Physical Properties of Aldehydes and Ketones

The physical properties of aldehydes and ketones sit, in a very literal sense, between those of non-polar alkanes and hydrogen-bonded alcohols of similar molar mass, and understanding why explains both their boiling points and their solubility trends.

Boiling point. The carbonyl group is polar -- the $\text{C=O}$ dipole gives every aldehyde and ketone molecule a permanent dipole moment -- so these molecules attract one another through dipole-dipole interactions, which are stronger than the weak, momentary dispersion forces that hold alkane molecules together. This alone gives aldehydes and ketones distinctly higher boiling points than an alkane of comparable molar mass. However, aldehydes and ketones have no $\text{O-H}$ or $\text{N-H}$ hydrogen of their own, so unlike an alcohol they cannot hydrogen-bond with a second molecule of themselves. Their boiling points are therefore consistently lower than an alcohol of similar molar mass. Concretely, comparing three roughly-isomass compounds: propan-1-ol (an alcohol, hydrogen-bonded, boiling point $\approx 97\,^\circ\text{C}$) boils distinctly higher than butanal (an aldehyde, dipole-dipole only, boiling point $\approx 75\,^\circ\text{C}$), which in turn boils distinctly higher than butane (a non-polar alkane, dispersion forces only, boiling point $\approx 0\,^\circ\text{C}$) -- giving the order alkane $<$ aldehyde/ketone $<$ alcohol for any set of molecules of comparable size. …