The correct order of pKa values for the following compounds is: — Solution | JEE Chemistry

MCQEasyJEE 2023Electronic Effects (Inductive, Resonance, Hyperconjugation)

The correct order of $pK_a$ values for the following compounds is:

Four substituted phenol compounds labeled a, b, c and d: phenol, p-dimethylaminophenol, p-nitrophenol and p-isopropylphenol.

A
c $>$ a $>$ d $>$ b
B
b $>$ d $>$ a $>$ c
C
b $>$ a $>$ d $>$ c
D
a $>$ b $>$ c $>$ d

Answer

Correct answer:B

Step-by-step solution

Standard Method

Given: Four substituted phenols (a), (b), (c) and (d) are to be arranged in the order of $pK_a$ values.

Find: The correct order of $pK_a$ values.

A $-NO_2$ group shows $-M$ effect, so it increases acidity. A $-N(CH_3)_2$ group shows $+M$ effect, so it decreases acidity. Hyperconjugation of isopropyl also decreases acidity.

Therefore, the order of acidic strength is

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

Since $pK_a$ is inversely related to acidic strength, the order of $pK_a$ values is

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

Therefore, the correct option is B.

Common mistakes

Confusing acidity order with $pK_a$ order. Greater acidity means smaller $pK_a$ , so the order must be reversed.
Ignoring the $-M$ effect of $-NO_2$ . This group stabilizes the phenoxide ion and increases acidity, so its $pK_a$ should be the lowest.
Assuming $-N(CH_3)_2$ increases acidity. Its $+M$ effect donates electron density and destabilizes the phenoxide ion, so it decreases acidity instead.

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Answer

Step-by-step solution

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