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CONDENSATIONS OF , ALDEHYDES AND KETONES The carbonyl group has been characterized as the central or key functional group in orgariic chemistry. If so, base-catalyzed condensations involving car- bonyl compounds are probably the most important set of reactions for forming carbon-carbon bonds. Among the many important base-catalyzed reactions are the Perkin , Claisen, and Knoevenagel condensations, illustrated below. ~CHO ~ ~ N a OAc ~CH=CH-COOH ~ + CH 3 -C-O-C-CH 3 ) ~ o 0 0 0 II II NaOC , H , II II CH 3 -C-OC 2 H s + CH 3 -C-OC 2 H s ) CH 3 -C-CH 2 -C-OC 2 H s Claisen condensation ~CiIO ~CH=CH-COOH ~ + HOOC-CH 2 -COOH ~ ~ All these related condensation reactions are characterized by two things: a carbonyl compound which serves as the electrophile, and a carbon acid fro , which a nucleophilic carbanion is formed. If a carbonyl compound is used . the carbon acid, the acidity is usually attributable to its ability to form a stab enolate ion. The delocalization of charge in the enolate ion affords it cons ' j
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erable stability. (Discussion of the effect of two adjacent carbonyl groups may be found in Sec. 10.5.) o OH 0 - 0 I I I -w I - II -CHz-C- :;;::::::: : : -CH=C- ~ -CH=C- ~ -CH-C- +H + Keto form En d l form Enol a te ion Once the enolate ion has been formed by deprotonation of the carbonyl compound, it reacts with some electrophile present in solution. If we consider , p wh a t reaction might occur between acetaldehyde (ethanal) and ba s e, we see that both the electrophile and the carbori acid are present in the same molecule. The reaction sequence can be formulated as follows: o O-Na + 0 OH II I H ,O + I I I H-C-CH z -C-CH 3 ~ H-C-CH z -CH-CH 3 ~ 3 - H y drox y but a nal (a ld o l ) The ba s e-catalyzed reaction of acetaldehyde with itself i s the simple s t b a se- cat a lyzed condensation. The product, 3-hydroxybutanal, is given the trivial name ald o l , and this n a me ha s b e en generalized to the reaction t y pe. Many reactions between a carbon acid and a carbonyl compound are referred to as ald o l c onden s ations , a lthough the term may not be strictly appropriate. The general mechanism for the reactions which are discussed in this chapter may
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This note was uploaded on 11/23/2010 for the course BIOL 6 taught by Professor Olsen,r during the Spring '08 term at Fresno City College.

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