In a Michael addition to an α,β-unsaturated carbonyl compound, where does the nucleophile attack?

• A. The β-carbon (conjugate addition)
• B. The carbonyl carbon
• C. The α-carbon
• D. The leaving group site

Answer: (A)

In a Michael addition, the nucleophile goes to the β-carbon of an α,β-unsaturated carbonyl. This is a conjugate (1,4) addition driven by the way electrons are delocalized through the conjugated system. The carbonyl withdraws electron density and, through resonance, places a partial positive character at the β-carbon, making it the most electrophilic site for a nucleophile with the right softness or polarizability. When the nucleophile attacks there, a new bond forms at the β-carbon and the π electrons shift toward the carbonyl, producing an enolate. After workup, the enolate is protonated to restore the carbonyl, giving a product in which the carbonyl group remains intact and a new substituent is added at the β position, with the original C=C bond converted to a single bond.

Attacking the carbonyl carbon would be a 1,2-addition, which is less favored in this conjugated system with typical Michael donors. Attacking at the α-carbon or involving a leaving group isn’t the characteristic pathway for a Michael addition.