Why do E and Z isomers occur?

1 Answer
anor277 · MeneerNask
Oct 13, 2016

Because an olefin does not allow free rotation around the double bond.

Explanation:

For even a simple olefin such as "2-butylene", geometric isomerism can occur because of the C=C bond. The methyl groups can be on the same side of the C=C bond to give the "cis" or "Z" isomer, or on opposite sides of the C=C bond to give the "trans" or "E" isomer.

Both isomers have the same C-C connectivity: C1 connects to C2......to......C4, yet they are manifestly different geometrically. They are thus diastereomeric, and should give rise to different chemical and physical properties, and indeed they do.
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