Question

If the ring closure is an `S_N2` reaction then what should be the arrangement of the `-O-C-C-X` system?

Answer 1

The O-C and C-X bonds must be anti to each other.

One way to prepare an epoxide is to react a halohydrin with a base.

An example is the conversion of 3-bromo-2,3-dimethylbutan-2-ol to 2,3-epoxy-2,3-dimethylbutane.

The reaction is really an intramolecular Williamson ether synthesis.

Step 1 is an acid-base reaction. The OH⁻ removes a proton from the alcohol. This converts it to an alkoxide ion and enhances the nucleophilicity of the oxygen atom.

Step 2 is an intramolecular `"S"_"N"2` substitution displacement of the bromine atom.

`"S"_"N"2` reactions involve "backside" attack on the carbon bearing the leaving group.

The C-O⁻ bond and the C-Br bond must therefore be anti to each other.