The C–N–C bond angle in trimethylamine is 108°. Which of the following best explains this value being less than the ideal tetrahedral angle of 109.5°?

Answer: B. Nitrogen is sp³ hybridised with one orbital holding the unshared lone pair; the lone pair–bond pair repulsion compresses the C–N–C angle to 108°, slightly less than 109.5°.

Which of the following is the correct IUPAC name of (CH₃CH₂)₃N?

Answer: B. Treating one of the ethyl groups as the parent ethanamine and the other two as N-substituents gives N,N-diethylethanamine. "Triethylamine" is the common name, not IUPAC.

The Gabriel phthalimide synthesis is not suitable for the preparation of aromatic primary amines because:

Answer: B. The phthalimide anion is a nucleophile and requires an SN2-active alkyl halide; aryl halides do not undergo this nucleophilic substitution, so aromatic primary amines cannot be made.

Amines — CUET Chemistry Notes & MCQs

📌 Snapshot

Amines are derivatives of ammonia obtained by replacing one, two or three H atoms by alkyl/aryl groups; nitrogen is sp³ hybridised, giving a pyramidal geometry.
Covers classification (1°, 2°, 3°), nomenclature (common + IUPAC), six preparation routes, basicity in gas vs aqueous phase, distinguishing tests (carbylamine, Hinsberg, HNO₂), and electrophilic substitution of arylamines.
Diazonium salts derived from aniline are introduced as versatile synthetic intermediates allowing replacement of –N₂⁺ by –Cl, –Br, –I, –F, –CN, –OH, –NO₂, –H, and azo-coupling to give dyes.
CUET regularly tests product prediction (Hofmann bromamide, Sandmeyer, Gabriel), basicity ordering with pKb logic, and identification of amines using the Hinsberg/carbylamine tests.

📖 Detailed Notes

2.1 Core concepts

Amines are formed by replacing one, two or three H atoms of NH₃ with alkyl/aryl groups; biologically they occur in proteins, vitamins, alkaloids and hormones; commercially used in medicines, dyes and polymers (NCERT Introduction, p. 259).
Nitrogen in amines is sp³ hybridised with one unshared lone pair; geometry is pyramidal and the C–N–E angle is less than 109.5° (108° in trimethylamine, Fig. 9.1) (NCERT §9.1, p. 259-260). The lone pair occupies the fourth sp³ orbital and is responsible for both the basicity and nucleophilicity of amines.
Classification: 1° (R–NH₂), 2° (R–NHR′), 3° (R₃N) depending on number of H atoms of NH₃ replaced; "simple" if all alkyl/aryl groups are identical, "mixed" if different (NCERT §9.2, p. 260). Aromatic amines may carry –NH₂ on the ring (aniline) or on a side chain (benzylamine, C₆H₅CH₂NH₂ — still aliphatic in nature).
Nomenclature: common system uses alkylamine (one word); IUPAC names primary amines as alkanamines (replace "e" of alkane by "amine"); secondary/tertiary use locant N (e.g., CH₃NHCH₂CH₃ = N-methylethanamine); C₆H₅NH₂ = aniline = benzenamine (NCERT §9.3, Table 9.1, p. 260-261).
Preparation routes: (1) Reduction of nitro compounds with H₂/Ni, Pd or Pt or with Fe/HCl (FeCl₂ hydrolyses to regenerate HCl, so only catalytic HCl needed); active metals + acidic medium (Sn/HCl, Fe/HCl) reduce ArNO₂ → ArNH₂; selective reduction with H₂S, polysulphide or Fe/HCl leaves another reducible group untouched (NCERT §9.4(1), p. 262).
(2) Ammonolysis of alkyl halides with ethanolic NH₃ in a sealed tube at 373 K — gives a mixture of 1°, 2°, 3° amines and quaternary ammonium salt (Hofmann's exhaustive alkylation); reactivity RI > RBr > RCl; large excess NH₃ favours primary amine while excess alkyl halide favours quaternary salt (NCERT §9.4(2), p. 262-263).
(3) Reduction of nitriles with LiAlH₄ or catalytic H₂/Ni, Pd, Pt gives 1° amines with one more carbon — used for "ascent of amine series" (Mendius reduction); (4) Reduction of amides with LiAlH₄ gives amines (NCERT §9.4(3-4), p. 263).
(5) Gabriel phthalimide synthesis: phthalimide + ethanolic KOH → potassium phthalimide → alkyl halide (SN2) → alkaline hydrolysis (or hydrazinolysis) → primary amine + phthalhydrazide; works ONLY for aliphatic 1° amines, since aryl halides do not undergo nucleophilic substitution with the phthalimide anion (NCERT §9.4(5), p. 264).
(6) Hofmann bromamide degradation: R/Ar–CONH₂ + Br₂ + 4 NaOH → R/Ar–NH₂ + Na₂CO₃ + 2 NaBr + 2 H₂O — gives a primary amine with one carbon less than the amide; involves migration of the alkyl/aryl group from carbonyl C to N through an isocyanate intermediate (NCERT §9.4(6), p. 264). Benzamide → aniline; acetamide → methanamine.
Physical properties: lower aliphatic amines are gases with fishy odour; higher ones smell like fish; aniline and arylamines are colourless but darken on storage due to atmospheric oxidation; lower amines are water-soluble via N–H···O hydrogen bonding, solubility falls with increasing alkyl mass; alcohols are more polar than amines (O is more electronegative than N) and form stronger H-bonds (NCERT §9.5, p. 265).
Boiling point order of isomeric amines: 1° > 2° > 3° because 1° has two N–H bonds available for intermolecular H-bonding while 3° has none; Table 9.2 shows n-C₄H₉NH₂ (350.8 K) > (C₂H₅)₂NH (329.3 K) > C₂H₅N(CH₃)₂ (310.5 K) > (C₂H₅)(CH₃)₂N (310 K) and n-C₄H₉OH (390.3 K) > n-C₄H₉NH₂ (NCERT §9.5, Table 9.2, p. 266).
Basicity: amines react with acids to form salts; aliphatic amines are stronger bases than NH₃ (+I effect of alkyl groups, pKb 3–4.22); aromatic amines are weaker than NH₃ (lone pair in conjugation with ring; aniline pKb = 9.38) (NCERT §9.6, Table 9.3, p. 266-267). Smaller pKb means stronger base; Kb = [R-NH₃⁺][OH⁻]/[R-NH₂].
Gas-phase basicity order: 3° > 2° > 1° > NH₃ (governed only by the +I effect of alkyl groups); aqueous-phase order is irregular due to interplay of (a) +I electron release, (b) solvation/H-bonding stabilisation of the alkyl-substituted ammonium cation, and (c) steric hindrance to solvation. NCERT gives the ethyl series as (C₂H₅)₂NH > (C₂H₅)₃N > C₂H₅NH₂ > NH₃ and the methyl series as (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃ (NCERT §9.6.1, p. 268-269).
Aniline is less basic than NH₃ because aniline has five resonance structures (lone pair delocalised into the benzene ring) but anilinium ion has only two — so the free amine is more stabilised than its conjugate acid; electron-releasing groups (–OCH₃, –CH₃) at p-position increase basicity, while electron-withdrawing groups (–NO₂, –SO₃H, –COOH, –X) decrease it. Order: p-toluidine > aniline > p-nitroaniline (NCERT §9.6.1(b), p. 269).
Alkylation: amines react with alkyl halides giving 2°, 3° amines and quaternary salts (Hofmann's exhaustive alkylation); acylation: 1° and 2° amines react with acid chlorides, anhydrides or esters in presence of a base (pyridine, removing HCl) to give N-substituted amides; benzoyl chloride gives benzoylated derivatives (Schotten-Baumann benzoylation) (NCERT §9.6.2-9.6.3, p. 270).
Carbylamine reaction: 1° aliphatic and aromatic amines + CHCl₃ + alcoholic KOH (Δ) → foul-smelling isocyanides (carbylamines) R–NC; 2° and 3° amines give no such reaction — diagnostic test for 1° amines: R–NH₂ + CHCl₃ + 3KOH → R–NC + 3KCl + 3H₂O (NCERT §9.6.4, p. 271).
Reaction with HNO₂ (nitrous acid generated in situ from NaNO₂ + dil. HCl): 1° aliphatic amines → unstable aliphatic diazonium salts which liberate N₂ quantitatively and form alcohols (Van Slyke method for amino-acid estimation); 1° aromatic amines at 273-278 K → stable aromatic diazonium salts (diazotisation); 2° amines give yellow oily N-nitrosoamines (R₂N–NO); 3° amines react differently (form a soluble salt with HCl, then react with HNO₂ to give an N-nitroso ammonium compound) (NCERT §9.6.5, p. 271).
Hinsberg test with benzenesulphonyl chloride (C₆H₅SO₂Cl): 1° amine → N-alkylbenzenesulphonamide bearing an N–H that is acidic (SO₂ group withdraws e⁻), hence soluble in alkali; 2° amine → N,N-dialkylbenzenesulphonamide (no acidic N–H), insoluble in alkali; 3° amine → no reaction (no N–H to react); p-toluenesulphonyl chloride is now preferred (NCERT §9.6.6, p. 271-272).
Electrophilic substitution on aniline: –NH₂ is a powerful o/p-directing activating group; bromination with bromine water at room temperature gives white 2,4,6-tribromoaniline (over-reaction); to obtain monosubstituted products, –NH₂ is protected by acetylation (acetic anhydride → acetanilide), substitution performed, then hydrolysed back to the amine — gives mainly p-bromoaniline (NCERT §9.6.7(a), p. 272-273).
Nitration of aniline directly gives tarry oxidation products and significant m-nitroaniline (anilinium ion in strong acid is meta-directing; ratio o : m : p ≈ 19 : 47 : 51); after acetylation, nitration gives p-nitroacetanilide as major product, hydrolysed to p-nitroaniline (NCERT §9.6.7(b), p. 273).
Sulphonation of aniline with conc. H₂SO₄ → anilinium hydrogensulphate → on heating at 453-473 K → p-aminobenzenesulphonic acid (sulphanilic acid, exists as a zwitterion); aniline does not undergo Friedel-Crafts reaction (alkylation or acylation) because it forms a salt with AlCl₃, making N positively charged (strong deactivator) (NCERT §9.6.7(c), p. 273).
Diazonium salts have formula R–N₂⁺X⁻ where R is aryl and X⁻ is Cl⁻, Br⁻, HSO₄⁻, BF₄⁻; named by attaching "diazonium" to the parent hydrocarbon name followed by anion name; arenediazonium ion is stabilised by resonance over the ring carbons (NCERT §II, p. 274).
Diazotisation: aniline + NaNO₂ + 2HCl at 273-278 K → C₆H₅N₂⁺Cl⁻ + NaCl + 2H₂O; diazonium salts are unstable in the dry state (explosive) and must be used immediately in solution (NCERT §9.7, p. 274).
Properties: benzenediazonium chloride is a colourless crystalline solid, water-soluble, stable in cold but reacts with warm water; benzenediazonium fluoroborate is water-insoluble and stable at room temperature (NCERT §9.8, p. 275).
Replacement reactions (loss of N₂): Sandmeyer with Cu₂Cl₂/Cu₂Br₂/CuCN gives Ar–Cl, Ar–Br, Ar–CN; Gatterman uses Cu powder + HX; KI gives Ar–I directly (no Cu needed); HBF₄ then heat (Balz-Schiemann) gives Ar–F; mild reducing agents (H₃PO₂ or ethanol) give Ar–H; warming in water (283 K) gives phenol; NaNO₂/Cu with diazonium fluoroborate gives Ar–NO₂ (NCERT §9.9 A, p. 275-276).
Coupling reactions (retention of diazo group): benzenediazonium chloride + phenol (in mildly alkaline medium) → p-hydroxyazobenzene (orange dye); + aniline (in mildly acidic medium) → p-aminoazobenzene (yellow); the resulting azo compounds (–N=N–) are coloured and used as dyes (NCERT §9.9 B, p. 276).
Importance: diazonium salts enable introduction of –F, –Cl, –Br, –I, –CN, –OH, –NO₂ into aromatic rings — aryl fluorides and iodides cannot be made by direct halogenation, and –CN cannot be introduced by direct nucleophilic substitution of chlorobenzene (NCERT §9.10, p. 276-277).

2.2 Definitions to memorise

Term	Definition	Page
Primary amine (1°)	R–NH₂ — one H of NH₃ replaced by alkyl/aryl group	260
Secondary amine (2°)	R–NHR′ — two H atoms of NH₃ replaced	260
Tertiary amine (3°)	R₃N — all three H atoms of NH₃ replaced	260
Simple vs mixed amine	All alkyl/aryl groups same = simple; different = mixed	260
Quaternary ammonium salt	R₄N⁺X⁻ formed by exhaustive alkylation of amine	263
Ammonolysis	Cleavage of C–X bond by NH₃ giving amine + HX (sealed tube, 373 K)	262
Hofmann bromamide degradation	Amide + Br₂ + NaOH → 1° amine with one C less	264
Gabriel phthalimide synthesis	Phthalimide → K-phthalimide → alkyl halide → hydrolysis to give 1° aliphatic amine	264
Mendius reduction	Reduction of nitrile R–CN with Na/C₂H₅OH or H₂/Ni → 1° amine with one C more	263
Diazotisation	Conversion of 1° aromatic amine to arenediazonium salt with NaNO₂/HCl at 273-278 K	274
Carbylamine reaction	1° amine + CHCl₃ + alc. KOH → R–NC (isocyanide); diagnostic for 1° amines	271
Hinsberg test	C₆H₅SO₂Cl distinguishes 1°/2°/3° amines by alkali-solubility of the sulphonamide	271
Sandmeyer reaction	Ar–N₂⁺X⁻ + Cu₂X₂ → Ar–X (X = Cl, Br, CN)	275
Gatterman reaction	Ar–N₂⁺X⁻ + Cu/HX → Ar–X (less efficient than Sandmeyer)	275
Balz-Schiemann reaction	Ar–N₂⁺BF₄⁻ Δ→ Ar–F + N₂ + BF₃	275
Coupling reaction	Ar–N₂⁺ + ArOH or ArNH₂ → Ar–N=N–Ar′ (azo dye)	276
Schotten-Baumann reaction	Benzoylation of 1°/2° amine with C₆H₅COCl in presence of NaOH	270
Acetanilide	C₆H₅NHCOCH₃ — N-protected aniline used to moderate ring substitution	272
pKb	–log Kb; smaller pKb = stronger base	267
Aniline	C₆H₅NH₂ = benzenamine; weakest of common amines (pKb 9.38)	269
Isocyanide	R–N≡C (carbylamine); foul-smelling; obtained from 1° amine + CHCl₃ + KOH	271
N-Nitrosoamine	R₂N–NO; yellow oil formed from 2° amine + HNO₂	271
Invert/azo dye	Coloured –N=N– compound from diazonium salt + activated arene	276

2.3 Diagrams / processes to remember

Fig. 9.1, p. 260 — pyramidal shape of trimethylamine, three C–N bonds with the lone pair occupying the fourth tetrahedral position; C–N–C angle = 108°, slightly less than tetrahedral 109.5° due to lone-pair compression.
Fig. 9.2, p. 266 — intermolecular hydrogen bonding in primary amines: N–H···N chains explain why 1° amines boil higher than 2° (one N–H per N) and 3° amines (no N–H); diagram contrasts the H-bonded network in CH₃CH₂NH₂ with the absence of such bonding in (CH₃)₃N.
Table 9.1, p. 261 — common vs IUPAC names of alkylamines and arylamines (methylamine = methanamine; ethylamine = ethanamine; aniline = benzenamine; N,N-dimethylmethanamine for trimethylamine).
Table 9.2, p. 266 — boiling points of amines, alcohols, alkanes of similar molar mass showing 1° > 2° > 3° amine order (n-butylamine 350.8 K > diethylamine 329.3 K > N,N-dimethylethylamine 310 K) and alcohol > amine (n-butanol 390.3 K).
Table 9.3, p. 267 — pKb values: methanamine 3.38, dimethylamine 3.27, trimethylamine 4.22, ethanamine 3.29, aniline 9.38, N-methylaniline 9.30, N,N-dimethylaniline 8.92 — useful for ordering basicity.
Resonance structures of aniline (five) vs anilinium ion (two), p. 269 — diagram of how the lone pair on N is donated into the ring in three Kekulé-equivalent canonical forms plus two ortho/para charge-separated forms, lowering the energy of the free amine more than the protonated ion → reduced basicity.
Sandmeyer / Gatterman reaction equations, p. 275 — Cu(I) catalysed replacements of –N₂⁺ by –Cl, –Br, –CN; mechanism involves radical-pair intermediate with Cu(II)X species; arrows showing C₆H₅N₂⁺ + CuCl → C₆H₅Cl + N₂ + Cu⁺.
Coupling reaction scheme, p. 276 — formation of p-hydroxyazobenzene (from phenol, mildly alkaline pH 9-10) and p-aminoazobenzene (from aniline, mildly acidic pH 4-5); electrophile is the arenediazonium ion; coupling para to the activating –OH or –NH₂ group.
Acetanilide protection scheme, p. 272-273 — flowchart: aniline → (Ac₂O / pyridine) → acetanilide → (Br₂ / CH₃COOH) → p-bromoacetanilide → (H₃O⁺) → p-bromoaniline; shows how moderation prevents tribromination.
Carbylamine test arrow-pushing, p. 271 — formation of dichlorocarbene from CHCl₃ + OH⁻, attack on R–NH₂, elimination of 2 HCl to give R–NC; only 1° amines have two replaceable N–H atoms.

2.4 Common confusions / NTA trap points

Gas-phase basicity follows pure +I logic (3° > 2° > 1° > NH₃), but aqueous phase order is irregular due to solvation + steric hindrance; do NOT generalise either way — NCERT specifies methyl series as (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃ (Table 9.3, p. 267-268).
Hofmann bromamide gives amine with one C less; reduction of nitriles gives amine with one C more — these two routes are commonly swapped in distractors (p. 263-264). Acetamide (C-2) → methanamine (C-1); methyl cyanide (C-2) → ethylamine (C-2).
Gabriel phthalimide synthesis works only for primary aliphatic amines; aromatic primary amines cannot be made because aryl halides do not undergo nucleophilic substitution with phthalimide anion (p. 264).
Hinsberg test: 1° gives sulphonamide soluble in alkali (acidic N–H); 2° gives sulphonamide insoluble in alkali (no N–H); 3° does not react at all — students often reverse 1° and 2° solubility (p. 271-272).
Carbylamine test is positive only for 1° amines (aliphatic and aromatic); 2° and 3° amines give negative result (p. 271).
Aniline does NOT undergo Friedel-Crafts alkylation/acylation because aniline + AlCl₃ forms a salt putting positive charge on N, deactivating the ring (p. 273); the salt is C₆H₅NH₂·AlCl₃.
Direct nitration of aniline gives substantial m-nitroaniline because in strong acid aniline is protonated to anilinium ion which is meta-directing — this contradicts the "amino is o/p-directing" rule and is a favourite trap (p. 273).
HNO₂ + 1° aliphatic amine → alcohol + N₂↑ (unstable RN₂⁺ decomposes); but HNO₂ + 1° aromatic amine → stable ArN₂⁺ at 273-278 K — students often confuse the two outcomes (p. 271).
HNO₂ + 2° amine → N-nitrosoamine (yellow oil, R₂N–NO); 3° aliphatic amine → quaternary ammonium nitrite (water-soluble); 3° aromatic amine (N,N-dimethylaniline) → p-nitroso derivative (ring attack). All three outcomes are different — memorise each (p. 271).
Diazonium coupling needs activated arenes (phenol, aniline, N,N-dimethylaniline); benzene itself does NOT couple. Students sometimes write coupling with toluene — wrong.
Bromination of aniline with Br₂/H₂O gives the 2,4,6-tribromo product (not the mono); to get the mono-Br product, the amine must first be acetylated (p. 272-273).
Reduction of ArNO₂ in acidic medium (Sn/HCl, Fe/HCl) gives ArNH₂; in neutral/alkaline medium gives intermediate products like ArN=NAr (azobenzene) or ArNHNHAr (hydrazobenzene) — easy to confuse with diazonium chemistry.

📊 Previous-Year Questions

Practise with real CUET Chemistry previous-year papers — every question solved, with the correct answer and a step-by-step explanation.

View solved CUET PYQ papers →

Ready to drill Chemistry?

Unlock all MCQs, chapter tests, mocks & PYQs for ₹199/year.

Get UniDrill Pro