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Chapter 12 Alkenes, Alkynes, and Aromatic Compounds

Chapter 12 Alkenes, Alkynes, and Aromatic Compounds. 12.2 Cis – Trans Isomers. Cis and Trans Isomers. In an alkene, cis and trans isomers are possible because the double bond is rigid cannot rotate

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Chapter 12 Alkenes, Alkynes, and Aromatic Compounds

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  1. Chapter 12 Alkenes, Alkynes, and Aromatic Compounds 12.2Cis–Trans Isomers

  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

  3. Cis–Trans Isomers 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)

  4. Cis–Trans Isomers (continued) 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 thetrans isomer, the groups are attached on opposite sides of the double bond.

  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-Dibromoethenetrans-1,2-Dibromoethene

  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)

  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

  8. Learning Check Name each, using cis–transprefixes when needed.

  9. Solution cis-1,2-Dibromoethene trans-2-Butene 1,1-Dichloropropene Identical atoms; no cis or trans

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