Physical and Bond Properties
Even though the acyl halide 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.
Carboxylic acid derivatives can be described by three resonance forms, their relative contribution depending on the nature of 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 methyl chloride and acetaldehyde as the reference measurement of true single C-Cl and double C=O bond lengths.
What conclusion can you draw?
There should be a negligible contribution of the third resonance form.
The contribution of this third resonance form in acyl halides is the lowest in the carboxylic acid derivatives.