Question

Whyis3-nitrophenol(m-nitrophenol)lessacidicthanits2-and4-isomers,butmoreacidicthan phenol itself?
(b) Rank in order of increasing acidity: phenol, A; 3,4-dimethylphenol, B; 3-hydroxybenzoic (m-hydroxybenzoic) acid, C; 4-(fluoromethyl)phenol [p-(fluoromethyl)phenol], D.

Answer 1

a) In disubstituted benzenes , at meta position there is no effect of +M or -M groups. The +M or =M effects work only at ortho and para positions not on meta position.

Thus in m-nitrophenol only -I effect operates while on 2 and 4- nitrophenols both -I and -M operate. thus they are more acidic than m-nitrophenol and all three nitrophenols are more acidic than phenol due to the -I and -I/-M effects of nitro groiups.