Question

State reasons for the following :
(i) pKb value for aniline is more than that for methylamine.
(ii) Ethylamine is soluble in water whereas aniline is not soluble in water.
(iii) Primary amines have higher boiling points than tertiary amines. ​

Answer 1

Answer:

(i) In aniline, the lone pair of electrons on the N atom is delocalised over the benzene ring. As a result, the electron density on the nitrogen atom decreases. Therefore, aniline is a weaker base than methylamine. Hence, its pKb value is higher than that of methylamine.

(ii) Ethylamine when added to water forms intermolecular H−bonds with water. Hence it is soluble in water. But aniline Can form H−bonding with water to a very small extent due to the presence of a large hydrophobic −C6H5 group. Ethylamine is soluble in water whereas aniline is not soluble in water.

(iii) Primary amines have higher boiling point than tertiary amines, because primary amine have replaceable hydrogen atoms which are available for hydrogen bonding. Hydrogen bonding in tertiary amine increases its boiling point as more amount of heat is needed to break these hydrogen bonds.