Physical and Bond Properties

Even though the anhydride function bears a certain polarity, its presence in a molecule does not make much difference concerning its boiling point as compared to the corresponding alkane, i.e. it doesn't make the intermolecular forces stronger.

Plot of boiling points against molecular weight for alkanes, esters, acyl halides, acids, amides and anhydrides: anhydrides boil like esters of similar molecular weight

Since we know that carboxylic acids form dimers, one should consider them with a "double" molecular weight. With that in mind, acids and anhydrides end up displaying similar boiling points.

Butanoic acid dimer held by two hydrogen bonds (b.p. 164 °C) compared with butanoic anhydride, without hydrogen bonds (b.p. 198 °C)

Carboxylic acid derivatives can be described by three resonance forms, their relative contribution depending on the nature of L group.

Resonance forms of an acid derivative R–CO–L: negative charge on the oxygen and positive charge on the carbon or on the L group

The C=O and C-L bond lengths can be understood by a higher or lower contribution of the first and third resonance forms.

Let's take methanol and acetaldehyde as the reference measurement of true single C-O and double C=O bond lengths.

C=O bond length in acetaldehyde: 1.231 Å

Bond lengths of acetic acid: C=O 1.234 Å and C–O 1.364 Å

What conclusion can you draw?

There should be a negligible contribution of the third resonance form because the bond lengths are very close to those of the reference compounds.

Bond lengths of acetic anhydride: C=O 1.229 Å and C–O 1.393 Å