The acidity of the carboxylic acid is highly effective by the inductive effect of substituents. The substituents which are attached to the alpha carbon of carboxyl group show inductive effects. There is great importance of inductive effect on carboxyl pKa in various reactions of carboxylic acid. Two different types of substituents attached show different effects on acidity.
Electron-withdrawing substituents exert a positive inductive effect on the carboxyl acidity. They increase the acidity of carboxylic acid.They disperse the negative charge of carboxylate ions and stabilze it. More stable the carboxylate ion more acidic the carboxyl. The important electron-withdrawing electrons are -Cl,-Br, -F, -OH, -CN, etc. The positive inductive effect on carboxyl increase when the electron-withdrawing substituents are close to alpha carbon.
Effect of halogens:
The electronegativity of halogens is directly proportional to the electron-withdrawing effect of halogens. The more electronegative the halogen is, the greater the positive inductive effect on carboxyl acidity. The effect of halogens on the acidity of the carboxylic acid decrease as the distance between the halogen and carboxyl group increase.
Number of electron-withdrawing substituents:
The positive inductive effect on carboxyl group also increases with the increase in the number of substitution attached to it. It causes the increase in the acidity of the carboxylic acid. The substitutions should be electron-withdrawing cause electron-donating substitutions decrease the acid strength. The carboxylic acid with a number of substitutions is more acidic than a monosubstituted carboxylic acid.
The substituents which have the ability to donate an electron to the carboxylic acid stabilize the carboxylate ion. They intensify the negative charge of carboxylate ions, which decreases the acid strength of carboxylic acid. They have a negative inductive effect on carboxyl acidity. The common electron-donating substituent of a carboxylic acid is the alkyl group.
Effect of alkyl group:
The acidity of carboxylic acid decrease with the increase of the size of the alkyl group. Alkyl group far from the alpha carbon does not decrease the acid strength.
The dicarboxylic groups also show the positive inductive effect on carboxyl acidity. They come close to each other.One carboxyl group show inductive effect on the other carboxyl group. This inductive effect increases the acidity of the whole acid such as malonic acid. The inductive effect on carboxyl group by the other carboxyl group is positive.