Arrange the following in decreasing order of reactivity towards nucleophilic acyl substitution: (I) ethanoyl chloride, (II) ethanoic anhydride, (III) ethyl ethanoate, (IV) ethanamide.

The reactivity decreases as the donation of a lone pair (+M) from the group attached to the carbonyl carbon increases. Chlorine (poor overlap, strong -I) donates least. In the anhydride the oxygen lone pair is shared between two carbonyls. In the ester it donates more, and nitrogen in the amide is the strongest donor. The order is I > II > III > IV.