Carboxylic Acids and their Derivatives - Result Question 3-1
3. The increasing order of the reactivity of the following with $\mathrm{LiAlH}_4$ is
(2019 Main, 12 Jan II)
(a) $(A)<(B)<(D)<(C)$
(b) $(A)<(B)<(C)<(D)$
(c) $(B)<(A)<(D)<(C)$
(d) $(B)<(A)<(C)<(D)$
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Answer:
Correct Answer: 3. (a)
Solution:
All the given compounds are acid derivatives, thus contain carbonyl group in them. $\mathrm{LiAlH}4$ reduces these compounds through nucleophilic substitution via addition elimination $\left(\mathrm{S}{N_A} E\right)$ reaction. The rate of reaction depends upon the following factors :
(i) Size of alkyl group.
(ii) Steric hinderance around the $>\mathrm{C}=\mathrm{O}$ group.
(iii) $(+)$ ve charge on the $C$ -atom of $>\mathrm{C}=\mathrm{O}$ group.
The alkyl groups are more or less same in the given compounds. Thus, the reactivity order of given compounds depends upon $2$nd and $3$rd factor written above. The cumulative effect of these two factors results to leaving group ability (LGA) of the substituents in the following order:
This leaving group ability (weak conjugate base) corresponds directly to the reactivity order.
Hence, the correct reactivity order is:
BETA
