MHTCET 13 Apr 2026 Shift 2 Paper

Concept:Acid strength of carboxylic acids depends on the stability of the carboxylate ion, which is influenced by electron-donating and electron-withdrawing groups.Explanation:Carboxylic acids ionise to give $\mathrm{RCOO}^{-}$ and $\mathrm{H}^{+}$.A more stable carboxylate ion means a stronger acid.Electron-withdrawing groups like $\mathrm{Cl}$ show the $-I$ effect and stabilise the ion, increasing acidity.Electron-donating alkyl groups show the $+I$ effect and destabilise the ion, decreasing acidity.In $\mathrm{Cl}_{3}\mathrm{CCOOH}$, three $\mathrm{Cl}$ atoms make it the strongest acid here.In $\mathrm{ClCH}_{2}\mathrm{COOH}$, one $\mathrm{Cl}$ atom makes it stronger than acetic acid.$\mathrm{CH}_{3}\mathrm{COOH}$ has one methyl group, so its acidity is decreased slightly.$\left(\mathrm{CH}_{3}\right)_{2}\mathrm{CHCOOH}$ has an isopropyl group, which has a stronger $+I$ effect than a methyl group.This destabilises the carboxylate ion the most, making it the weakest acid.Therefore, the weakest acid is $\left(\mathrm{CH}_{3}\right)_{2}\mathrm{CHCOOH}$.Answer:Option A: $\left(\mathrm{CH}_{3}\right)_{2}\mathrm{CHCOOH}$