Unformatted text preview: le bond have restricted rotation about that double bond. This restricted rotation in a mo lecule gives rise to a type of iso mer known as geometric isomer.
· Isomers that differ fro m each other only in the geometry of their mo lecules and not in the order of their atoms are known as geometric isomers. They are also called cistrans isomers. Cl H Cl C C C H Cl C Cl H cis1,2dichloroethene (bp= 60.1°C) H trans1,2dichloroethene (bp= 48.4°C) · An alkene shows cistrans iso merism when each carbon atom of the double bond has two different kinds of groups attached to it.
a a a C b b C C C b b cis isomer a trans isomer · An alkene does not show cistrans iso merism if one carbon of the double bond has two ident ical groups attached to it. H H C H H C 3 two groups the same C H C H C 3 3 CH 3
C H Chemistry 52 Chapter 20 GEOMETRIC ISOMERS
· Shown below are some examples o f alkenes that do not have cis/trans iso mers. C H 3 C H 3 H H C H 2 C H 3 H C 3 C H 3 H C H C H 3
2 Examples: 1. Identify each of the mo lecules below as cis or trans iso mers. a) b)...
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