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

1 Answer
Ernest Z.
Jan 9, 2015

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.

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

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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.

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