Carbanions

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Overview:

Carbanions introduces the characteristic reactions occurring at the alpha hydrogen of carbonyls. Reactions studied include bromination, aldol and crossed aldol condensation, the Cannizzaro reaction, the Wittig reaction, the Claisen and crossed Claisen reactions, the malonic ester synthesis, and the acetoacetic ester synthesis.

To a chemist experienced with application of mechanisms to biochemistry, the placement of carbanion study late in the organic chemistry course seems to indicate a lesser degree of importance of that mechanism when compared to free radical and carbocation. The problem is the organic student’s lack of readiness for the study of carbanions prior to this point in the Chemistry Professor’s opinion. This unit requires a bit more chemistry sophistication than the other mechanisms, hence the delay in introducing the unit.

This unit on carbanions begins with a treatment of the alpha hydrogen and its significantly acidic character, a factor which permits reaction with a particularly strong base, such as lithium diisopropylamide. This immediately brings into question the character of strong bases and the reaction which subsequently follows to produce the carbanion. The factors which stabilize or destabilize this carbanion are addressed and the order of carbanion stability deduced. The rearrangement of the carbanion under appropriate conditions to form the familiar enolate ion and subsequently the keto-enol tautomers is addressed.

Bromination is the first reaction addressed using attack at the alpha hydrogen. The difference between base promoted bromination and acid catalyzed bromination are addressed both from the difference in the terms and the mechanism each uses. The student is directed to notice, again, the ability to direct reactions to desired locations by appropriate use of reagents and conditions.

The aldol condensation is outlined and its mechanism detailed. The ability of the beta-hydroxy aldehyde to readily dehydrate, forming an alpha-beta unsaturated carbonyl is discussed and several practice reactions presented. From this study the question arises of controlling the reaction when more than one aldehyde is present. At that point, the crossed aldol condensation is introduced.

The Cannizzaro reaction in which an aldehyde is both oxidized to an acid and reduced to an alcohol is introduced. The student is directed to understand the impact of having no alpha hydrogen on the carbonyl.

The Wittig reaction is an elegantly simple method of forming an alkene whose double bond placement is never in doubt. The reaction and its interesting intermediates are presented.

The formation of the acetoacetic ester and its use in the Claisen condensation is introduced. The limitations of this condensation are discussed and the crossed Claisen condensation introduced. The unit concludes with treatment of the malonic ester synthesis and additional notations regarding acetoacetic ester use.