Elimination, ElcB versus

In this section, we examine the effects of changes in the substrate, base, leaving group, and medium on (i) overall reactivity, (2) El versus E2 versus ElcB, and (3) elimination versus substitution. 

Effect on El versus E2 versus ElcB. In the El mechanism, an external base is generally not required The solvent acts as the base. Hence, when external bases are added, the mechanism is shifted toward E2. Stronger bases and higher base concentrations cause the mechanism to move toward the ElcB end of the El-E2-ElcB spectmm."° However, weak bases in polar aprotic solvents can also be effective in elimination reactions with certain substrates (the E2C reaction). Normal E2 elimination has been accomplished with the following bases " HjO, NR3, OH, OAc, OR, OAr, NHj, COa ,... 

Ejfect of Solvent on El versus E2 versus ElcB. With any reaction a more polar environment enhances the rate of mechanisms that involve ionic intermediates. Eor neutral leaving groups, it is expected that El and ElcB mechanisms will be aided by increasing polarity of solvent and by increasing ionic strength. With certain substrates, polar aprotic solvents promote elimination with weak bases (the E2C reaction). 

VII. j3-Elimination-Addition Reactions E2 versus ElcB Mechanisms. 397... 

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