Contrasting E2, El, and ElcB

To understand the interdependence of the extent of leaving group departure and deprotonation, let s examine what makes a reaction such as E2 concerted. Jencks has described concerted reactions such as E2 as forced to be concerted. This term is used to describe a situation when no barrier separates a possible intermediate from the product. If there is no barrier to reaction of an intermediate, it has no lifetime, and does not represent a stable structure on the potential energy surface. For example, if a carbenium ion is too unstable to exist, an El reaction will become an E2 reaction. Furthermore, if a carbanion is too unstable to exist, an ElcB reaction will convert to E2. Stated another way, once we create a reactant that can achieve a carbanion or carbenium ion structure that is stable enough to exist, an E2 mechanism will convert to ElcB or El, respectively. 

A More O Ferrall-Jencks plot allows one to predict how deprotonation and leaving group departure in an E2 reaction change as a function of each other. We show what happens if the leaving group is made better. 

If we increase the leaving group ability in an E2 reaction, is there more leaving group departure in the transition state Actually, there is not. Making the leaving group better lowers 

Effect on the E2 Transition State of Various Structural Changes 

Greater R group substitution on the carbon with the leaving group a. Less deprotonation b. More leaving group departure c. More carbenium ion character 

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