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.
Alkenes are unsaturated since they have a double covalent carbon bond. Alkenes have the general formula CnH2n.
Here is a table of the first Alkenes. Note there is no 'methene' since, in order to have a carbon double bond you need at least 2 carbon atoms. Once you get to butene, there are two locations the double bond could be placed (between the first and second, and third and fourth are considered the same since one could be flipped to make the other). This gives rise to isomerism.
This is the process of breaking down large fractions with high boiling points that are difficult to sell into smaller, more useful molecules.
An example of a cracking reaction:
Hexadecane Petrol + Butene + Ethene
C16H34 C8H18 + C4H8 + 2C2H4
This reactionis done either by passing the vapour over a catalyst at high pressure (catalytic cracking) or by the use of heat alone (thermal cracking) depending on the fraction used.
Cracking produces smaller alkanes, but also alkenes!
|.||Thermal Cracking||Catalytic Cracking|
|How?||Increased vibrations in atoms from heat splits the alkane. The higher the temperature, the further towards the end of the chain it splits at.||Zeolite catalyst has pores in its crystal where the rection takes place. branched alkanes cannot fit into this.|
|Product Uses||Smaller alkanes can be sold easily, and alkenes can be used to make plastics in a further process.||This process produced branched alkanes, which burn more easily and therefore make good fuels.|
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.
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.