How do geometric isomers differ from structural isomers?

1 Answer
anor277
May 7, 2016

Structural isomers differ in connectivity. Geometric isomers have the same connectivity, but differ in their geometry.

Explanation:

This is an old example, but still it is worthwhile in that illustrates the difference.

Consider #"but-2-ene"#. Now #"but-2-ene"# has cis and trans isomers, and I assume you can identify them. In each isomer, #C1# connects to #C2#, connects to #C3#, connects to #C4#. And thus the cis and trans isomers are structurally equivalent, because they have precisely the same connectivity. Nevertheless because #C1# can be on the opposite (or the same side) as #C4#, this simple organic molecule can generate a pair of geometric isomers in their #"cis"# and #"trans"# varieties.

The introduction of a pair of chiral centres offers further opportunities for geometric isomerism.

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