A conjugated carbonyl group has a ##"C=C"## double bond adjacent to it; an unconjugated carbonyl group has no adjacent double bond

A conjugated carbonyl group has a ##"C=C"## double bond adjacent to it; an unconjugated carbonyl group has no adjacent double bond.

Examples of unconjugated carbonyl are propanal and propanone.

Typical conjugated (or α,β-unsaturated) carbonyl compounds are propenal

and

butenone

In addition to a normal ##"C=O"## contributor, an α,β-unsaturated carbonyl compound has another contributor with a positive charge on the β-carbon.

It can be attacked by nucleophiles at either the β-carbon or the carbonyl carbon.

Note that the product of 1,4-addition is an enol that will tautomerize to the more stable carbonyl compound.