Chapter 11 Unsaturated Hydrocarbons 11.2 Cis–Trans Isomers

Cis and Trans Isomers In an alkene, the double bond is rigid holds attached groups in fixed positions makes cis–trans isomers possible

Cis–Trans Isomers 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 Two isomers are possible when the groups attached to each side of the double bond are different. In a cis isomer, the alkyl groups are attached on one side of the double bond and H atoms are on the other side. In the trans isomer, the groups and H atoms are attached on opposite sides. Ball-and-stick models of the cis and trans isomers of 2-butene.

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

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. Pheromones allow insects to attract mates from a great distance.

Naming Cis–Trans Isomers The prefix of cis or trans is placed in front of the alkene name when the compound is a cis or trans isomer. cis trans cis-1,2-dibromoethene trans-1,2-dibromoethene

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

Solution Name each, using cis or trans prefixes when needed. cis-1,2-dibromoethene trans-2-butene 1,1-dichloropropene