Which of the following reagents will selectively reduce the $\text{-CHO}$ group in the presence of a $\text{C=O}$ group in a molecule containing both?

Solution

1. ${\text{NaBH}}_3\text{CN}$ is a selective reducing agent whose reactivity is highly dependent on the $\text{pH}$ of the medium.
2. At $\text{pH }6$, the aldehyde is protonated to an iminium-like species or activated, making it more electrophilic than the ketone.
3. ${\text{NaBH}}_4$ and ${\text{LiAlH}}_4$ reduce both functional groups indiscriminately.
4. Hence the answer is (D).