Arrange the following in acidic character

What is acidic character?

Acidic character is the ability to or a tendency to lose a proton. Carboxylic acid is highly acidic in nature.

Factors responsible for acidic character (acidity)

• Inductive effect
• Mesomeric effect
• Hyperconjugation effect
• Ortho effect (especially for benzoic acid)

Dependency of acidity on the above effects

Acidity is directly proportional to -I, -M, -H effect and is inversely proportional to +I, +M, +H effect.

i.e., acidity increase when a group show -I, -M, -H effect and decrease when a group show +I, +M, +H effect. Ortho effect increase the acidic character of benzoic acid, means ortho substituted benzoic acid, irrespective of the group present at ortho position, is highly acidic in nature.

Arrange the following in acidic character: benzoic acid, 2-methyl benzoic acid , 3-methyl benzoic acid and 4-methyl benzoic acid.

The methyl group show +I and +H effect, means it decreases the acidity. Methyl group at ortho and para position show +I and +H effect but at meta position show only +I effect.

It indicates that the meta isomer is more acidic than para isomer, while the ortho isomer is highly acidic because of ortho effect.

Due to ortho effect, the carboxylic acid group being out of the plane of the benzene ring. The carboxylic acid group being out of the plane in o-methylbenzoic acid, make two equivalent resonating structures of carboxylic acid leading to its highest acidic character.

In case of benzoic acid, there is no group, so no effect applied. So, it is highly acidic than meta and para isomer.

Hence the acidic character order is:

2-Methylbenzoic acid > Benzoic acid > 3-Methylbenzoic acid > 4-Methylbenzoic acid

Remember: At meta position, only inductive effect occurs, while at ortho and para position all three effects, i.e., mesomeric/resonance, inductive and hyperconjugation effect operates.

Read More: Why is alkene more reactive than alkane?

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