Alpha-Substitution Reactions

Alpha-substitution reactions occur at the position next to the carbonyl group—the a position—and involve the substitution of an cv hydrogen atom by an electrophile, E, through either an enol or eriolate ion intermediate. Let s begin by learning more about these two species. 

Reactivity of Ends The Mechanism of Alpha-Substitution Reactions 84... 

There is no simple answer to this question, but the exact experimental conditions usually have much to do with the result. Alpha-substitution reactions require a full equivalent of strong base and are normally carried out so that the carbonyl compound is rapidly and completely converted into its enolate ion at a low temperature. An electrophile is then added rapidly to ensure that the reactive enolate ion is quenched quickly. In a ketone alkylation reaction, for instance, we might use 1 equivalent of lithium diisopropylamide (LDA) in lelrahydrofuran solution at -78 °C. Rapid and complete generation of the ketone enolate ion would occur, and no unreacled ketone would be left so that no condensation reaction could take place. We would then immediately add an alkyl halide to complete the alkylation reaction. 

The haloform reaction (Problem 22.23d) is an alpha-substitution reaction in which a methyl ketone is trihalogenated at the alpha position, and the trihalomethyl group is displaced by -OH. It is a test for methyl ketones. 

Alpha-substitution reactions, on the other hand, use one equivalent of base. 

Reactivity of Ends The Mechanism ol Alpha-Substitution Reactions 905... 

Carbonyl Condensation Reactions versis Alpha-Substitution Reactions 941... 

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