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JEE Advanced levelSingle correct optionDifficulty 3 of 3g1100

The pKa values of phenol and 4-nitrophenol in water at 298 K are 9.99 and 7.15 respectively. By what factor is $K_{a}$ of 4-nitrophenol greater than $K_{a}$ of phenol?

Aabout $6.9 \times 1 0^{3}$
Babout $2.8$
Cabout $6.9 \times 1 0^{2}$
Dabout $6.9 \times 1 0^{1}$

Answer: C

The ratio of acid dissociation constants is

K_{a} (4-nitrophenol) / K_{a} (phenol) = 1 0^{9.99 - 7.15} = 1 0^{2.84} \approx 6.9 \times 1 0^{2}

. The large increase comes from delocalisation of the negative charge onto the nitro group in the 4-nitrophenoxide ion by -M and -I effects.

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