AIPMT 2010 Physics and Chemistry Paper

Higher the tendency to give a proton, higher is the acidic character, and tendency to lose a proton depends upon the stability of intermediate, i.e., carbanion formed. 2,4,6-trinitrophenol after the loss of a proton gives $2,4,6$-trinitro phenoxide ion which is stabilised by resonance, $-1$ 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 $- l$ effects but is less stabilised as compared to $2,4,6$-trinitro phenoxide ions. Thus, it is less acidic as compared to $2,4,6$ - trinitrophenol. $(\mathrm{CH}_3 \mathrm{COOH})$ after losing a proton gives acetateion 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, Hence, the correct order of acidic strength is 2, 4, 6-trinitrophenol > acetic acid > phenol > cyclohexanol