Amines - Physical and Chemical Properties

Amines are derivatives of $NH_3$ obtained by replacing one or more $H$ atoms with alkyl or aryl groups. Their physical state ranges from gases (lower aliphatic amines like $CH_3NH_2$) to liquids and solids.

Boiling point order: Primary ($1^\circ$) > Secondary ($2^\circ$) > Tertiary ($3^\circ$). This is because primary and secondary amines can form intermolecular hydrogen bonds due to the presence of $H$ atoms on $N$, whereas tertiary amines lack such $H$ atoms.

Solubility: Lower aliphatic amines are soluble in $H_2O$ because they can form hydrogen bonds with water molecules. Solubility decreases as the size of the hydrophobic alkyl part increases.

Basicity: Amines are Lewis bases because of the lone pair of electrons on the $N$ atom. In the gaseous phase, basicity follows the order: $3^\circ > 2^\circ > 1^\circ > NH_3$ due to the $+I$ effect of alkyl groups.

Basicity in Aqueous Solution: The order is influenced by the $+I$ effect, solvation effect, and steric hindrance. For methyl-substituted amines, the order is $(CH_3)_2NH > CH_3NH_2 > (CH_3)_3N > NH_3$. For ethyl-substituted amines, the order is $(C_2H_5)_2NH > (C_2H_5)_3N > C_2H_5NH_2 > NH_3$.

Aniline is less basic than $NH_3$ because the lone pair on $N$ is involved in resonance with the benzene ring, making it less available for protonation.

Carbylamine Reaction: Only aliphatic and aromatic primary amines react with $CHCl_3$ and ethanolic $KOH$ to form foul-smelling isocyanides ($R-NC$). This is a test for $1^\circ$ amines.

Hinsberg's Reagent: Benzenesulphonyl chloride ($C_6H_5SO_2Cl$) is used to distinguish between $1^\circ$, $2^\circ$, and $3^\circ$ amines. $1^\circ$ amines form a precipitate soluble in alkali, $2^\circ$ amines form a precipitate insoluble in alkali, and $3^\circ$ amines do not react.

Electrophilic Substitution: In Aniline, the $-NH_2$ group is highly activating and ortho/para directing. Bromination with bromine water yields $2,4,6-tribromoaniline$.