[Charlie_]

In an aldol reaction why does the reaction seem to progress such that the enolate formed by a ketone reacts with an aldehyde rather than another ketone?

Is it that it does happen but all the ketones get used up to form enolates? Is it that in the ketone the polarity caused by the carbonyl group is less severe than in the aldehyde becuase there are two neighboring carbons to spread the effect out rather than one? Or something else?

Thanks.

[Kate]

Aldehydes are more reactive than ketones. Their carbonyl carbon has a more positive partial charge than the respective carbon in ketones. So in an aldol reaction, the enolate having a negative charge will react preferably with an aldehyde.

Also, for an aldol reaction to be synthetically useful, certain precautions are taken. For instance, the aldehyde shouldn't have α-hydrogens and the ketone should have only one type of α-hydrogens. Otherwise, you're going to get a mixture of many different possible products.