Geometric ISOMERS of Alkenes Chapter 12 Section 3

 The CARBON – CARBON double bond is rigid (no free rotation) because of the shapes of the orbitals involved in its formation Pi bond Sigma bond 2 p-orbital electronsthe electrons lie lie above and belowin a line between the 2 carbons the 2 carbon nuclei Rotation around the double bond is restricted because the pi bond would have to be broken to allow for rotation. THUS, THE DOUBLE BOND IS RIGID ALKENES

In cycloalkanes, cis/trans applied to the branches trans = on OPPOSITE sides of the ring cis = on the SAME side of the ring Geometric Isomers

In alkenes, cis/trans applies to the location of the parent chain around the double bond, NOT the branches Geometric Isomers

If one of the carbons of the double bond has 2 identical substituents there are NO cis/trans isomers. Geometric Isomers

1,1-dichloroethene no cis/trans because both carbons have identical substituents 1,2-dichloroethene cis/trans isomers DO exist in this molecule Example:

 1-pentene  3-ethyl-3-hexene  3-methyl-2-pentene Which exist as geometric isomers?

 Cis-3-octene  Trans-5-chloro-2-hexene  Trans-2,3-dichloro-2-butene Draw these molecules.

 3 RD EDITION: P351 #19, 23, 24, 26  4 TH EDITION: P359 #21, 25, 26, 28 HOMEWORK