Why do some alkenes have geometric (cis-trans) isomers?

1 Answer
anor277
May 21, 2016

Why? Well, simply because they can.

Explanation:

Take a simple olefin, #"2-butene"#. Clearly its formula is #C_4H_8# or #H_3C-CH=CH-CH_3#. Because the double bond extends above and below the plane of the molecule, the pendant methyl groups cannot rotate with respect to each other.

Thus this simple formula generates a pair of geometric isomers, compounds that have identical connectivity (i.e. #C_1# connects #C_2......# to #C_4#, but have different geometry.

Just to illustrate that this does make a difference, the melting point of #"cis-2-butene"# is #-138.9# #""^@C# versus #"trans-2-butene"# is #-105.5# #""^@C#. These are certainly different compounds with different physical and chemical properties even though their formulae are identical.

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