Question
Arrange the following compounds in increasing order of their property as indicated:
Benzoic acid, 4-Nitrohenzoic acid, 3, 4-Dinitrobenzoic acid, 4-Methoxy benzoic acid (acidstrength).
Solution
The +I effect (electron –donating) groups decrease the strengths of acids, while –I (electron withdrawing) groups increase the strength of acids. As methoxy group is an electron-donating group, 4-methoxybenzoic acid is a weaker acid than benzoic acid. Nitro groups is an electron withdrawing groups and will increase the strength of acids. As 3, 4-dintrobenzoic acid contains two nitro groups, it is a slightly stronger acid than 4-nitrobenzoic acid. Hence, the strength of the given acid increase as:
4-Methoxy benzoic acid < Benzoic acid < 4-Nitrobenzoic acid < 3, 4-Dinitrobenzoic acid.
