15.20:
C–C Bond Cleavage: Retro-Aldol Reaction
Recall that the aldol addition reaction is unfavorable for ketones under basic conditions.
The retro-aldol reaction is the exact reverse of the aldol addition, where the β-hydroxy ketone in the presence of an aqueous base is preferably cleaved into two ketone molecules as the retro-aldol products.
The reaction mechanism consists of three distinct steps.
In step one, the deprotonation of the β-hydroxy group generates an alkoxide ion intermediate.
In the following step, the alkoxide's carbon–carbon bond is cleaved to form an enolate ion and a ketone molecule.
In the final step, the protonation of the enolate ion produces the second ketone molecule.
Similarly, the β-hydroxy aldehyde undergoes the retro-aldol reaction yielding the corresponding carbonyl precursors.
15.20:
C–C Bond Cleavage: Retro-Aldol Reaction
The reverse of the aldol addition reaction is called the retro-aldol reaction. Here, the carbon–carbon bond in the aldol product is cleaved under acidic or basic conditions to form two molecules of carbonyl compounds. The mechanism of the reaction consists of three steps.
In the first step, as depicted in Figure 1, the base deprotonates the β-hydroxy ketone at the hydroxyl group to form an alkoxide ion.
Figure 1. The deprotonation of a β-hydroxy ketone to form an alkoxide ion.
Figure 2 shows the second step, which involves the cleavage of the carbon–carbon bond to yield a ketone molecule and an enolate ion.
Figure 2. The formation of an enolate ion.
Finally, as illustrated in Figure 3, the enolate ion is protonated to form the second ketone molecule.
Figure 3. The protonation of enolate generates a second ketone.
Similarly, the β-hydroxy aldehyde in the presence of a base undergoes the retro-aldol reaction to produce two aldehyde molecules.