Structure and Bonding
Alkenes are part of an homologous series. They have the funtional group of a C=C (double bond between two carbons). and the general formula of CnH
There are two types of bond in alkenes because of the different electron sub-shells. There is a σ bond (sigma) which is when two p suborbitals overlap, and the π bond (pi) is above and below the atoms, it tends to be weaker because of this and it also has no rotation which is why cis/trans isomerism exists.
Because of this bonding, the double bond is a centre of high electron density, meaning it is a great place for electrophilles to attack.
Hydrogenation
This means adding hydrogen, and if you do this to alkenes then they will turn into alkanes. This is the process that is done to produce margarine. The conditions are: 150°C and a nickel catalyst. Here is the reaction.
Electrophillic Addition
Because of this double bond, HBr for example can attack the alkene and the mechanism for this is outlined below.
Not only HBr, but also other molecules will do this exact same process. For this reason, the two parts of the molecules have been colour coded and to get the mechanism for H2SO4 and Br2 you merely substitute it.
The test for alkenes is the bromine water test, the mechanism for this is the one with Br-Br. A solution of Br will be coloured orange, but when this reaction happens with an alkene it will turn colourless because it becomes a haloalkane.
In the middle of this reaction, a carbocation (that is a cation of carbon) is formed; this is a C+. The position of this is important, depending on whether this is primary, secondary, or tertiary (see this page), determines the likeliehood of different product being formed. A teritiary carbocation is the most stable, and therefore will form the major product, secondary and primary carbocations will make the minor product.
Polymerisation
Polymerisation if the process of making polymers which are long chains of molecules. Alkenes can be made into polymers and these are commonly known as plastics in the following process:
For more about polymers see this page.