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Answer:
Product A: cis; no
Product B: cis: no
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 <em>cis</em>-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 <em>cis</em>-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.
