Acid derivatives

      The compounds which are obtained by replacing the -OH of the carboxylic group by other atoms or groups such as X^-, NH₂, - OR and    are known as acid derivatives.

      ·    group is general to all the derivatives and is known as acyl group and these derivatives are termed as acyl compound.

      · The important derivatives are given below :

Reactivity

      Acyl derivatives are characterised by nucleophilic substitution reactions.

      The relative reactivities of several acyl elements have been found to be in the following order:

      Out of acid halides, the acid chlorides are more important ones.

      The overall order of reactivity can be accounted for in terms of the following three factors:

         (i) Basicity of the leaving group (ii) Resonance effects and (iii) Inductive effects.

      (i) Basicity of the leaving group : Weaker bases are good leaving groups. Hence, the acyl derivatives with weaker bases as leaving groups are more reactive. Chloride ion is the weakest base while - NH₂ is the strongest base. Thus, acyl chlorides are most reactive and amides are least reactive.

      (ii) Resonance effect : The leaving group in each case has an atom with lone pair of electrons adjacent to the carbonyl group. The compound exists, therefore, as a resonance hybrid.

      This makes the molecule more stable. The greater the stabilization, the smaller is the reactivity of the acyl compound.

      However, acyl chlorides are least affected by resonance. Due to lower stabilization, the acid chlorides are more reactive as the loss of -Cl is easier. Greater stabilization is achieved by resonance in esters and amides and thus, they are less reactive.

(iii) Inductive effect : Higher the -I effect, more reactive is the acyl compound. Inductive effect of oxygen in ester is greater than nitrogen in amide, hence ester is more reactive than an amide.

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