Chemistry · Ch 9 — Amines

Summary

Amines are derivatives of ammonia ( $NH_3$ ) where one or more hydrogen atoms are replaced by alkyl/aryl groups ( $R$ or $Ar$ ). Classified as primary (1°), secondary (2°), or tertiary (3°) based on the number of $H$ atoms replaced.
Nomenclature: Common names use suffix -amine (e.g., $CH_3NH_2$ = methylamine). IUPAC names replace -e of alkane with -amine (e.g., $CH_3CH_2NH_2$ = ethanamine). For aryl amines, aniline ( $C_6H_5NH_2$ ) is the parent.
Preparation:
- Reduction of nitro compounds: $RNO_2 \xrightarrow{Sn/HCl} RNH_2$ (or $H_2/Pd$ ).
- Gabriel phthalimide synthesis: Only gives primary amines via alkyl halide + phthalimide $\rightarrow$ hydrolysis.
- Hoffmann bromamide degradation: Amide ( $RCONH_2$ ) + $Br_2/NaOH$ $\rightarrow$ primary amine with one less carbon.
- Ammonolysis of alkyl halides: $RX$ + $NH_3$ (sealed tube, 373 K) $\rightarrow$ mixture of 1°, 2°, 3° amines + quaternary salt; primary amine predominates with excess $NH_3$ .
- Reduction of nitriles: $RCN$ + $LiAlH_4$ (or $H_2$ /Ni) $\rightarrow$ $RCH_2NH_2$ — one carbon more (ascent of series).
- Reduction of amides: $RCONH_2$ + $LiAlH_4$ $\rightarrow$ $RCH_2NH_2$ — same carbon count.
Physical properties: Lower amines are gases with fishy odor; higher are liquids/solids. They form hydrogen bonds (weaker than alcohols), so boiling points: 1° > 2° > 3° (for similar molar mass). Soluble in water due to $H$ -bonding.
Basicity: Amines are Lewis bases (lone pair on $N$ ). In water no single order holds — the inductive (+I) effect, solvation of the cation and steric hindrance act together: $(C_2H_5)_2NH > (C_2H_5)_3N > C_2H_5NH_2 > NH_3$ but $(CH_3)_2NH > CH_3NH_2 > (CH_3)_3N > NH_3$ . Aromatic amines (e.g., aniline) are weaker than $NH_3$ due to resonance delocalization of the lone pair into the ring.
Chemical reactions:
- Alkylation: $RNH_2 + R'X \rightarrow$ mixture of 1°, 2°, 3° amines and quaternary ammonium salt ( $R_4N^+X^-$ ).
- Acylation: $RNH_2 + R'COCl \rightarrow$ amide ( $RCONHR'$ ) — only 1° and 2° amines react.
- Carbylamine reaction: Only primary amines ( $RNH_2$ ) + $CHCl_3$ + $KOH$ $\rightarrow$ foul-smelling isocyanide ( $RNC$ ).
- Hinsberg test: 1° amines give soluble sulfonamide with benzenesulfonyl chloride ( $C_6H_5SO_2Cl$ ); 2° give insoluble; 3° do not react.
- Diazotization: Aromatic primary amines ( $ArNH_2$ ) + $NaNO_2$ + $HCl$ at 0–5°C $\rightarrow$ diazonium salt ( $ArN_2^+Cl^-$ ).
Diazonium salts ( $ArN_2^+X^-$ $A r N_{2}^{+} X^{-}$ ): Versatile intermediates. Key reactions:
- Substitution: Replace $N_2^+$ with $Cl$ , $Br$ , $CN$ , $OH$ , $I$ , $H$ (e.g., Sandmeyer reaction: $ArN_2^+Cl^- \xrightarrow{CuCl} ArCl$ ).
- Coupling: With phenols or aromatic amines $\rightarrow$ azo dyes ( $Ar-N=N-Ar'$ ), colored compounds.
Key distinction: Aliphatic amines are stronger bases than aromatic amines. Aniline is less basic than $NH_3$ due to resonance. …

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