Question

Arrange the following compounds in increasing order of their boiling points.

CH₃CHO, CH₃CH₂OH, CH₃OCH₃, CH₃CH₂CH₃

Answer 1

The molar masses of the compounds are comparable: CH₃CHO (44), CH₃CH₂OH (46), CH₃OCH₃ (46) and CH₃CH₂CH₃ (44). CH₃CH₂OH shows a lot of intermolecular hydrogen bonding; hence, it is found in the form of combined molecules. Hence, it has the highest boiling point (351 K). The value of the dipole moment (2.72 D) of CH₃CHO is higher than CH₃OCH₃ (1.18 D); hence, dipole-dipole interactions in CH₃CHO are stronger than CH₃OCH₃. Hence, the boiling point of CH₃CHO is higher than CH₃OCH₃. CH₃CH₂CH₃ shows only weak van der Waal forces. CH₃OCH₃ has some strong dipole-dipole interactions. Hence, the boiling point of CH₃OCH₃ is higher than CH₃CH₂CH₃, hence, the increasing order of boiling points of the compounds is as follows:

CH₃CH₂CH₃ < CH₃OCH₃ < CH₃CHO < CH₃CH₂OH