α,β-unsaturated carbonyl

Meaning: A carbonyl compound with a C=C between the α and β carbons, so C=C–C=O is conjugated (an enone, enal, or unsaturated ester or acid). Resonance pulls electron density from the alkene toward oxygen, leaving partial positive charge on the β carbon. The molecule therefore has two electrophilic sites: the carbonyl carbon and the β carbon.

On the MCAT: Attack at the carbonyl carbon is 1,2-addition. Attack at the β carbon is 1,4-addition (conjugate or Michael addition), and the resulting enolate is protonated to give a saturated carbonyl. Strong, basic nucleophiles such as Grignard reagents and LiAlH₄ tend to favor 1,2; weaker, softer ones such as enolates, amines and thiols tend to favor 1,4. Aldol condensation makes these compounds when a β-hydroxy carbonyl loses water, driven by the stability of conjugation. Conjugation also lowers the IR C=O stretch slightly (about 1680 cm⁻¹ versus about 1715 cm⁻¹ for a saturated ketone) and moves UV absorption to longer wavelength.

Common confusion: Students treat the carbonyl carbon as the sole electrophile, as in a simple ketone. Resonance makes the β carbon electrophilic too, so the site of attack depends on the nucleophile. Also, α and β name the carbons next to the carbonyl, not a stereochemical label.