Question

The correct order of boiling points of the following isomeric amine is
${C_4}{H_9}N{H_2},{\left( {{C_2}{H_5}} \right)_2}NH,{C_2}{H_5}N{\left( {C{H_3}} \right)_2}$.
A. ${C_2}{H_5}N{\left( {C{H_3}} \right)_2} > {\left( {{C_2}{H_5}} \right)_2}NH > {C_4}{H_9}N{H_2}$
B. ${\left( {{C_2}{H_5}} \right)_2}NH > {C_2}{H_5}N{\left( {C{H_3}} \right)_2} > {C_4}{H_9}N{H_2}$
C. ${C_4}{H_9}N{H_2} > {\left( {{C_2}{H_5}} \right)_2}NH > {C_2}{H_5}N{\left( {C{H_3}} \right)_2}$
D. ${\left( {{C_2}{H_5}} \right)_2}NH > {C_4}{H_9}N{H_2} > {C_2}{H_5}N{\left( {C{H_3}} \right)_2}$

Answer 1

Whenever there is an association of several molecules by a bond then it led to the higher boiling point. This is because a large amount of energy is needed to overcome these forces.

Complete step by step answer:
Amines are classified as primary (1° ), secondary (2°) and tertiary (3° ) depending upon the number of hydrogen atoms replaced by alkyl or aryl groups in ammonia molecules.
Primary and secondary amines are engaged in intermolecular association due to hydrogen bonding between nitrogen of one and hydrogen of another molecule. This intermolecular association is more in primary amines than in secondary amines as there are two hydrogen atoms available for hydrogen bond formation in it. Tertiary amines do not have intermolecular association due to the absence of hydrogen atom available for hydrogen bond formation. Therefore, the order of boiling points of isomeric amines is as follows:
Primary > Secondary > Tertiary

Hence option (C) is correct.

Note: Intermolecular association leads to the association of the molecule which further results in the increment of boiling point.