Question

The decreasing order of boiling points of ethyldimethylamine, n-butylamine and diethylamine is n-Butylamine > Diethylamine > Ethyldimethylamine. This trend of boiling point can be explained as

• A. boiling point increases with increase in molecular mass
• B. tertiary amines have highest boiling point due to highest basicity
• C. intermolecular hydrogen bonding is maximum in primary amines and absent in tertiary amines
• D. intramolecular hydrogen bonding is present in tertiary amines

Answer 1

Answer: (C)

The decreasing order of boiling points:
n-butylamine(primary amine) $>$ diethylamine(secondary amine) $>$ ethyldimethylamine (tertiary amine).
$CH _{3} CH _{2} CH _{2} CH _{2} NH _{2}>\left( C _{2} H _{5}\right)_{2} NH > C _{2} H _{5} NH \left( CH _{3}\right)_{2}$
In primary amines only one hydrogen is substituted by an alkyl group, in secondary amines, two hydrogens are substituted by alkyl groups and in tertiary amines, all the three hydrogens are substituted by alkyl groups thereby resulting in the absence of intermolecular hydrogen bonding. Higher the hydrogen bonding difficult is to break the bonds thereby high boiling point.
Intermolecular hydrogen bonding is maximum in primary amines and absent in tertiary amines.