Chemical Reactions of Amines 🎯
Amines love to react because their nitrogen atom carries a lone-pair, making them eager nucleophiles. The N–H bond is also quite polar, and primary amines form strong H-bonds, so their boiling points rise compared with similar alkanes 🌡️:contentReference[oaicite:0]{index=0}.
1 Basic character 🚩
Amines grab protons from acids and give you ammonium salts:
$$\mathrm{RNH_2 + H^+ \;\longrightarrow\; RNH_3^{+}}$$
You can measure strength with $$K_b=\frac{[\mathrm{RNH_3^{+}}][\mathrm{OH^-}]}{[\mathrm{RNH_2}]}$$ and $$pK_b=-\log K_b$$. A bigger Kb (or smaller pKb) means a stronger base 💪:contentReference[oaicite:1]{index=1}.
- Gas phase order: tertiary > secondary > primary > NH3.
- Water order: primary > secondary > tertiary (solvation + steric effects flip the trend).
- Aromatic amines (e.g., aniline) are weaker because resonance pulls the lone pair into the ring:contentReference[oaicite:2]{index=2}.
2 Alkylation ⚡
Slip an alkyl halide into the mix and you lengthen the carbon chain:
$$\mathrm{RNH_2 + R’X \;\xrightarrow[\;]{\;KOH\;}\; RR’NH + HX}$$
Keep going with excess halide to reach quaternary ammonium salt (handy surfactants):contentReference[oaicite:3]{index=3}.
3 Acylation (including Benzoylation) 🧥
Primary or secondary amines trade one hydrogen for an acyl group when they meet acyl chlorides, anhydrides, or esters (pyridine mops up HCl):
$$\mathrm{CH_3NH_2 + C_6H_5COCl \;\longrightarrow\; CH_3NHCOC_6H_5 + HCl}$$
The product is an amide. Using benzoyl chloride gives benzoylation:contentReference[oaicite:4]{index=4}.
4 Carbylamine (Isocyanide) test 😷
Heat a primary amine with CHCl3 and ethanolic KOH:
$$\mathrm{RNH_2 + CHCl_3 + 3KOH \;\xrightarrow{\Delta}\; RNC + 3KCl + 3H_2O}$$
The nasty-smelling isocyanide proves you started with a primary amine:contentReference[oaicite:5]{index=5}.
5 Reaction with nitrous acid 🌬️
- Primary aliphatic amine → unstable alkyldiazonium salt → N2 (vigorous fizz) + alcohol (quantitative N2 lets chemists measure amino acids):contentReference[oaicite:6]{index=6}.
- Primary aromatic amine (273–278 K) → stable arenediazonium salt (gateway to dyes and more).
e.g. Aniline + HNO2 + HCl → C6H5N2Cl + 2 H2O:contentReference[oaicite:7]{index=7}. - Secondary and tertiary amines behave differently (no N2 burst).
6 Hinsberg test with benzenesulphonyl chloride 🧪
| Amines meet C6H5SO2Cl | Product | Alkali solubility? |
|---|---|---|
| Primary | N-ethylbenzenesulphonyl amide | Yes (acidic H) |
| Secondary | N,N-diethylbenzenesulphonamide | No |
| Tertiary | No reaction | No product |
This elegant test separates and identifies the three classes of amines:contentReference[oaicite:8]{index=8}.
7 Electrophilic substitution on aniline 🎨
The –NH2 group pumps electrons into the ring, so ortho/para positions light up with charge density:
- Bromination: Br2(aq) instantly gives 2,4,6-tribromoaniline (white precipitate):contentReference[oaicite:9]{index=9}.
- Nitration: Direct nitration is messy, so first protect –NH2 by acetylation (make acetanilide), nitrate, then hydrolyse to get mostly p-nitroaniline:contentReference[oaicite:10]{index=10}.
- Sulphonation: Aniline → anilinium hydrogensulphate → heat → sulphanilic acid (p-aminobenzenesulphonic acid):contentReference[oaicite:11]{index=11}.
Because AlCl3 forms a salt with aniline, Friedel–Crafts alkylation/acylation simply doesn’t happen.
8 A quick look at diazonium salts ❄️
Arenediazonium ions (R–N2+X−) stay stable only around 273 K; they form by diazotisation of a primary aromatic amine with NaNO2 + HCl. These salts unlock gorgeous azo-dyes and coupling reactions :contentReference[oaicite:12]{index=12}.
High-Yield NEET Ideas 🔥
- Understand pKb trends—gas vs. aqueous—plus factors like solvation and steric hindrance.
- Use the carbylamine test to spot primary amines instantly.
- Remember the Hinsberg test for separating primary, secondary, and tertiary amines.
- Master diazotisation conditions (273 K, NaNO2/HCl) because diazonium ions appear in many synthesis questions.
- Know why the –NH2 group sends electrophiles to ortho/para positions and how acetyl protection tames its hyperreactivity.
Happy studying and keep practicing those reaction pathways! 🎓✨