question_answer 59)

  Make the correct order of decreasing acid strength of the following compounds. (a)            (b) (c)            (d)

Answer:

  (b) Since electron-withdrawing groups increase the acidity of phenols, therefore, both p-nitrohenol (II) and m-nitrophenol (IV) are stronger acids than phenol (I). However, due to the combined -I and -R effect of the  group at p-position, p-nitrophenol (II) is a much stronger acid than m-nitrophenol (IV), which exerts only -I-effect at w-position. Usually electron-donating groups decrease the acidity of phenols. But group at m-position cannot exert its +R-effect but can exert only its -I-effect, therefore, m-methoxyphenol (in) is a stronger acid than phenol (I). In contrast, at p-posidon,  group exerts its +R-effect which makes p-methoxyphenol (V) a weaker acid than phenol (I). Thus, overall acid strength of these five phenols decreases in the order: II > IV > HI > I > V, i.e., option (b) is correct.