CHEMCA Free Chemistry Notes for JEE NEET Chemistry MadeEasy: Properties of Amines

Amines are basic in nature due to the presence of a lone pair of electrons on the nitrogen atom. Their reactivity is governed by their nucleophilic character and the number of hydrogen atoms attached to nitrogen.

1. Basic Character of Amines

Trends in Basicity

Amines act as Lewis bases. The basicity depends on the availability of the lone pair and the stability of the conjugate acid (ammonium ion).

A. Aliphatic Amines vs Ammonia: Alkyl groups ($+I$ effect) increase electron density on Nitrogen.

$$ R-NH_2 > NH_3 $$

B. Aqueous Phase Basicity Order (Important): Depends on $+I$ effect, Solvation effect, and Steric hindrance.

Methyl Substituted: $2^\circ > 1^\circ > 3^\circ > NH_3$
$(CH_3)_2NH > CH_3NH_2 > (CH_3)_3N > NH_3$
Ethyl Substituted: $2^\circ > 3^\circ > 1^\circ > NH_3$
$(C_2H_5)_2NH > (C_2H_5)_3N > C_2H_5NH_2 > NH_3$

C. Aryl Amines vs Ammonia: Aniline is much less basic than ammonia because the lone pair is delocalized into the benzene ring via resonance.

$$ NH_3 > C_6H_5NH_2 $$

2. Chemical Tests

A. Carbylamine Reaction (Isocyanide Test)

Specific test for Primary Amines (Aliphatic & Aromatic). Heating with Chloroform and ethanolic KOH produces a foul-smelling Isocyanide.

$$ R-NH_2 + CHCl_3 + 3KOH(alc) \xrightarrow{\Delta} \underset{\text{Foul Smell}}{R-NC} + 3KCl + 3H_2O $$

B. Reaction with Nitrous Acid ($HNO_2$)

Prepared in situ ($NaNO_2 + HCl$).

$1^\circ$ Aliphatic Amines: Form unstable diazonium salts which liberate $N_2$ gas and form Alcohols.
$$ R-NH_2 + HNO_2 \rightarrow [R-N_2^+Cl^-] \xrightarrow{H_2O} R-OH + N_2 \uparrow $$
$1^\circ$ Aromatic Amines (Aniline): Form stable Benzene Diazonium Chloride at low temp (0-5°C).
$$ C_6H_5NH_2 \xrightarrow{NaNO_2/HCl, 273K} C_6H_5N_2^+Cl^- $$

C. Hinsberg Test

Uses Benzenesulphonyl chloride ($C_6H_5SO_2Cl$) to distinguish $1^\circ, 2^\circ, 3^\circ$ amines.

$1^\circ$ Amine: Forms precipitate soluble in alkali.
$2^\circ$ Amine: Forms precipitate insoluble in alkali.
$3^\circ$ Amine: Does not react.

3. Electrophilic Substitution of Aniline

High Reactivity

The $-NH_2$ group is strongly activating and ortho-para directing.

A. Bromination: Reaction with Bromine water gives 2,4,6-Tribromoaniline (White ppt). To get mono-substituted product, the $-NH_2$ group is protected by acetylation.

$$ C_6H_5NH_2 + 3Br_2(aq) \rightarrow \text{2,4,6-Tribromoaniline} $$

B. Nitration: Direct nitration yields a significant amount of meta-nitroaniline (47%) along with para (51%) and ortho (2%).
Reason: In acidic medium, aniline gets protonated to anilinium ion ($Ph-NH_3^+$), which is meta-directing.

C. Sulphonation: Heating with conc. $H_2SO_4$ forms Sulphanilic acid, which exists as a Zwitterion.

$$ C_6H_5NH_2 + H_2SO_4 \xrightarrow{453K} \underset{\text{Sulphanilic Acid (Zwitterion)}}{p\text{-NH}_3^+C_6H_4SO_3^-} $$

4. Acylation

Aliphatic and aromatic primary/secondary amines react with acid chlorides, anhydrides, or esters to form Amides.

$$ C_2H_5-NH_2 + CH_3COCl \xrightarrow{\text{Base}} C_2H_5-NH-CO-CH_3 + HCl $$

Product: N-Ethylethanamide

Properties of Amines | chemca

Properties of Amines