Proton removal nucleophilic substitution, competition with

One important point regarding relative reactivities of Af-unsubstituted and Af-substituted 2-chloroimidazoles with powerful nucleophiles such as alkoxide is that in the former there is competition between proton abstraction from N-1 (which severely hampers any nucleophilic attack at C-2) and the nucleophilic substitution at the 2-position. Thus, chloride is not displaced from 2-chlorobenzimidazole by alkoxide ions, whereas 2-chloro-l-methyl-benzimidazole reacts readily. That steric effects can also be important is evidenced by the lack of reactivity of 2-chloro-l-methylbenzimidazole with t-butoxide, and similar resistance to nucleophilic attack of 2-chloro-l-isopropylbenzimidazole. Alkoxide readily displaces chloride from 2-chloro-1 -isopropenylbenzimidazole, and the N- substituent may be removed subsequently by oxidative cleavage (74CRV279). 

A close look at the nature of a nucleophile will emphasize that it shares common features with a Lewis hase (see Chapter 18). Indeed, a nucleophilic species can act as such a base if the reaction conditions are appropriate - it can remove a proton (H ion) from a halc enoalkane and thereby initiate an elimination reaction. In this type of reaction HX is eliminated from the halogenoalkane and an alkene is produced. It is essential to realize that, given the similarity of the reagents involved, the two processes of nucleophilic substitution and elimination are generally in competition with each other. If a primary halogenoalkane is reacted with aqueous alkali (OH (aq)) then the substitution reaction we have discussed earlier is favoured. However, if ethanolic alkali (OH (ethanol)) is used, then the elimination reaction is favoured. 

Elimination reactions compete with substitution reactions. The competition occurs because the nucleophile is also a base. When it reacts as a base, it removes a proton from the carbon adjacent to the leaving group, resulting in the formation of the elimination product. 

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