Naming Alkenes and Alkynes

Recall:

Alkenes General Formula = C n H 2n Contain one or more double bonds Names will end in “–ene” Alkynes General Formula = C n H 2n-2 Contain one or more triple bonds Names will end in “-yne”

Example #1 What is the formula of an alkene with 5 “C” atoms? Example #2 What is the formula of an alkyne with 10 “H” atoms?

Special Isomers for Alkenes: Cis and Trans Geometric isomers Remember that isomers are molecules with the same molecular formula with different arrangements of the atoms. Cis and Trans isomers (also called geometric isomers) occur when groups of atoms are arranged in different ways around a double bond.

Special Isomers for Alkenes: Cis and Trans Geometric isomers They occur because double bonds cannot rotate. This forces atoms attached to double bonded carbons to stay in the same position relative to one another.

In order to have cis and trans isomers, each carbon in the C=C double bond must have two different “groups” (also called substituents) attached to it. CH 3 and H are different groups (substituents) Special Isomers for Alkenes: Cis and Trans Geometric isomers

Cis isomers (or “Z” isomers) have the two larger substituents on the same side of the double bond. Trans isomers (or “E” isomers) have the two larger substituents on opposite sides of the double bond. (or Z-2-butene) (or E-2-butene) Special Isomers for Alkenes: Cis and Trans Geometric isomers

No cis or trans isomer because both substituents on carbon #2 are the same!

Steps for Naming Alkenes and Alkynes

Naming Alkenes and Alkynes Step 1: Identify the longest continuous chain of carbon atoms that contains the double or triple bond

Step 2: Use the smallest possible number to indicate the position of the double or triple bond. (Regardless of where branches are found!!) The location number goes in front of the parent chain alkene name heptene Naming Alkenes and Alkynes

Step 3: Name branches using same rules for alkane branches methyl 3-heptene Naming Alkenes and Alkynes

Step 4: Number the branches from the same end used to number the double/triple bond methyl 3-heptene Naming Alkenes and Alkynes

Step 5: (Alkenes only!!) Include “cis” or “trans” in front of the compound name if possible: *cannot have two of the same substituents on a carbon *must be able to see the shape in the diagram methyl 3-heptene “trans” (opposite sides) Naming Alkenes and Alkynes

Final Name: trans-6-methyl-3-heptene methyl 3-heptene “trans” (opposite sides) Naming Alkenes and Alkynes

*Note: the trans / cis does not always exist or is not always possible to know! Possible to know the arrangement: Not Possible: Which side???? Naming Alkenes and Alkynes

cis-3-hexene Trans - Naming Alkenes and Alkynes cis-2-hexene

Sample #2: Provide the name for the following compound using IUPAC rules. Word file Naming Alkenes and Alkynes CH 2

Another example Word File Naming Alkenes and Alkynes

Practice makes perfect! Worksheet #5a *Fix questions!!

Sample Problem #3: Draw a condensed structural formula for the compound below: 3-butyl-5,7-dimethyl-1-hexyne Word File Word File Work Backwards through the name -Draw parent chain of carbons first (add H’s later) -Number the carbons -Add your branches Naming Alkenes and Alkynes