Arrange the following phenols in increasing order of ${p K a}_{1}$ value.

I < I I < I I I

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I I I < I < I I

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I I I < I I < I

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I < I I I < I I

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Solution

The correct option is A $I < I I < I I I$
Higher the acidity, higher the $p K a_{1}$ value.

Nitro group has both $- I$ and $- R$ effect with the $- R$ effect being more dominant than the $- I$ effect. Since $- R$ effect does not affect the meta position, only the weaker $- I$ effect is responsible for withdrawing electron in the meta position. Hence m-nitrophenol is the weakest acid among the following.

Among ortho and para-nitrophenol, due to intramolecular hydrogen bonding, the loss of proton becomes more difficult in case of o-nitrophenol making it a weaker acid than p-nitrophenol.

Hence, A is the correct answer

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Arrange the following phenols in increasing order of pKa1 value.

Arrange the following phenols in increasing order of ${p K a}_{1}$ value.