Question

Which one of the isomeric C₃H₉N amines would you expect to have the lowest boiling point? Explain your choice.

CH3CH2CH2NH2, CH3CH2NHCH3, (CH3)3N, (CH3)2CHNH2

Please explain why.

Answer 1

Boiling point of an organic compound is decided by the secondary intermolecular forces in the molecule like H-bonding(if any), dipole-dipole interactions, or vander Waals' forces if they are non- polar.

The vander waals' forces in turn depend upon the surface area, molecular weight etc.

Herre in the given example , all are isomeric, that is with same molecular weight, but the more symmetric the moleccule , less will be the surface area. Thus more branching, more symmetric, less surface area leads to less vander Waals' forces ultimately lower the boiling point.

From this aspecct, the more the branches(substitution) the less is the boiling point of compound.

ii) in terms of H- bonding, the higher the extent of H- Bonding, higher is the boiling point, thus primary> secondary> tertiary amines with respect to H- bonding. Thus the lower extent of H- Bonding also leads to lower boiling point for tertiary amine.

Thus tertiary amine that is (CH₃)₃ N is the one with lowest boiling point.

The relative order of boiling point would be propylamine> isopropylamine> ethylmethylamine> trimethylamine