Given are cyclohexanol (I), acetic acid (II), 2, 4, 6-trinitrophenol (III) and phenol (IV). In these, the…

1. III $>$ II $>$ IV $>$ I
2. II $>$ III $>$ I $>$ IV
3. II $>$ III $>$ IV $>$ I
4. III $>$ IV $>$ II $>$ I

Solution

Higher the tendency to give a proton, higher is the acidic character, and tendency to lose a proton depends upon the stability of intermediate, ie, carbanion formed. 2, 4, 6-trinitrophenol after the loss of a proton gives 2,4,6-trinitrophenoxide ion which is stabilised by resonance, $-I$ effect and $-M$ effect, thus is most acidic among the given compounds. Phenol after losing a proton form phenoxide ion which is also stabilised by resonance, $-M$ and $-I$ effects but is less stabilised as compared to 2, 4, 6-trinitrophenoxide ions. Thus, it is less acidic as compared to 2 , 4, 6-trinitrophenol. $\left(\mathrm{CH}_3 \mathrm{COOH}\right)$ after losing a proton gives acetate ion which is stabilised by only resonance. However, it is more resonance stabilised as compared to a phenoxide ion, thus more acidic as compared to phenol. 2,4, 6-trinitrophenol, however, is more acidic than acetic acid due to the presence of three electron withdrawing $-\mathrm{NO}_2$ groups. Cyclohexanol gives an anion that is least stable among the given, thus, it is least acidic. Hence, the correct order of acidic strength is $2,4,6$-trinitrophenol $>$ acetic acid $>$ phenol $>$ cyclohexanol $\text { III } > \text { II } > \text { IV } > \text { I }$

Asked in: NEET 2010 (Screening)

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