Physical Properties

Physical State and Odour

Among the carbonyl compounds, methanal (formaldehyde) is a gas at ordinary room temperature, while ethanal (acetaldehyde) is a liquid so volatile that it boils just above room temperature. All the remaining aldehydes and ketones exist as liquids or solids under normal conditions.

Smell follows a similar trend with size. The lower-molecular-mass aldehydes have a sharp, pungent odour, but as the hydrocarbon part of the molecule grows longer, the smell turns milder and increasingly pleasant — many naturally occurring aldehydes and ketones are, in fact, valued for their fragrance and are used in perfumery and as flavouring agents.

Boiling Points

The carbonyl group ($\text{C=O}$) is strongly polar, and this polarity lets neighbouring carbonyl molecules weakly associate through dipole-dipole interactions. As a result:

• Aldehydes and ketones boil at higher temperatures than hydrocarbons or ethers of comparable molecular mass. A hydrocarbon or an ether has no comparable permanent dipole to hold its molecules together, so only weak van der Waals forces act between them, whereas the dipole-dipole attraction in a carbonyl compound needs extra thermal energy to overcome.

• Aldehydes and ketones boil at lower temperatures than alcohols of comparable molecular mass. An alcohol molecule carries an $\text{O–H}$ bond, so alcohol molecules can hydrogen-bond directly with one another. A carbonyl compound has no such $\text{O–H}$ hydrogen to donate, so this stronger, intermolecular hydrogen bonding is simply not available between two aldehyde or ketone molecules — dipole-dipole attraction is the strongest force they can offer each other.

So, for molecules of similar mass, the order of increasing boiling point runs roughly as:

This pattern can be checked against a short set of compounds of nearly identical molecular mass (58–60 u) — an alkane, an ether, an aldehyde, a ketone, and an alcohol — whose measured boiling points, taken together, confirm exactly this ordering:

Solubility in Water

Even though a carbonyl compound cannot hydrogen-bond with another molecule of itself, it can hydrogen-bond with water. The oxygen of the $\text{C=O}$ group carries lone pairs of electrons and a partial negative charge ($\delta^-$), which lets it accept a hydrogen bond from the partially positive ($\delta^+$) hydrogen of a water molecule:

Because of this interaction with water, the lower members of the series — methanal, ethanal, and propanone — are miscible with water in all proportions. …