Reaction of stabilized carbanions with carbonyl compounds

2 Reaction of stabilized carbanions with carbonyl compounds Aldol condensation 

The aldol condensation can also be brought about with acid catalysis. There are many aldol-like reactions which involve an aldehyde or ketone. However, the word aldol is a common name for the product of these reactions. Generally, the word aldol is used to refer to any P-hydroxyaldehyde or (3-hydroxyketone. For example, treatment of acetone with a base results in the aldol reaction. The product, -hydroxyketone 3.14, is separated from the reaction mixture as it is formed. 

Aldol condensation reaction is a reversible reaction and equilibrium is favoured towards the product (aldol) only for simple aldehydes, greater substitution favours the starting compounds at equilibrium. 

The aldol reaction is often combined with dehydration of the initial aldol product, forming a conjugated double bond (an enone). Thus, (3-hydroxyaldehyde and (3-hydroxyketone undergo dehydration to produce a, 3-unsaturated aldehyde and a, -unsaturated ketone, respectively. In fact, it is difficult to isolate (3-hydroxyaldehydes and (3-hydroxyketones because they are very prone to dehydration. 

Mixed or crossed aldol condensation Aldol condensations between different carbonyl reactants are called crossed (or mixed) reactions. Crossed aldol condensation works well if one carbonyl compound has no a-hydrogen(s). For example, acetone reacts with furfural in a crossed-aldol reaction to give the corresponding a,P-unsaturated ketone 3.15. 

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