Enolizable aldehydes and enolizable ketones, in the presence of an acid or base catalyst in aqueous medium at high temperature, undergo a reaction, giving an α, β-unsaturated aldehyde or an α, β-unsaturated ketone, respectively, as the product.  This reaction is known as aldol condensation.  The base-catalyzed aldol condensation, in which the catalyst is usually the hydroxide ion, is more common.

eg:

Mechanism:
Step 1:  The hydroxide ion deprotanates the aldehyde reversibly.

Step 2:  Enolate ion 1 adds to the unreacted aldehyde.

Step 3:  Alkoxide ion 2 is protonated by water.

Step 4:  Aldol 3 is an enolizable aldehyde.  A small amount of it is converted to the corresponding enolate ion (4) by the hydroxide ion.

Step 5:  Enolate ion 4 loses a hydroxide ion.

Step 1 through 3 are an aldol reaction, steps 4 and 5 a 1,2-elimination via E1cB mechanism.  Thus, aldol condensation is aldol reaction followed by 1,2-elimination.

 

 

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