Organic Chemistry : Alkene

Alkene is similar to alkane. The only one difference is one of the bond between carbons are double, not single. This makes the number of Hydrogen atom twice of number of carbons. (CnH2n). Below is the alkene, with 6 carbon, called Hexene.

http://www.green-planet-solar-energy.com/alkene.html

However, as an example shown above, the position of double bond can be at many possible places. This gives the isomerism of alkene depending on double bond, which is important when we are learning how to call its name.

IUPAC Name
The difference of alkene and alkane is
- when we are finding the longest carbon chain, the chain we choose MUST contain the double bond.
- when we are numbering in order the carbon in the chain, the number we give to carbons with double bond must be as low as possible, regardless to any other brunches.
- The last word "alkene" such as "Hexene" must also tell where the double bond is. E.g. Hex-3-ene tells that the double bond is at third carbon. (see picture above.)
- When we look at the double bond, two carbons here are connected with other alkyls or hydrogens. Some maybe big or some maybe small. If the bigger ones are at the same side, use prefix "cis". If they are opposite, use "tran".

http://www.elmhurst.edu/~chm/vchembook/209cistrans.html

 Reference : BCC M.5 Chemistry Book.