SolutionsPreparation
1. Which combination?
To prepare tert-butyl methyl ether by Williamson synthesis, would you use sodium tert-butoxide and methyl iodide, or sodium methoxide and tert-butyl bromide? Why?
Sodium tert-butoxide and methyl iodide. The methyl halide undergoes SN2 without competing elimination; with tert-butyl bromide, methoxide would act as a base and 2-methylpropene would be obtained (E2).
2. Cyclic ethers
Which ether is formed when 4-chloro-1-butanol is treated with NaOH? Why does it form easily?
Tetrahydrofuran, by intramolecular SN2 of the alkoxide on the carbon bonded to chlorine. Five-membered rings form very quickly: they have little strain and the ends of the chain meet easily.
3. Epoxides from halohydrins
Why does trans-2-bromocyclohexanol form the epoxide with base while the cis isomer does not?
Because the alkoxide must attack the carbon from the side opposite the bromine (SN2). In the trans isomer this is possible in the diaxial conformation; in the cis isomer the O− and the Br are never anti.