Arrange the following compounds in decreasing order of acid strength. (i) `CH_3-CH_2-OH` (ii)`(CH_3)_3COH (iii) `C_6H_6OH` (iv) P-NO_2C_6H_4OH.

Explanation

Compounds (iii) and (iv) are phenols and therefore are more acidic than the alcohols (i) and (ii). The acidic strenghts of compounds depend upon stabilization of the corresponding conjugate bases. Hence let us compare electronic effects in the conjugate bases of these compounds :
Alcohols :

The conjugate base of the alcohol (i) is destabilized by + I effect of one alkyl group, where as conjugate base of the alcohol (ii) is destabilized by + I effect of three alkyl groups. Hence (ii) is weaker acid than (i)

Phenols : The conjugate base of p-nitrophenol (iv) is better resonance stabilized due to six resonance structures compared to the five resonance structures of conjugate base of phenol (iii) (see Fig.). The resonance structure VI has -ve charge on only electronegative oxygens. Hence the phenol (iv) is stronger acid than (iii). Thus the decreasing order of acid strength is `(iv)gt (iii) gt (i)gt(ii)`