Chapter 9: Amines
Key Topics: Structure and Classification, Preparation Methods (Gabriel Phthalimide & Hofmann Bromamide), Basicity Trends ($pK_b$ values), Distinction Tests (Hinsberg & Carbylamine), and Diazonium Salt Chemistry.
1. Structure & Classification of Amines
Amines can be considered derivatives of ammonia ($\text{NH}_3$) obtained by replacement of one, two, or all three hydrogen atoms by alkyl or aryl groups.
- Primary ($1^\circ$) Amines: One hydrogen atom of $\text{NH}_3$ is replaced ($\text{R-NH}_2$).
- Secondary ($2^\circ$) Amines: Two hydrogen atoms are replaced ($\text{R-NH-R'}$).
- Tertiary ($3^\circ$) Amines: All three hydrogen atoms are replaced ($\text{R-N(R')-R''}$).
- Structure: Nitrogen is $sp^3$ hybridized with a pyramidal geometry holding an unshared electron pair.
2. Key Methods of Preparation
Reduction of Nitro Compounds:
$$\text{R-NO}_2 + 6[\text{H}] \xrightarrow{\text{Sn/HCl or Fe/HCl}} \text{R-NH}_2 + 2\text{H}_2\text{O}$$
Gabriel Phthalimide Synthesis (Exclusive for Pure $1^\circ$ Aliphatic Amines):
$$\text{C}_6\text{H}_4(\text{CO})_2\text{N}^-\text{K}^+ + \text{R-X} \rightarrow \text{N-Alkylphthalimide} \xrightarrow{\text{NaOH (aq)}} \text{R-NH}_2 + \text{C}_6\text{H}_4(\text{COO}^-\text{Na}^+)_2$$
Hofmann Bromamide Degradation (Amides to Amines with Loss of 1 Carbon):
$$\text{R-CONH}_2 + \text{Br}_2 + 4\text{NaOH} \rightarrow \text{R-NH}_2 + \text{Na}_2\text{CO}_3 + 2\text{NaBr} + 2\text{H}_2\text{O}$$
3. Basicity Comparison & Physical Properties
| Phase / Property | Aliphatic Amines | Aromatic Amines (Aniline) |
|---|---|---|
| Aqueous Base Strength ($pK_b$) | Stronger bases due to $+I$ inductive effect of alkyl groups. | Weaker bases because lone pair is delocalized into the benzene ring via resonance. |
| Gas Phase Basicity Order | $3^\circ > 2^\circ > 1^\circ > \text{NH}_3$ (due to cumulative electron-releasing inductive effect). | Significantly lower basicity compared to aliphatic counterparts. |
| Solubility in Water | Lower members are soluble due to hydrogen bonding with water molecules. | Sparingly soluble due to the large hydrophobic benzene ring. |
Basicity Trend in Aqueous Solution (Aliphatic):
$$\text{For methyl series: } (\text{CH}_3)_2\text{NH} > \text{CH}_3\text{NH}_2 > (\text{CH}_3)_3\text{N} > \text{NH}_3$$
$$\text{For ethyl series: } (\text{C}_2\text{H}_5)_2\text{NH} > (\text{C}_2\text{H}_5)_3\text{N} > \text{C}_2\text{H}_5\text{NH}_2 > \text{NH}_3$$
4. Distinction Tests & Chemical Reactions
Carbylamine Test (Specific for Primary Amines - Aliphatic & Aromatic):
$$\text{R-NH}_2 + \text{CHCl}_3 + 3\text{KOH (alc.)} \xrightarrow{\Delta} \text{R-NC (Foul-smelling isocyanide)} + 3\text{KCl} + 3\text{H}_2\text{O}$$
Hinsberg Test (Benzenesulphonyl chloride, $\text{C}_6\text{H}_5\text{SO}_2\text{Cl}$):
- $1^\circ$ Amine: Forms N-alkylbenzenesulphonamide which is soluble in alkali ($\text{KOH}$).
- $2^\circ$ Amine: Forms N,N-dialkylbenzenesulphonamide which is insoluble in alkali.
- $3^\circ$ Amine: Does not react with Hinsberg's reagent.
Electrophilic Substitution of Aniline (Acetylation for Monobromination):
$$\text{C}_6\text{H}_5\text{NH}_2 + (\text{CH}_3\text{CO})_2\text{O} \xrightarrow{\text{Pyridine}} \text{C}_6\text{H}_5\text{NHCOCH}_3 \xrightarrow{\text{Br}_2 / \text{CH}_3\text{COOH}} \text{p-Bromoacetanilide} \xrightarrow{\text{H}_3\text{O}^+} \text{p-Bromoaniline}$$
5. Diazonium Salts and Their Reactions
Preparation of Diazonium Salts (Diazotisation):
$$\text{C}_6\text{H}_5\text{NH}_2 + \text{NaNO}_2 + 2\text{HCl} \xrightarrow{273-278\text{ K}} \text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- + \text{NaCl} + 2\text{H}_2\text{O}$$
Sandmeyer Reaction (Replacement of $-\text{N}_2^+\text{Cl}^-$):
$$\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- \xrightarrow{\text{CuCl / HCl}} \text{C}_6\text{H}_5\text{Cl} + \text{N}_2$$
$$\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- \xrightarrow{\text{CuBr / HBr}} \text{C}_6\text{H}_5\text{Br} + \text{N}_2$$
$$\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- \xrightarrow{\text{CuCN / KCN}} \text{C}_6\text{H}_5\text{CN} + \text{N}_2$$
Azo Coupling Reaction (Synthesis of Dyes):
$$\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- + \text{C}_6\text{H}_5\text{OH} (\text{Phenol in NaOH}) \xrightarrow{273-278\text{ K}} \text{p-Hydroxyazobenzene (Orange Dye)}$$