JEE Main · 2022 · 26 Jun 2022 · Shift 2 · Aldehydes Ketones and Carboxylic Acids

Stopping a carboxylic acid at the aldehyde demands a reagent selective enough not to overshoot. Lithium aluminium hydride is far too powerful and carries the reduction past the aldehyde all the way to the primary alcohol, delivering benzyl alcohol rather than benzaldehyde; sodium borohydride lies at the opposite extreme and is too mild to reduce a carboxylic acid at all; and potassium permanganate is an oxidising agent, pushing in the wrong direction entirely. Eliminating these three leaves $\mathrm{MnO}$ as the only listed reagent consistent with the single-step conversion of benzoic acid to benzaldehyde.