chemoselectivity

Meaning: Chemoselectivity is the preference of a reagent to react with one functional group over another in a molecule that has more than one possible reactive site. It usually reflects differences in electrophilicity or nucleophilicity between the groups. Aldehydes and ketones are typically more electrophilic than esters and amides, because resonance donation from the adjacent oxygen or nitrogen lone pair stabilizes the carbonyl in the latter. So NaBH₄ reduces a ketone but usually leaves an ester alone, while the stronger LiAlH₄ reduces both.

On the MCAT: A passage shows a molecule with two functional groups, such as a ketone and an ester, and asks which reagent converts only one, or what the product is. Expect NaBH₄ versus LiAlH₄, and protecting groups: converting the ketone to an acetal lets LiAlH₄ reduce just the ester, and aqueous acid then restores the ketone. Mild H₂ with Pd/C usually reduces a C=C bond but typically not a C=O.

Common confusion: Chemoselectivity is not regioselectivity or stereoselectivity. Chemoselectivity asks which functional group reacts. Regioselectivity asks which position reacts or where a group adds (Markovnikov versus anti-Markovnikov). Stereoselectivity asks which stereoisomer forms. Ask first whether the competing sites are different functional groups.