Q. Given are cyclohexanol (I), acetic acid (II), 2, 4, 6-trinitrophenol (III) and phenol (IV). In these, the order of decreasing acidic character will be
AIPMTAIPMT 2010Aldehydes Ketones and Carboxylic Acids
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,
1. e.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.$(CH_3COOH)$ 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 $-NO_2$ 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
$III > II > IV > I$
