question_answer

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                       

A) III > II > IV > I  

B)        II > III > I > IV     

C)        II > III > IV > I  

D)        III > IV > II > I

Correct Answer: A

Solution :

Key Idea 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 stabilized 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 -1 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. \[(C{{H}_{3}}COOH)\] 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 \[-N{{O}_{2}}\] 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