Given are cyclohexanol $(I)$ , acetic acid $(I I) 2, 4, 6 -$ trinitrophenol $(I I I)$ and phenol $(I V)$ . In these the order of decreasing acidic character will be:

I I I > I I > I V > I

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I I > I I I > I > I V

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I I > I I I > I V > I

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I I I > I V > I I > I

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Solution

The correct option is A $I I I > I I > I V > I$
Acidic strength depends on the stability of anion formed after the removal of acidic hydrogen from the compound.
Phenol ( $p K_{a}$ =10) will be more acidic than cyclohexanol ( $p K_{a}$ =16)because phenolate ion is stable due to resonance.
After acetic acid ( $p K_{a}$ =4.8) loses $H^{+}$ , negative charge will resonate between two $O$ atoms that makes it more acidic than phenol, but not more acidic than picric acid.
2,4,6-trinitrophenol (picric acid, $p K_{a}$ =0.4) is most acidic because $- N O_{2}$ group present in ortho and para positions will make the resonating negative charge more stable due to its $- M$ effect.
This order can also be seen from their $p K_{a}$ . Lower the $p K_{a}$ , the stronger is the acid.
Hence correct option is A.

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