donating). Tertiary amines may be strong bases in gas phase but weaker in water due to poor solvation.
• Aromatic vs aliphatic amines: Aromatic amines (aniline and derivatives) are usually much less basic than aliphatic amines because the lone pair is conjugated with the aromatic ring. Substituents on the ring modify this via resonance and inductive effects; ortho substituents can also introduce steric (ortho) effects.
• Substituent position and type (electronic effects): Para and ortho electron-donating groups increase basicity of aniline (but steric ortho effects may reduce it); electron-withdrawing groups (NO2, CN) drastically lower basicity. Hammett correlation (σ constants) often describes these trends quantitatively.
• Gas phase vs solution-phase behavior: In the gas phase, solvation is absent, so inductive/hyperconjugation factors dominate (tertiary > secondary > primary > ammonia). In aqueous solution, solvation and hydrogen-bonding reverse or modify these orders.
• Difference between basicity and nucleophilicity: Basicity is a thermodynamic property (equilibrium position for protonation), while nucleophilicity is a kinetic property (rate of reaction with electrophiles). Steric hindrance and solvent affect nucleophilicity more strongly than basicity.

How to apply this in problems: Identify whether resonance or inductive effects dominate, check hybridization, consider steric and solvation effects, and know whether the question expects gas-phase or aqueous behavior.

Fig 14 — Educational Diagram: Applications

Amines are nitrogen-containing organic compounds (RNH2, R2NH, R3N, ArNH2) whose basicity, nucleophilicity and ability to form salts and derivatives make them extremely useful in industry, biology and daily life. Their main practical applications arise from three properties: (1) they act as bases and nucleophiles in synthesis, (2) they form stable salts (including quaternary ammonium salts) with diverse physical properties, and (3) aromatic amines can be converted into diazonium salts that are precursors to dyes and pigments.

Major application areas:

• Pharmaceuticals: Many drugs contain amine groups (local anesthetics like procaine and lidocaine, neurotransmitters such as dopamine and epinephrine, antihistamines and many antidepressants). The amine functionality affects solubility, receptor binding and metabolic behaviour.
• Dyes and pigments: Primary aromatic amines (e.g., aniline) are converted into diazonium salts (ArN2+Cl−) which couple with phenols or other aromatic compounds to give azo dyes used in textiles and inks.
• Polymers and resins: Diamines (e.g., hexamethylenediamine) react with dicarboxylic acids to give polyamides (nylon-6,6). Melamine (a triamine) forms melamine–formaldehyde resins used in laminates, adhesives and kitchenware.
• Industrial auxiliaries and gas treatment: Amines such as monoethanolamine (MEA) and diethanolamine (DEA) are employed to remove CO2 and H2S from natural gas and flue gases (amine scrubbing).
• Surfactants and disinfectants: Long-chain aliphatic amines and quaternary ammonium salts (e.g., benzalkonium chloride) are used as fabric softeners, detergents, emulsion stabilizers and antimicrobial agents.
• Agrochemicals and dyes intermediates: Amines serve as building blocks for herbicides, insecticides and agrochemical intermediates.
• Catalysts and phase-transfer agents: Quaternary ammonium salts (e.g., tetrabutylammonium salts) act as phase-transfer catalysts accelerating reactions between reagents in different phases.

Practical notes: the chemical reactivity of amines — alkylation, acylation, diazotization (for primary aromatic amines), quaternization — is exploited to synthesize complex molecules. The choice of an amine type (aliphatic vs aromatic, primary/secondary/tertiary) is guided by desired basicity, solubility and steric profile.

Fig 15 — Educational Diagram: Safety and Environmental Aspects

Overview: Amines are organic derivatives of ammonia (RNH2, R2NH, R3N). They are widely used in dyes, pharmaceuticals, rubber, pesticides, and water treatment. Their basicity, volatility (for low molecular-weight amines), and chemical reactivity create specific safety and environmental issues that must be managed.

Health and safety hazards:

• Irritation and corrosion: Many amines irritate skin, eyes and respiratory tract. Concentrated amines can cause chemical burns.
• Toxicity: Aromatic amines (e.g., aniline, benzidine derivatives) can be systemically toxic and many are carcinogenic or cause methemoglobinemia. Short-chain aliphatic amines (methylamine, ethylamine, dimethylamine) can be acutely toxic by inhalation.
• Volatility and inhalation risk: Low molecular-weight amines are volatile and malodorous; inhalation exposure is a common route.
• Flammability: Some amines and their vapours are flammable; safe storage away from ignition sources is required.
• Reactivity: Amines react with nitrites under acidic conditions to form N-nitroso compounds (nitrosamines), many of which are potent carcinogens. Amines can also react with chlorine or chloramine during water treatment to form hazardous by-products (e.g., NDMA).

Safe handling and emergency measures:

• Use local exhaust ventilation or closed systems for volatile amines. Monitor air concentrations where applicable.
• Personal protective equipment (PPE): chemical-resistant gloves, goggles/face shield, lab coat/coveralls, and, if inhalation risk exists, appropriate respirators.
• Storage: keep cool, well‑ventilated, in tightly closed containers, segregated from strong oxidizers and strong acids; store combustible amines away from heat sources.
• First aid: for inhalation remove to fresh air; for skin contact wash with water and remove contaminated clothing; for eye exposure flush with water and seek medical care. For ingestion do not induce vomiting; seek immediate medical attention.
• Spill control: contain with inert absorbent, use appropriate PPE, ventilate area, collect wastes for proper disposal—avoid release to drains.

Environmental impacts:

• Nitrosamines and carcinogenic by-products: Secondary amines react with nitrite under acidic conditions to form N-nitrosamines (R2N–N=O), many of which (e.g., NDMA) are carcinogenic and persist at trace levels with serious health implications in drinking water and food.
• Water treatment interactions: Amines present in source water or produced in processes can react with chlorine/chloramine to produce regulated disinfection by-products (e.g., chloramines, nitrosamines). Dimethylamine is a known precursor of NDMA during chloramination.
• Aquatic toxicity and biodegradability: Some amines and quaternary ammonium compounds (QACs) are toxic to aquatic organisms and can be persistent; they may increase BOD/COD and harm wastewater treatment biological systems.
• Odour and air emissions: Volatile amines contribute to foul odours (e.g., from animal waste, sewage) and can impact local air quality and nuisance complaints.

Waste management and mitigation:

• Minimize formation of nitrosamines: control nitrite levels, pH, and avoid contact between secondary amines and nitrosating agents; choose alternative disinfectants when appropriate.
• Treatment technologies for amine-containing effluents: acid scrubbing (converts amines to ammonium salts), steam stripping, activated carbon adsorption, advanced oxidation processes (ozone, UV/H2O2), and biological treatment where biodegradable.
• Disposal: follow regulations—neutralize and collect wastes for chemical treatment or licensed hazardous waste disposal; do not discharge untreated amine wastes to surface waters or drains.
• Substitution and process control: use less hazardous amine alternatives, closed systems, reagent recovery, and solvent/amine reclamation to reduce releases.

Regulatory and monitoring: Many amines and nitrosamines are regulated in workplace exposure standards and drinking water. Regular monitoring of workplace air, process streams, and effluents is recommended. Where nitrosamine risk exists, monitor precursor amine and nitrite levels and measure specific nitrosamine concentrations (trace-level analysis).