Self-condensation

Self-condensation is an organic reaction in which a chemical compound containing a carbonyl group acts both as the electrophile and the nucleophile in an aldol condensation. It is also called a symmetrical aldol condensation as opposed to a mixed aldol condensation in which the electrophile and nucleophile are different species.

For example, two molecules of acetone condense to a single compound mesityl oxide in the presence of an ion exchange resin:

For synthetic uses, this is generally an undesirable, but spontaneous and favored side-reaction of mixed aldol condensation, and special precautions are needed to prevent it.

In many cases, self-condensation is an unwanted side-reaction. Therefore, chemists have adopted many ways to prevent this from occurring when performing a crossed aldol reaction.

If acetophenone and benzaldehyde are put together in the presence of aqueous NaOH, only one product is formed:

This occurs because benzaldehyde lacks any enolizable protons, so it cannot form an enolate, and the benzaldehyde is much more electrophilic than any unenolized acetophenone in solution. Therefore, the enolate formed from acetophenone will always preferentially attack the benzaldehyde over another molecule of acetophenone.

When nitromethane and acetophenone are combined using aqueous NaOH, only one product is formed:

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