Carboxylic Acids and their Derivatives 2 Question 3
3. The increasing order of the reactivity of the following with $\mathrm{LiAlH}_{4}$ is
(2019 Main, 12 Jan II)
(A)
(C)
(B)
(D)
(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 $\left(\mathrm{S}{N_{4}} E\right)$ 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 $\mathrm{C}$-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 2nd and 3rd factor written above. The cumulative effect of these two factors results to leaving group ability (LGA) of the substituents in the following order :
$$ \underset{\substack{\text { Good leaving } \ \text { group }}}{\mathrm{Cl}^{-}}>\mathrm{O}^{-}-\stackrel{\mathrm{O}}{\mathrm{C}}-\mathrm{CH}{3}>\overline{\mathrm{O}}-\mathrm{CH}{3}>\underset{\substack{\text { Poor leaving } \ \text { group }}}{\mathrm{NH}_{2}} $$
This leaving group ability (weak conjugate base) corresponds directly to the reactivity order. Hence, the correct reactivity order is:
$(A)$
Note The $-I$ effect of $-\mathrm{Cl}$ and $+m$ effects of
$-\mathrm{NH}{2}-\mathrm{O}-\stackrel{\mathrm{O}}{\mathrm{C}}-\mathrm{C}{2} \mathrm{H}{5}$ and $-\mathrm{OCH}{3}$ groups while attached to $-\underset{O}{\mathrm{C}}-$ group to be reduced. Also add to the group leaving ability.
