Answer to Question #158330 in Organic Chemistry for sally

Question

Explain the following substituent effects on acidity.

(i)

Acetic acid pKa = 4.74

Chloroacetic acid pKa = 2.86

Dichloroacetic acid pKa = 1.26

(ii)

p-methoxy pKa = 4.46

m-nitro pKa = 3.47

p-nitro pKa = 3.41

Answer 1

Electronegative substituents increase acidity by inductive electron withdrawal. As expected, the higher the electronegativity of the substituent the greater the increase in acidity (F > Cl > Br > I), and the closer the substituent is to the carboxyl group the greater is its effect

An electron withdrawing group increases the acidity carboxylic acid. It disperses negative charge by inductive/ resonance effect and stabilizes the carboxylate ion. Thus, p-nitro benzoic acid (pKa 3.41) is more acidic than benzoic acid (pKa 4.19). Nitro group is electron withdrawing group.