Rank the following compounds in order of decreasing acidity, putting the most acidic first. III > II > I > IV IV > I > II > III IV > III > II > I II > III > IV > I
The Correct Answer and Explanation is:
The correct ranking of the compounds in order of decreasing acidity is III > II > I > IV.
To determine acidity, consider the substituents on the benzene ring and their electronic effects on the carboxyl group. The acidity of benzoic acid derivatives depends on the ability of the substituent to stabilize or destabilize the negative charge on the conjugate base after deprotonation.
Compound III, 4-nitrobenzoic acid, is the most acidic because the nitro group is a strongly electron-withdrawing group through both inductive and resonance effects. It significantly stabilizes the carboxylate anion, making the molecule more likely to lose a proton.
Compound II, 4-chlorobenzoic acid, is less acidic than III but more acidic than I and IV. The chlorine atom withdraws electron density via the inductive effect, stabilizing the carboxylate anion to a lesser extent than the nitro group.
Compound I, benzoic acid, lacks additional substituents, meaning its acidity is determined solely by the intrinsic properties of the carboxyl group. Since it lacks an electron-withdrawing or donating group, its acidity is moderate compared to the others.
Compound IV, 4-methylbenzoic acid, is the least acidic because the methyl group donates electron density via the inductive effect, destabilizing the carboxylate anion. This electron donation makes deprotonation less favorable, lowering the acidity.
The ranking demonstrates the role of electronic effects in organic acids. Electron-withdrawing groups enhance acidity by stabilizing the conjugate base, while electron-donating groups reduce acidity by destabilizing it. This concept is widely applied in predicting the behavior of organic molecules in various chemical reactions.
