Conjugate addition

This explains these reactions:

The different products are a result of two competing mechanisms:

The dominant mechanism can depend on other minor variables, such as temperature:

Or the presence of trace compounds:

A conjugated ketone can also be called an "α,β-unsaturated carbonyl compound:

Resonance helps explain why the beta carbon can be considered partially positive:

Carbon NMR gives some evidence for this. The beta-carbon above absorbs around 140 ppm, rather than around the 120 expected for an unconjugated double bond.