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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 studentís lack of readiness for the study of carbanions
prior to this point in the Chemistry Professorís 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 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.