Q5(e). E2 of 3-bromo-2,3-dimethylpentane: Saytzeff products

Introduction. 3-Bromo-2,3-dimethylpentane, (CH₃)₂CH-CBr(CH₃)-CH₂CH₃, is a tertiary bromide. Hot alcoholic KOH brings about dehydrohalogenation (E2 with some E1); substitution is minor. There are three kinds of β-hydrogen, so three alkenes (four with E/Z) can form. The Saytzeff rule sets their order: the more substituted alkene is formed more.

Scheme: Middle pair: 3,4-dimethylpent-2-ene, E and Z.

1. In E2 the C–H and C–Br bonds break together and the transition state has partial double-bond character; the alkyl groups that stabilise the product alkene (hyperconjugation) also stabilise this transition state. Hence the rate of formation follows alkene stability.
2. Order of yield: tetrasubstituted > trisubstituted (E > Z) > disubstituted: 2,3-dimethylpent-2-ene > (E)-3,4-dimethylpent-2-ene > (Z)-3,4-dimethylpent-2-ene > 2-ethyl-3-methylbut-1-ene.
3. The Z isomer of 3,4-dimethylpent-2-ene is less stable than E because its CH₃ on C-2 is cis to the isopropyl group on C-3.
4. Statistics alone (1: 2: 3 β-H) would favour the terminal alkene; product stability outweighs it with a small base such as OH⁻/EtO⁻. A bulky base (KOC(CH₃)₃) would raise the share of the terminal (Hofmann) alkene.
5. A small amount of substitution product (2,3-dimethylpentan-3-ol or its ethyl ether, by SN1) may also form.

Conclusion. Three alkenes form; 2,3-dimethylpent-2-ene is the major (Saytzeff) product, then (E)- and (Z)-3,4-dimethylpent-2-ene, and 2-ethyl-3-methylbut-1-ene is the least.

Application. Saytzeff and Hofmann orientation rules are used to plan the position of a C=C in synthesis (choosing ethoxide or tert-butoxide as the base).