Question

The two reactions above, show routes for conversion of an alkene into an oxirane. If the starting alkene is cis-3-hexene the configurations of the oxirane products, A and B are Product A: _______ Product B: _______ Will either of these two oxirane products rotate the plane of polarization of plane polarized light? _____

Answer 1

Product A: cis; no

Product B: cis: no

Step-by-step explanation:

Two common methods of forming oxiranes from alkenes are:

• Reaction with peroxyacids
• Formation of a halohydrin followed by reaction with base

1. Reaction with peroxyacids

(a) Stereochemistry

The reaction with a peroxyacid is a syn addition, so the product has the same stereochemistry as the alkene.

The starting alkene is cis, so the product is cis-2,3-diethyloxirane.

(b) Configuration

The product is optically inactive because it has an internal plane of symmetry.

It will not rotate the plane of polarized light.

2. Halohydrin formation

(a) Stereochemistry

The halogenation of the alkene proceeds via a cyclic halonium ion.

The backside displacement of halide ion by alkoxide is also stereospecific, so a cis alkene gives a cis epoxide.

The product is cis-2,3-diethyloxirane.

(b) Configuration

The cyclic halonium ion has an internal plane of symmetry, as does the product (meso).

The oxirane will not rotate the plane of polarized light.