Figure 7.7. Aldol Condensation, Step 2: Dehydration of the Aldol
The –OH is removed as water (dehydration), forming a double bond.Aldol condensations are most useful if we only use one type of aldehyde or ketone. If there are
multiple aldehydes or ketones, we cannot easily control which will act as the nucleophile and which
will act as the electrophile, and a mixture of products will result. This can be prevented if one of the
molecules has no α-hydrogens because the α-carbons are quaternary (like benzaldehyde).
This reaction is referred to as a condensation reaction because two molecules are joined with the
loss of a small molecule. This type of reaction is also a dehydration reaction because the small
molecule that is lost is water.
THE RETRO-ALDOL REACTION
The reverse of this reaction is called a retro-aldol reaction. To push the reaction in a retro-aldol
direction, aqueous base is added and heat is applied. The retro-aldol reaction is useful for breaking
bonds between the α-and β-carbons of a carbonyl, as shown in Figure 7.8. This reaction is facilitated
if the intermediate can be stabilized in the enolate form, just as in the forward reaction.
Figure 7.8. Retro-Aldol Reaction
The bond between the α- and β-carbons of a carbonyl is broken.KEY CONCEPT
In a retro-aldol reaction, a bond is broken between the α-and β-carbons of a carbonyl,
forming two aldehydes, two ketones, or one aldehyde and one ketone.