The ElcB mechanism of 1,2-eliminations

2-Elimination from substituted hydrocarbons to give olefins 

Rate = k1k2 [B] [Reactant] j(k i [BH+] + k2) where B and BH + buffer the reaction solution. 

At low concentrations of BH+ such that k2 k j [BH+], the observed first-order rate coefficient for conditions where buffer is present in excess over reactant is kx [B]. The rate of reaction is determined by a slow proton removal by base from carbon. At high concentrations of BH+, the observed first-order rate coefficient is (k1fe2/k-i)[B]/[BH+]. In this case, if the reaction is carried out in aqueous solution, the rate of reaction depends upon the hydroxide ion concentration and is independent of the buffer concentration at a fixed buffer ratio (specific base catalysis). The mechanism under these conditions consists of rapid pre-equilibrium formation of a carbanion followed by a slow step. Over the whole range of buffer concentration the first-order rate coefficient (M,hs) measured at fixed buffer ratio first increases (/ bs = kl [B]) with buffer concentration but reaches a limiting value (kohs = (ki k2 /k-i) [B] /[BH+]). This change in mechanism has been observed for a limited number of reactions [58]. Reactions (38) [58(a)] and (39) [58(b)] occurring in ethanol and reaction (40) [58(c)] in aqueous 

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