Alkenes
The alkenes are aliphatic hydrocarbons in which one of the carbon to carbon bonds is a double bond (double covalent bond).
The alkenes are considered to be unsaturated hydrocarbons because the double bond could be broken allowing for the addition of more hydrogen.
The prefix “ALK-“ once again refers to the aliphatic hydrocarbon but the suffix “-ENE” refers to the presence of at least one double bond between carbons.
The characteristic type of reaction for the alkenes is by addition since it would be possible to add across the double bond when combining with another substance.
There are several other names for this series:
Olefin series.
Ethene series.
The ethene series gets its name from the very first member of its class, ethane.
The general molecular formula of this class is CnH2n.
As with the alkanes, you simply insert the number of carbons for the integer ”n” to produce the molecular formula.
Naming the Alkenes
As with the alkanes, there are rules for naming the alkenes:
First, find the longest continuous chain of carbons containing the double bond.
Give the address of the double bond by using smallest numbers. You can omit this step if the address cannot be confused.
Number the carbons in the longest chain so that the carbons on either side of the double bond have the smallest addresses.
The address of the carbon next to the bond which has the smallest numbered address is the address of the double bond.
If the double bond is between carbons three and four then the address would be three.
Name the carbon sidechain(s) from largest to smallest. Give addresses for all side chains using the addresses for the carbons as determined by the address of the double bond.
The name is always written from right to left but is read from left to right.
Carbon chains are always identified by the appropriate prefix for the number of carbons in the chain. The longest chain would have a number preceding the prefix identifying the address of the double bond and would have the suffix “-ENE” because of the double bond.
Carbon sidechains are also called radicals and they are identified by the appropriate prefix followed by the suffix “-YL” just like in the alkanes.
I will not cover naming two or three carbon alkanes as I feel these are fairly self explanatory.
Naming the Alkenes
With the four carbon alkene, we see there is the potential that the double bond could be located between carbons one and two or carbons two and three.
Don’t over think this: always use the lowest number. If it is between one and two then it is 1-Butene and obviously if it is between two and three it is 2-Butene.
Now let’s try naming the big alkene:
Follow the rules, first count the total number of carbons in the longest chain. The chain has six carbons and the prefix for six is hex. We know that we are dealing with hexene.
Next we determine the double bond, and it is either between carbons four and five or two and three. We always use the lowest numbers so it would be between two and three. Using the smallest number we get the number two. At this point we know we have 2-hexene.
Now we have to determine the side chain. This is the same as the last example so we know that we are dealing with one carbon which gives us methyl.
Naming the Alkenes
Now comes the fun part, we have to count from the same side where we have the double bond. In this case it is on the fifth carbon from the right. This gives us the number 5.
Do not fall into the trap of starting from the closest end. Always start from whichever end the covalent bond starts.
The formal name is 5-methyl 2-hexene.