A compound with the C=C-C=O arrangement is called an a,b-unsaturated aldehyde or ketone. Such compounds are usually prepared by an aldol/dehydration sequence. In the aldol reaction, an aldehyde or ketone enolate is condensed with another aldehyde or ketone molecule to form a b-hydroxy aldehyde or ketone. If the two carbonyl molecules are not the same, the reaction is called a crossed aldol, and may give up to four different products. Condensation of an enolate with an aldehyde or ketone that does not have any hydrogens on the carbon next to the carbonyl group solves this problem. The b-hydroxy carbonyl compound can then be dehydrated to give an a,b-unsaturated compound. The driving force for this dehydration is the formation of a conjugated molecule. The dehydration can be acheived with acid or base, and often occurs in the same reaction flask as the aldol reaction (Section 14.2).