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It was noted previously (here) that there were great similarities in SN and E in terms of the starting materials and reagents – both involve a molecule containing a leaving group, and a reagent that is either a nucleophile or a base (it was also noted in the Nucleophilic Substitution chapter that these two share similar properties). So why do some reactions go as substitutions and some as eliminations.
The issue of which proton is removed when there is a choice is not as simple in E2 as it is in the other mechanisms. Two empirical rules have been formulated which apply to different systems, and essentially say the opposite things.
The stereochemistry of E1 and E1CB is relatively easily to dissect. In terms of the first step the regiochemistry is reasonably clear (and there should be no stereochemistry to consider) – in E1CB the most acidic proton (adjacent to the leaving group) is taken and in E1 the leaving group leaves.
The similarity in rate determining steps between E1 and SN1 helps to explain why the same factors govern whether a mechanism is E1 or E2 (E1CB disregarded for the time being) – those being; possibility of a stabilised cation and a polar solvent which can solvate said cation.
An elimination reaction involves the removal of two species from a molecule.