Why are the boiling points of aldehydes less than those of ketones?

Question

Both aldehydes and ketones have a carbonyl group, but since the carbonyl group of ketone is between alkyl groups, wouldn't it be more difficult for it to form hydrogen bonds than aldehydes? Why then, do aldehydes have a higher boiling points than ketones?

Answer 1

Ketones have a higher boiling point than aldehydes. The reason for this is because the carbonyl group of a ketone is more polarized than that of an aldehyde. Put differently, the oxygen of the ketone carbonyl has a greater partial negative than the oxygen of the aldehyde carbonyl.

Since ketones have two alkyl groups attached to the carbonyl carbon, compared to one for an aldehyde, the additional alkyl group can stabilize the partial positive on the carbonyl carbon through hyperconjugation. As a result of this stabilization, the carbonyl oxygen of a ketone can "steal" electron density from the carbonyl carbon more so than in an aldehyde. Hence, the greater polarization ---> greater boiling point