Alkenes are organic molecules containing a carbon-carbon double bond. The carbons involved in the double bond are usually singly bonded to hydrogens or carbons.
The rules for naming alkenes (and all other functional groups) build on the rules covered in the tutorial on the nomenclature of branched alkanes. While these rules are described on this page, there is a more detailed explanation in the previous tutorial.
As before, the first step of nomenclature is to identify the parent chain. In an alkene, the parent chain is the longest chain of carbons that contains both carbons in the double bond – this isn’t always the longest chain of carbons.
The name of an alkene parent chain ends with the suffix –ene (instead of the –ane suffix used for alkanes). For example, an alkene with a parent chain that is three carbons long is called a propene (while the corresponding alkane is called a propane).
Next, we need to number the carbons of the parent chain. For an alkene, we number the parent chain (which contains the double bond) so that the double bond carbons have the lowest numbers possible. While we wanted to give the lowest numbers possible to parent chain carbons bonded to alkyl and halogen substituents for the branched alkanes and alkyl halides, the double-bonded carbons are of higher priority in alkenes. Therefore, the positions of substituents are not usually considered when naming an alkene. However, if we have an alkene in which the double bond carbons are given the same numbering starting from either end of the parent chain, then number the parent chain so as to give lower numbers to carbons with substituents.
Now we are ready to start putting the name together. Like before, we assemble the alkene name by listing the substituents in alphabetical order, and then finish with the parent chain name.
While the naming of alkyl and halogen substituents is the same as we covered previously, the name given to the alkene parent chain must describe the location of the double bond. This is done by including the number of the first double bond carbon (that is, the lowest numbered carbon) in the name of the parent chain. This can be done two different ways. IUPAC rules say that this number should be placed in the middle of the alkene name (e.g., but-2-ene). However, it is also common to place the number at the beginning of the alkene name (e.g., 2-butene). It’s a good idea to check what format your instructor uses.
Let’s practice these rules by assigning a name to the following alkene:
First step, as always, is to identify the parent chain. Remember that this is the longest chain of carbons which includes both double bond carbons. The parent chain carbons are highlighted here:
Note that here, it doesn’t matter which of the two methyl groups bonded to the alkene carbons we choose to be part of the parent chain. Now, we need to number the parent chain. While naming alkenes, we want to give the lowest possible numbers to the double bond carbons:
Next, we name the substituents on the parent chain. We have two methyl groups on carbons 2 and 5, and a propyl group on carbon 4. These are given the names: 2,5-dimethyl and 4-propyl. If we list these in alphabetical order (remember that prefixes indicating numbers, such as di- and tri-, are ignored), we get: 2,5-dimethyl-4-propyl.
As the parent chain is an alkene with eight carbons, it is an octene. However, we also have to indicate the position of the double bond: the lowest numbered double bond carbon is carbon 2. Therefore, the parent chain name is oct-2-ene. Finally, if we assemble the substituent names and the parent chain name, we get: 2,5-dimethyl-4-propyloct-2-ene.
Now, let’s go through the nomenclature of a more complicated alkene:
First, identify the parent chain. As this molecule has two double bonds, we need to try and find a parent chain that contains both of them. Note that it isn’t always possible to include all of the double bonds in the parent chain (for example, if there are three double bonds). Here are the carbons in the parent chain, including both double bonds:
Next up is numbering the parent chain. We have to decide which end of the parent chain to start numbering. As with simpler alkenes, we want to give the lowest numbers possible to the double bond carbons:
Numbering from the left gives us double bonds starting at carbons 1 and 5, while starting from the right would give us double bonds starting on carbons 2 and 6.
The parent chain has three substituents: an ethyl group on carbon 3, a bromo group on carbon 4, and a methyl group on carbon 6. We can assemble these alphabetically to: 4-bromo-3-ethyl-6-methyl.
Our parent chain is seven carbons long, which corresponds to the alkane heptane. As this parent has two double bonds, we use the suffix –adiene instead of the suffix –ene. Thus, our molecule would be a heptadiene. If we had three double bonds, we would use –atriene, and so on. We must also indicate the positions of the double bonds in the name, using the lowest numbered carbons of each double bond (so, carbons 1 and 5). Putting this together, we have: hepta-1,5-diene. Adding the substituents to the front, we get: 4-bromo-3-ethyl-6-methylhepta-1,5-diene.
Depending on how the substituents on the double-bond of an alkene are oriented, different stereoisomers are possible. The names given to alkenes should distinguish between these stereoisomers, using cis-trans notation or E–Z notation.
This tutorial covered the nomenclature of alkenes in which the double bond is part of the parent chain. However, a different nomenclature system is used if the alkenes are substituents.
The nomenclature of alkenes is also different if there are other functional groups in the molecule (such as alkynes).
The steps for the nomenclature of alkenes are:
- Identify the parent chain (the longest chain of carbons that contains both alkene carbons).
- Number the parent chain (giving the lowest numbers possible to the double bond carbons).
- List the substituents in alphabetical order.
- Finish with the name of the parent chain, using the suffix –ene. If the molecule has multiple double bonds, use the suffixes –adiene, -atriene, etc. Indicate the position of the double bond with the number of the lowest numbered double bond carbon (e.g., but-2-ene).