SolutionsUnimolecular Elimination
1. Substitution and elimination
What products are obtained by heating tert-butyl chloride in methanol?
Methyl tert-butyl ether, (CH3)3COCH3, by SN1, and 2-methylpropene (isobutylene), CH2=C(CH3)2, by E1. Both come from the same tert-butyl carbocation.
2. The leaving group
tert-Butyl chloride, bromide and iodide give the same ether/alkene ratio in methanol. Why?
Because the partition between SN1 and E1 is decided after the halogen has left, at the carbocation, which is the same in all three cases.
3. Two alkenes
Which alkenes are formed by E1 from 2-bromo-2-methylbutane? Which is the major one?
2-Methyl-2-butene, (CH3)2C=CHCH3, the major one because it is the more substituted (more stable), and 2-methyl-1-butene, CH2=C(CH3)CH2CH3.
4. Kinetics
Write the rate law of an E1 reaction and explain its order.
v = k[RX]: first order, because the rate-limiting step is formation of the carbocation, in which only the halide is involved.