Amines MCQs for NEET — Chemistry Questions with Answers

The text says: 'However, primary amine is obtained as a major product by taking large excess of ammonia.'

The text states: 'Nitriles on reduction with lithium aluminium hydride ($LiAlH_4$) or catalytic hydrogenation produce primary amines. This reaction is used for ascent of amine series, i.e., for preparation of amines containing one carbon atom more than the starting amine.'

The context explains: 'Lower aliphatic amines are soluble in water because they can form hydrogen bonds with water molecules.'

The context provides a hint: 'Considering the electronegativity of nitrogen of amine and oxygen of alcohol as 3.0 and 3.5 respectively, you can predict the pattern of solubility of amines and alcohols in water.' And then it explicitly states: 'You may remember that alcohols are more polar than amines and form stronger intermolecular hydrogen bonds than amines.' Stronger hydrogen bonding with water implies better solubility.

The text mentions: 'Aniline and other arylamines are usually colourless but get coloured on storage due to atmospheric oxidation.'

The text clearly states: 'Moreover, amines behave as nucleophiles due to the presence of unshared electron pair.'

The NCERT table (Table 9.3) provides $pK_b$ values. A smaller $pK_b$ value indicates a stronger base. Among the given options, N-Ethylethanamine has the lowest $pK_b$ of 3.00.

According to the NCERT text, 'Aromatic primary amines cannot be prepared by this method [Gabriel phthalimide synthesis] because aryl halides do not undergo nucleophilic substitution with the anion formed by phthalimide.' Ammonolysis of aryl halides, conceptually similar to Gabriel phthalimide synthesis in its reliance on nucleophilic substitution, also faces limitations due to the low reactivity of aryl halides towards nucleophilic substitution. Reduction of nitro compounds, reduction of amides, and Hoffmann bromamide degradation are all viable methods for preparing aromatic primary amines.

The NCERT states, 'Hoffmann developed a method for preparation of primary amines by treating an amide with bromine in an aqueous or ethanolic solution of sodium hydroxide. In this degradation reaction, migration of an alkyl or aryl group takes place from carbonyl carbon of the amide to the nitrogen atom. The amine so formed contains one carbon less than that present in the amide.' This perfectly describes the Hoffmann bromamide degradation reaction.

The NCERT text mentions, 'Alkylamines are found to be stronger bases than ammonia. In alkylamines, a combination of electron releasing, steric and H-bonding factors influence the stability of the substituted ammonium cations in protic polar solvents and thus affect the basic nature of amines.' It also specifically states, 'Aliphatic amines are stronger bases than ammonia due to +I effect of alkyl groups leading to high electron density on the nitrogen atom.'