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

Question

Lucy Alessi · Accepted Answer

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.

Ruby Allen · Answer

undefined In an SN2 reaction, the leaving group (X) and the attacking nucleophile (O-) are on opposite sides of the carbon atom undergoing nucleophilic substitution. This arrangement is known as an anti configuration. Therefore, in the -O-C-C-X system, the leaving group (X) and the attacking nucleophile (-O) should be in a trans arrangement.