Naming alkenes
Alkenes are hydrocarbons containing one or more double bonds.
The name of an alkene contains three parts: prefix(es) stem ending
The name of an alkene contains three parts: prefix(es) stem ending
meth - 1 carbon
eth - 2 carbons
prop - 3 carbons
but - 4 carbons
pent - 5 carbons
hex - 6 carbons
eth - 2 carbons
prop - 3 carbons
but - 4 carbons
pent - 5 carbons
hex - 6 carbons
The ending is -ene (if no other functional groups are present)
The stem depends on the number of carbons in the longest chain containing the double bond.
In the alkene below, the longest chain containing the double bond has five carbon atoms. If you have trouble seeing this move your mouse over the image.
3-methylpent-2-ene
Prefixes are used to indicate the type and position of any substituents on the longest chain.
Any atom or group of atoms replacing H at a carbon is called a substituent.
The carbon of the double bond closest to the end of the chain is given the lowest number. This determines the number associated with any other substituents on the chain.
The stem depends on the number of carbons in the longest chain containing the double bond.
In the alkene below, the longest chain containing the double bond has five carbon atoms. If you have trouble seeing this move your mouse over the image.
3-methylpent-2-ene
Prefixes are used to indicate the type and position of any substituents on the longest chain.
Any atom or group of atoms replacing H at a carbon is called a substituent.
Common substituents are shown below.
Note that saturated hydrocarbon substituents have the same name as the parent alkaneexcept with ane substituted by -yl
| methyl | CH3– | bromo | Br– | |
| ethyl | CH3CH2– | chloro | Cl– |
Note that saturated hydrocarbon substituents have the same name as the parent alkaneexcept with ane substituted by -yl
The carbon of the double bond closest to the end of the chain is given the lowest number. This determines the number associated with any other substituents on the chain.
Count from the end (right OR left) of the chain closest to the doubly-bonded carbon.
In the example, the right-hand end of the chain is closest to the doubly-bonded carbon, and hence this is a pent-2-ene.
the number of the methyl group is 3 because it is the third carbon from the end of the chain determined by the position of the double bond.
In the example, the right-hand end of the chain is closest to the doubly-bonded carbon, and hence this is a pent-2-ene.
the number of the methyl group is 3 because it is the third carbon from the end of the chain determined by the position of the double bond.