The Aldol Reaction of Aldehydes

THE ALDOL REACTION OF ALDEHYDES

So far, we have been talking about mechanisms that affect ketones; however, some of these reactions have also applied to aldehydes as the R chain talked about could also be hydrogen. In the aldol reaction of aldehydes, the alpha carbon can be acted on with a base, leaving behind an enolate that can form a beta-hydroxyaldehyde, as is shown in figure 86:

Figure 86.

This is done under sodium hydroxide or potassium hydroxide basic conditions. This environment is necessary to make the enolate out of the aldehyde. In such cases, there needs to be at least one alpha hydrogen molecule on the aldehyde, which gets pulled off to make an enolate. This is highly neutrophilic and binds with another molecule of aldehyde to make a beta hydroxyaldehyde, in which the new beta carbon atom is also an alcohol.

In such cases, the carbonyl carbons on those aldehydes not already an enolate become good electrophiles, binding to the nucleophilic alpha enolate carbon atom. The end product, as mentioned is a beta-hydroxyaldehyde but, because it is an alcohol and an aldehyde, it is called an aldol. The simplest reaction is that of ethanal (CH3CHO) and itself.

Condensation/dehydration happens to these aldols under usual circumstances to yield an unsaturated aldehyde, with a double bond between the alpha and beta carbon atom and water as an end product. This is referred to as an enal. Figure 87 describes an enal molecule: