CH 6-8: Regioselectivity in E2 Reactions A reaction is regioselective if only “select” products are formed from all possible constitutional isomers. How many alkene products can be formed in an E2 reaction with the alkyl halide below? b2 b1 a Locate the “a” carbon Locate ALL “b” carbons and “b” hydrogens A double bond may form between the “a” and “b” carbons, with “elimination” of a b-hydrogen and the leaving group.

How many alkene products? = 3 b2 b1 a b1 b2 - trans b2 - cis Major Product Which is the “major” product? The most stable alkene! This is the most highly substituted alkene (fewest sp2 H’s), with the largest sp2 groups trans (least steric hindrance).

Alkyl Halide Reactivity in E2 Reactions If the most highly substituted alkene is the most stable (major product), which of the following alkyl halides is the most reactive in an E2 elimination? The order of reactivity is: 3o > 2o > 1o Note that this is opposite to the order of alkyl halide reactivity for SN2 reactions.

Structure of the Base in E2 Reactions HO- gives: b2 b1 WHY? ……..Steric Hinderance A large, bulky base will grab the most accessible b-hydrogen (usually a methyl b-hydrogen). A small base grabs the b-hydrogen that gives the most stable alkene.