JEE Main · 2020 · 08 Jan 2020 · Shift 1 · Aldehydes Ketones and Carboxylic Acids

The transformation reduces a carboxylic acid group ($-\mathrm{COOH}$) to a primary alcohol ($-\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$) while leaving the nitrile ($-\mathrm{CN}$) and amide ($-\mathrm{CON}{\mathrm{H}}_2$) untouched. Diborane, ${\mathrm{B}}_2{\mathrm{H}}_6$, is just such a selective reagent, reducing carboxylic acids to alcohols without attacking nitriles or amides. Lithium aluminium hydride would reduce all three groups, sodium borohydride does not touch carboxylic acids, and ${\mathrm{H}}_2/Pd$ would reduce the nitrile, so diborane is the suitable choice.