Why do geometric isomers exist in some alkenes?

Question

Emilia Alvarez · Accepted Answer

Because there is no possibility of free rotation around an olefinic bond...

Consider #"2-butylene"#...there are TWO GEOMETRIC ISOMERS POSSIBLE....

For BOTH isomers, the #C-C# connectivity is PRECISELY the same...i.e. we got #H_3stackrel(1)C-stackrel(2)CH=stackrel(3)CH-stackrel(4)CH_3#...but CLEARLY, the geometry of each species is different...and this results in DIFFERENT chemical and physical properties....for instance normal boiling points of #3.7# #""^@C# (cis), and #0.90# #""^@C# (trans)...

Such isomerism is also possible for disubstituted rings..and given #"1-3, dimethylcyclohexane"#, how would you represent this?

Madison Anderson · Answer

undefined Geometric isomers exist in some alkenes due to restricted rotation around the carbon-carbon double bond, leading to different spatial arrangements of substituents. This results in cis and trans isomers, where distinct geometric configurations affect properties and reactions.

Lucy Alessi · Answer

undefined Geometric isomers exist in some alkenes due to the restricted rotation around the carbon-carbon double bond. This limited rotation prevents the two substituent groups attached to the double-bonded carbons from freely rotating relative to each other. As a result, different spatial arrangements, or isomers, are possible. In geometric isomers, the relative positions of the substituent groups differ, leading to distinct geometric structures despite having the same molecular formula. This phenomenon occurs when two different groups are attached to each carbon atom of the double bond, resulting in cis and trans isomers. The cis isomer has similar substituent groups on the same side of the double bond, while the trans isomer has them on opposite sides.