7.3 Aldol Condensation

LEARNING GOALS

After   Chapter 7.3,    you will    be  able    to:

Identify    the species that    act as  nucleophiles    and electrophiles   in  aldol   condensations

Describe    the conditions, reactants,  and products    involved    in  a   retro-aldol reaction

List    the reaction    types   associated  with    aldol   condensation

The aldol condensation is another vital reaction for the MCAT. This reaction follows the same
general mechanism of nucleophilic addition to a carbonyl as previously described. In this case,
however, an aldehyde or ketone acts both as an electrophile (in its keto form) and a nucleophile (in
its enolate form), and the end result is the formation of a carbon–carbon bond.

As shown in Figure 7.6, when acetaldehyde (ethanal) is treated with a catalytic amount of base, an
enolate ion is produced. The enolate is more nucleophilic than the enol because it is negatively
charged.

This nucleophilic enolate ion can react with the electrophilic carbonyl group of another
acetaldehyde molecule. The key to this reaction is that both species are in the same flask. The
product is 3-hydroxybutanal, which is an example of an aldol (a molecule that contains both
aldehyde and alcohol functional groups). Note that mechanism is still called an aldol reaction even
when the reactants are ketones.