MP Board · Class 12 · Chemistry · Aldehydes, Ketones and Carboxylic AcidsExplain the acidity of carboxylic acids in detail compared to alcohols and phenols. Discuss the effect of various electron-withdrawing and electron-donating substituents on the acid strength of substituted benzoic acids or acetic acids, supported with appropriate examples.

Acidity of Carboxylic Acids\nCarboxylic acids are significantly more acidic than alcohols and phenols. To understand this, we need to look at the stability of their conjugate bases:

• Carboxylate Ion vs. Alkoxide/Phenoxide Ion: When a carboxylic acid loses a proton, it forms a carboxylate ion ($RCOO^-$). This ion is resonance-stabilized because the negative charge is delocalized over two electronegative oxygen atoms. In contrast, the alkoxide ion ($RO^-$) derived from alcohols has localized negative charge, making it less stable. Phenoxide ions ($C_6H_5O^-$) do show resonance, but the negative charge is delocalized over carbon atoms of the benzene ring, which are less electronegative than oxygen. Therefore, the carboxylate ion is far more stable than both alkoxide and phenoxide ions, driving the equilibrium forward and making carboxylic acids stronger acids.

Effect of Substituents on Acid Strength\nThe acid strength of carboxylic acids is profoundly influenced by the nature of the substituents attached to the alkyl or aryl group:

• Electron-Withdrawing Groups (EWG): Groups such as $-NO_2$, $-CN$, $-X$ (halogens), etc., exhibit $-I$ (inductive) and/or $-M$ (mesomeric) effects. They withdraw electron density from the carboxylate group, dispersing the negative charge and stabilizing the carboxylate ion. This increases the acid strength ($pK_a$ decreases). For example, trichloroacetic acid is much stronger than acetic acid due to the strong $-I$ effect of three chlorine atoms.
• Electron-Donating Groups (EDG): Groups such as alkyl groups ($-CH_3$, $-C_2H_5$), $-OH$, $-OCH_3$, etc., exhibit $+I$ and/or $+R$ effects. They release electrons toward the carbonyl carbon, intensifying the negative charge on the carboxylate ion and destabilizing it. This decreases the acid strength ($pK_a$ increases). For example, acetic acid is a stronger acid than propionic acid because the methyl group in acetic acid has a lesser $+I$ effect compared to the ethyl group in propionic acid.

Effect of Position in Benzoic Acids\nIn substituted benzoic acids, substituent effects depend strongly on their position (ortho, meta, para):

• Ortho Effect: Regardless of whether an ortho substituent is electron-donating or electron-withdrawing, ortho-substituted benzoic acids are almost always stronger acids than benzoic acid itself due to steric inhibition of resonance and stabilization factors.