What is the correct order of acidity of the protons marked A-D in… — Solution | JEE Chemistry

MCQMediumJEE 2023Electronic Effects (Inductive, Resonance, Hyperconjugation)

What is the correct order of acidity of the protons marked $A-D$ in the given compounds?

A benzene ring bearing two side chains: one carboxylic acid with proton C and proton D on adjacent carbon, and one propargyl side chain with proton A on terminal alkyne and proton B on benzylic carbon.

A
$H_C > H_D > H_B > H_A$
B
$H_C > H_D > H_A > H_B$
C
$H_D > H_C > H_B > H_A$
D
$H_C > H_A > H_D > H_B$

Answer

Correct answer:D

Step-by-step solution

Standard Method

Given: The compounds contain protons $H_A, H_B, H_C$ and $H_D$ marked in the structure.

Find: The correct order of acidity of these protons.

From the solution text, the $-M$ effect of the $-NO_2$ group increases acidity, while the $+M$ effect of $N(CH_3)_2$ decreases acidity. It is also stated that hyperconjugation of isopropyl decreases acidity.

Therefore, the order of acidic strength concluded in the solution is:

(c) > (a) > (d) > (b)

So, the acidity order is $H_C > H_A > H_D > H_B$ .

Therefore, the correct option is D.

Common mistakes

Assuming acidity depends only on bond type is incorrect. Resonance effects such as $-M$ and $+M$ can dominate the acidity order. Check how the conjugate base is stabilized.
Ignoring hyperconjugation leads to wrong comparison between alkyl-substituted positions. Greater electron donation by hyperconjugation decreases acidity, so compare substituent effects carefully.
Confusing the option numbering with proton labels is a common error. First determine the order among $H_A, H_B, H_C, H_D$ , then match it to options A-D.

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Answer

Step-by-step solution

Common mistakes

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