. Explain Crossed aldol condensation of Aldehyde and Ketone


Best Answer

When two different carbonyl compounds (with alpha-H atoms) are used in an aldol condensation, four products are formed because each carbonyl compound can react with itself (self aldol) as well as with the other carbonyl compound (crossed aldol). For example, when two carbonyl compounds, A and B are treated with dil OH-, both can lose a proton from the alpha-carbon to form carbanion (acting as nucleophiles) A- and B- respectively. A- can either react with A or B and B- can react with either B or A. The reaction of A- with B or B- with A is called crossed aldol addition. All the four products have similar physical properties, making them difficult to separate. Consequently, crossed aldol addition is not a useful synthetic preparation.

All the four products are beta-hydroxy carbonyl compounds. Under certain conditions, a mixed aldol reaction can lead primarily to one product. When one of the carbonyl compound does not have any alpha-hydrogen, it cannot form carbanion and number of possible products reduces to two. A greater amount of one of the two products will be formed if the compound without alpha-hydrogen is present in excess. The carbanion will be more likely to attack the carbonyl compound without hydrogen because there is more of it in the solution.

Note that when mixture of an aldehyde and a ketone with alpha-hydrogen atom are used, the carbanion is exclusively formed by ketone and the carbanion generated attacks the carbonyl carbon of aldehyde (as it is more electrophilic).

Another way to obtain a single aldol product is to convert one carbonyl compound completely into carbanion. This cannot be done by using a weaker base (dil. OH-). To achieve this, we make use of a much stronger base like LDA (lithium diisopropyl amide). Thus, this carbanion attacks over other carbonyl compound to give only one product. For example,

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