Aldol Dehydration (Elimination)

The Aldol products can eliminate water to form α,β-unsaturated carbonyl compounds.  The entire process from aldehyde to α,β-unsaturated carbonyl is called the Aldol condensation.  A condensation reaction is when two bigger molecules react (two aldehydes in this case) and eliminate a small molecule (usually water).  Normally these are conducted by heating the aldehyde in basic solution.

The elimination proceeds by an E_cb mechanism.  E_cb indicates its an elimination reaction that goes through a conjugate base (cb).  This is different from the eliminations (E1/E2) we've seen in the past.  In E_cb, the alcohol is deprotonated and then the hydroxide leaves.  The more stable trans-alkene is normally the major product as you would expect.

Examples

1)  Remember aldehydes are more reactive electrophiles than ketones.

2)  You can do it twice.

3) Intramolecular too!