General

What is the IUPAC convention for naming alkenes?

1. Find and name the longest continuous carbon chain, label it as the base name and use the appropriate suffix: ``-ene,'' indicating an alkene or cycloalkene.
   • The base chain must include the carbon atoms involved in the double bond.
2. Number the longest chain from the end that is closest to the double bond.
   • If the double bond is in the center of the chain, begin numbering from an end that has a substituent closest to the end of the root chain.
3. Identify any double bonds using the lowest-numbered carbon, e.g. 2-pentene for a double between the 2nd and 3rd carbon.
   • If there is more than one double bond, use the same numbering approach, i.e. choose the lowest-numbered carbon to identify the location of the double bond.
   • Alkenes with multiple double bonds have special designations: e.g. dienes have 2 C=C bonds, trienes have 3 C=C bonds, etc.
     • Identify their locations by using the lowest-numbered carbons. For example, 2,3-pentadiene is the name for a five carbon diene with double bonds between carbons 2 and 3; and 3 and 4.

Figure 43.2: Alkenes: Ethene (ethylene), left, and butene (butylene), right. Note: ethene and butene are the IUPAC names and ethylene and butylene are the common names.

How are cycloalkenes named?

For cycloalkenes, the double bond carbon numbering is similar to that of straight-chained alkenes:

1. Find the carbon which (a) is part of the double bond, and (b) is closest to ring substituents (if there are any)
   • If there is more than one substituent, choose the longest one or the one with heavier atoms (i.e. the higher melecular weight).

What are two types of substituents that contain double bonds?

1. Vinyl groups, H$_{2}$C=CHÐ
2. Allyl groups, H$_{2}$C=CHÐCH$_{2}$Ð

Note: The nomenclature for how groups are arranged around the double bond - cis-, and trans-, - will be covered in the Covalent Bond Chapter on page .