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SolutionsElimination Reactions

1. Alcohols and bromides

Explain the following facts: the cis and trans isomers of 4-tert-butylcyclohexanol dehydrate at very similar rates in H2SO4 at high temperature. However, the cis isomer of 1-bromo-4-tert-butylcyclohexane eliminates HBr with a strong base much faster than its trans isomer.

Schemes of the elimination in the cis and trans isomers: the alcohol dehydrates by E1 through the same carbocation, whereas in the bromide only the cis isomer, with the axial Br, has the anti H needed for E2; the cis rate is much higher than the trans rate

2. Saytzev or Hofmann

Give the major olefin in the elimination of 2-bromo-2-methylbutane with: a) sodium ethoxide in ethanol; b) potassium tert-butoxide in tert-butanol.

  1. 2-Methylbut-2-ene, the more substituted olefin (Saytzev): the small ethoxide removes the hydrogen of the CH2.
  2. 2-Methylbut-1-ene, the less substituted one (Hofmann): the bulky tert-butoxide removes the most accessible hydrogen, one of a CH3.

3. E2 stereochemistry

Which alkene is formed in the E2 elimination of (2R,3S)-2-bromo-3-methylpentane?

(Z)-3-Methylpent-2-ene. In E2 the H and the Br must be anti; when they are placed that way in the (2R,3S) isomer, the CH3 on C2 and the ethyl on C3 end up on the same side of the new double bond.

E2 is stereospecific: each stereoisomer of the substrate gives one particular isomer of the alkene.

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