NCERT MCQ Solutions for Class 12 Chemistry Chapter 9 Amines

Amines is derivatives of ammonia (NH₃), obtained by replacing H atoms with alkyl/aryl groups.

Due to the sp³ hybridization of nitrogen and the presence of an unshared electron pair, the geometry of amines is described as pyramidal.

We know that amines based on the number of H atoms replaced; replacement of two H atoms results in a secondary (2°) amine.

According to IUPAC rules, the longer chain (ethane) is the parent. The methyl group on the nitrogen is designated with the locant N, giving N-methylethanamine.

We know that catalytic hydrogenation (H₂/Pd) and reduction with metals in acid (Sn/HCl or Fe/HCl) as methods for reducing nitro compounds to amines.

Ammonolysis is the process of cleavage of the C-X bond by an ammonia molecule, leading to substitution by an amino group.

Ammonolysis is to have the disadvantage of yielding a mixture of products because the amine formed can further react with the alkyl halide.

Gabriel synthesis is a method for preparing primary amines. It notes that aromatic primary amines cannot be prepared this way because aryl halides don’t undergo the required nucleophilic substitution.

In this degradation reaction, the amine formed contains one carbon less than the parent amide due to the loss of the carbonyl carbon.

Lower amines are water-soluble because their nitrogen atom (with its lone pair) and N-H hydrogens can form hydrogen bonds with water molecules.

Tertiary amines lack N-H bonds and cannot form intermolecular hydrogen bonds with themselves, resulting in lower boiling points compared to primary and secondary amines of similar mass.

The presence of an unshared electron pair on the nitrogen atom allows amines to donate electrons, making them Lewis bases.

The +I effect of alkyl groups increases electron density on nitrogen, making the lone pair more available. Stabilization of the resulting ammonium ion by both +I effect and solvation also contributes to higher basicity compared to ammonia.

The lower basicity of aniline to the involvement of the nitrogen lone pair in resonance with the benzene ring, making it less available for protonation.

This specific reaction, used as a test for primary amines, as the carbylamine reaction or isocyanide.

Primary aliphatic amines form unstable diazonium salts with nitrous acid, which decompose to liberate nitrogen gas and form alcohols.

Benzenesulphonyl chloride is as Hinsberg’s reagent, which reacts differently with 1°, 2° and 3° amines, allowing for their distinction.

The -NH₂ group is a powerful activating group and is ortho- and para-directing due to resonance increasing electron density at these positions. Bromination with bromine water gives 2,4,6-tribromoaniline.

The Sandmeyer reaction as the replacement of the diazonium group by Cl⁻, Br⁻ or CN⁻ using the corresponding cuprous salt.

Reactions involving retention of the diazo group, where the diazonium salt couples with phenols or anilines, are called coupling reactions and produce azo compounds (Ar-N=N-Ar’), often used as dyes.