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 [AIPMT (S) 2010]

A) III > II > IV > I

B) II > III > I >IV

C) II > III > IV > I

D) III > IV > II > I

Correct Answer: A

Solution :

[a] 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.

(CH3COOH) after losing a proton gives acetate

Ion  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  —NO2   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