Alkyl hahde dehydrohalogenation

The E2 mechanism is followed whenever an alkyl hahde—be it primary, secondary, or tertiary—undergoes elimination in the presence of a strong base. If a strong base is absent, or present in very low concentration, elimination can sometimes still occur by a uni-molecular mechanism (El). The El mechanism for dehydrohalogenation will be described in Section 5.18. 

The El dehydration of 2° and 3° alcohols with acid gives clean elimination products without by-products formed from an SnI reaction. This makes the El dehydration of alcohols much more synthetically useful than the El dehydrohalogenation of alkyl hahdes (Section 8.7). Clean elimination takes place because the reaction mixture contains no good nucleophile to react with the intermediate carbocation, so no competing SnI reaction occurs. 

The dehydrohalogenation of an alkyl hahde is a good laboratory method for the synthesis of alkenes because alkyl hahdes are readily available from reactions of several other starting materials. Considered by itself, this reaction has a very unfavorable eqmhbrium constant. However, if we use a strong base such as alkoxide anion to extract a proton on the carbon adjacent to the bromine, the reaction becomes highly favorable. 

In Chapter 9, we saw that we can prepare alkenes by dehydrohalogenation. The dehydration of alcohols also gives alkenes, but this reaction occurs with rearrangement of the carbocation, and gives mixtures of products having different carbon skeletons. The elimination of a hydrogen hahde from an alkyl halide is a complex process. We must consider both rcgiochemistry and stereoelectronic effects. These effects are related to the mechanism of the reaction, which may be either E2 or El. 

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