1 Chapter 11 Unsaturated Hydrocarbons 11.2 Cis-Trans Isomers

2 Cis and Trans Isomers In an alkene the double bond, is rigid. holds attached groups in fixed positions. makes cis/trans isomers possible. CH 3 CH 3 CH 3 CH = CH CH = CH cis trans CH 3

3 In cis-trans isomers there is no rotation around the double bond in alkenes. groups attached to the double bond are fixed relative to each other. You can make a “double bond” with your fingers with both thumbs on the same side or opposite from each other. Cis-Trans Isomers

4 Two isomers are possible when groups are attached to the double bond are different. In a cis isomer, groups are attached on the same side of the double bond. In the trans isomer, the groups are attached on opposite sides. Cis-Trans Isomers

5 Cis-Trans Isomerism Alkenes cannot have cis-trans isomers if a carbon atom in the double bond is attached to identical groups. Identical 2-bromopropene 1,1-dibromoethene (not cis or trans) (not cis or trans) H H H Br

6 Cis-Trans Isomers in Nature Insects emit tiny quantities of pheromones, which are chemicals that send messages. The silkworm moth attracts other moths by emitting bombykol, which has one cis and one trans double bond.

7 Naming Cis-Trans Isomers The prefixes cis or trans are placed in front of the alkene name when there are cis-trans isomers. cis cis-1,2-dibromoethene trans-1,2-dibromoethene

8 Learning Check Name each, using cis-trans prefixes when needed.

9 Solution cis-1,2-dibromoethene trans-2-butene 1,1-dichloropropene