5. Amines
Amines are bases and nucleophiles — their lone pair does everything. Their basicity depends on how available that lone pair is, which is why aromatic amines are far weaker bases than aliphatic ones.
Basicity
Aliphatic amines (pKb ~3–4) are much more basic than aniline (pKb ~9.4): aniline's lone pair is delocalized into the aromatic ring. Electron-donating groups on the ring raise basicity; electron-withdrawing groups lower it.
Key Reactions
- Alkylation: amines + alkyl halides → ammonium salts (excess halide gives tetraalkylammonium salts).
- Acylation: amines + acid chlorides → amides.
- Diazotization: primary aromatic amines + NaNO2/HCl at 0–5 °C → diazonium salts, which couple with phenols or anilines to give azo dyes.
- Hofmann elimination: exhaustive methylation then Ag2O/heat gives the least substituted alkene — the opposite of Zaitsev.
Example: Aniline + NaNO2/HCl (0 °C) → benzenediazonium chloride; coupling with phenol gives an orange azo dye.
Practice Problems
- What is the product of CH3NH2 with excess CH3I?
- Why is aniline a weaker base than cyclohexylamine?
- What reagents diazotize an aromatic amine?
- What is the major Hofmann elimination product of the 2-butyltrimethylammonium ion?
Answers: 1. Tetramethylammonium iodide 2. Aniline's lone pair is delocalized into the ring, so it's less available to accept a proton 3. NaNO2, HCl, 0–5 °C 4. 1-butene — the least substituted alkene