Question

The decreasing order of boiling points of ethyldimethylamine, n- butylamine and diethylamine is n- butylamine> diethylamine> ethyldimethylamine. This trend of the 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

Hint:Intermolecular hydrogen bonding is responsible for the high boiling point of many organic compounds.

Complete step by step answer:
The given three organic compounds are all amines. They are ethyldimethylamine, n- butylamine and diethylamine. And their boiling point order is found to decrease in the following order: n- butylamine> diethylamine> ethyldimethylamine.
Now, n- butylamine has the following structure: ${\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{N}}{{\text{H}}_{\text{2}}}$, diethylamine has the following structure: ${\left( {{{\text{C}}_{\text{2}}}{{\text{H}}_{\text{5}}}} \right)_{\text{2}}}{\text{NH}}$ and ethyldimethylamine has the following structure: ${{\text{C}}_{\text{2}}}{{\text{H}}_{\text{5}}}{\text{NH}}{\left( {{\text{C}}{{\text{H}}_{\text{3}}}} \right)_{\text{2}}}$. Since in primary amines, only one hydrogen of the amine group is substituted by an alkyl group, in secondary amines, two hydrogens of the amine group are substituted by two alkyl groups and in tertiary amines, all the three hydrogens of the amine group are substituted by three alkyl groups, therefore, from the structures of the given three amines, it can be well understood that, n- butylamine is a primary amine, diethylamine is a secondary amine and ethyldimethylamine is a tertiary amine.
As a result, the tertiary amines are left with no hydrogen attached to the nitrogen and so they cannot form hydrogen bonds among themselves or in other words, they cannot exhibit intermolecular hydrogen bonding. So, ethyldimethylamine will not exhibit intermolecular hydrogen bonding. Therefore intermolecular forces of attraction will be weak and so it will have the lowest boiling point.
On the other hand, secondary and primary amines have one and two hydrogen atoms respectively attached to the nitrogen and so they can form intermolecular hydrogen bonding. More the number of hydrogen, more will be the intermolecular hydrogen bonding and higher will be the boiling point. So, n- butylamine will have higher boiling point than diethylamine.

So, the correct option is C.

Note:
Tertiary amines do form hydrogen bonds with other molecules like water using the lone pair on nitrogen, but that is ‘intramolecular’ hydrogen bonding, not intermolecular.