Addition-dehydration the aldol reaction

We now have examples of the generation of enolate anions from carbonyl compounds, and their potential as nucleophiles in simple Sn2 reactions. However, we must not lose sight of the potential of a carbonyl compound to act as an electrophile. This section, the aldol reaction, is concerned with enolate anion 

The namesake aldol reaction is the formation of an addition compound, aldol, from two molecules of acetaldehyde, when this aldehyde is treated with aqueous sodium hydroxide. The terminology aldol comes from the functional groups in the product, aldehyde and alcohol. 

This is easiiy formuiated as production of an enoiate anion foiiowed by nucieophiiic attack of this anion on to the carbonyi group of a second moiecuie of acetaidehyde. Aidoi is then produced when the addition anion abstracts a proton from soivent. The reaction is reversibie, and it is usuaiiy necessary to disturb the equiiibrium by some means. Removai of product is possibie, but, as seen beiow, the dehydration part of the sequence may be responsibie for pushing the reaction to compietion. 

In the reverse reaction, the addition anion reforms the carbonyi group by expeiiing the enoiate anion as ieaving group. This reverse aldol reaction is sufficientiy important in its own right, and we shaii meet exampies. Note that, as we saw with simpie aidehyde and ketone addition reactions, aidehydes are better eiectrophiies than ketones (see Section 7.1.1). This arises from the extra alkyl group in ketones, which provides a further inductive effect and extra steric hindrance. Accordingly, the aldol reaction is more favourable with aldehydes than with ketones. With ketones, it is absolutely essential to disturb the equilibrium in some way. 

The aldol reaction as formulated above involves two molecules of the starting substrate. However, by a consideration of the mechanism, one can see that different carbonyl compounds might be used as nucleophile or electrophile. This would be termed a mixed aldol reaction or crossed aldol reaction. However, if one merely reacted, say, two aldehydes together under basic conditions, one would get a 

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