Carboxylic Acids and Derivatives
12.0 Relative Reactivity of Acid Derivatives
12.0 Relative Reactivity of Acid Derivatives
The reactivity of the acid derivatives towards nucleophilic substitution has the following order.
This extent of reactivity of acid derivatives towards nulceophilic substitution can be explained on the basis of two facts.
(i) basicity of leaving group and
(ii) resonance effects
Acid derivatives undergo nucleophilic substitution in two steps.
(a) First step involves addition of nucleophile to electron deficient carbon atom of carbony group leading to formation of tetrahedral intermediate.
(b) In second step: The intermediate formed from an acyl compound usually eliminates a leaving group. Elimination leads to regeneration of carbon oxygen double bond and to a substitution product. The overall process in case of acyl substitution are nucleophilic addition elimination.
Step (1) is same as the nueleophitic addition to aldehyde and ketone so it should be favoured by electron withdrawal and hindered by +I effect or presence of bulky groups.
Second step depends on the basicity of the leaving group. The weaker a base, the better it is as leaving group.
Basicity of leaving groups: Weaker the basic character of the leaving group more will be the ease with which the leaving group leaves the compound and hence more is the reactivity $C{l^--}$ ion being weakest base are most reactive leaving group.
The order of basicity of the leaving group and their leaving tendency follows the order.
