General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.1 Chapter 12 Alkenes, Alkynes, and Aromatic Compounds 12.2 Cis–Trans Isomers

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.2 Cis and Trans Isomers In an alkene, cis and trans isomers are possible because the double bond  is rigid  cannot rotate  has groups attached to the carbons of the double bond that are fixed relative to each other

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.3 Cis–trans isomers can be modeled by  making a “double bond” with your fingers with both thumbs on the same side (cis isomer)  or fingers with thumbs on opposite sides from each other (trans isomer) Cis – Trans Isomers

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.4 Cis–trans isomers occur when different groups are attached to the double bond.  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 of the double bond. Cis – Trans Isomers (continued)

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.5 Naming Cis – Trans Isomers  The prefixes cis- or trans- are placed in front of the alkene name when there are cis–trans isomers. cis- trans- cis-1,2-Dibromoethene trans-1,2-Dibromoethene

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.6 Cis – Trans Isomerism  Cis–trans isomers do not occur if a carbon atom in the double bond is attached to identical groups. (not cis or trans)(not cis or trans)

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.7 Pheromones A pheromone  is a chemical messenger emitted by insects in tiny quantities  called bombykol is emitted by the silkworm moth to attract other moths and has one cis and one trans double bond

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.8 Learning Check Name each, using cis–trans prefixes when needed.

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.9 Solution cis-1,2-Dibromoethene trans-2-Butene 1,1-Dichloropropene Identical atoms; no cis or trans