1. 20.7 Naming Alkenes & Alkynes Alkene formula = C n H 2n Alkyne formula = C n H 2n-2 geometric isomers cis-2-butenetrans-2-butene

2. 1. Select the longest continuous chain of C atoms that contains the double or triple bond. 2. For an alkene, the root name of the carbon chain is the same as for the alkane, except that the –ane is replaced by –yne. For example, for a 2-C chain we have: CH 3 CH 3 CH 2 =CH 2 CH CH EthaneEtheneEthyne

3. 3. Number the parent chain, starting at the end closest to the double or triple bond. The location of the multiple bond is given by the lowest-numbered carbon involved in the bond. CH 2 =CHCH 2 CH 3 CH 3 CH=CHCH 3 1 2 3 4 1-butene2-butene

4. 4. Substituents on the parent chain are treated the same way as in naming alkanes. For example, the molecule ClCH=CHCH 2 CH 3 is called 1-chloro-1- butene. 5. When the carbons are in a circle use the prefix cyclo- cyclobutane

5. Reactions Key characteristic of alkenes & alkynes – very reactive around double or triple bond. Hydrogenation – Use H 2 to add H to each C formerly in a double bond

6. Reactions Halogenation – involves addition of halogen (group 7) to double bond. CH 2 =CHCH 3 + Br 2  CH 2 BrCHBrCH 3 Polymerization – add together many small molecules (ethene) to get a long chain.