Functional derivatives of carboxylic acids
Compounds such as acid chlorides, amides, esters etc., are called carboxylic acid derivatives because they differ from a carboxylic acid only in the nature of the group or atom that has replaced the -OH group of carboxylic acid.
The reactivity of the acid derivatives follows the order
The above order of reactivity can be explained in terms of
i) Basicity of the leaving group ii) Resonance effect
Weaker bases are good leaving groups. Hence acyl derivatives with weaker bases as leaving groups (L) can easily rupture the bond and are more reactive. The correct order of the basicity of the leaving group is
Hence the reverse is the order of reactivity.
Lesser the electronegativity of the group, greater would be the resonance stabilization as shown below.
This effect makes the molecule more stable and reduces the R reactivity of the acyl compound. The order of electronegativity of the leaving groups follows the order – Cl > - OCOR > - OR > - NH2
Hence the order of reactivity of the acid derivatives with nucleophilic reagent follows the order
acid halide > acid anhydride > esters > acid amides