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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