9.2 Alkenes

Alkenes have one or more double bonds between carbon atoms. They have the general formula $C_nH_{2n}$. When two carbon atoms share two pairs of electrons, they form a double bond between the carbon atoms. How many electrons are left on each carbon atom? Doubly bonded carbon atoms form a single bond with two other atoms.

9.2.1 Preparation of Alkene

1. By the cracking of large alkanes

A large alkane molecule breaks up into smaller hydrocarbons when heated at high temperatures such as 450-750C and high pressure. This process is called thermal cracking. This process produces a mixture of alkanes and alkenes. For example, when the decane is heated at high temperatures and pressure, it breaks into octane and ethene.

Large alkane molecules can also be broken into smaller pieces by catalytic cracking. In catalytic cracking, alkanes are heated at 550C in the presence of a catalyst such as zeolite. Catalytic cracking gives a higher percentage of hydrocarbons containing 5 to 10 carbon atoms.

Cracking gives a mixture of smaller hydrocarbons containing a higher proportion of alkenes. These hydrocarbons are more volatile and possess lower boiling points, making them easier to use as better fuels. Moreover, their combustion can be controlled and they produce large amounts of heat per gram.

For example, the following reaction occurs when alkanes are burned.

2. By Dehydration of Alcohols

Dehydration means loss of water. Alcohols dehydrate when their vapour are passed over heated alumina to give alkenes.

Phosphoric acid $({\mathrm{H}}_3{\mathrm{P}\mathrm{O}}_4)$$(\mathrm{H}_{3}\mathrm{PO}_{4})$ and phosphorus pentaoxide $({\mathrm{P}}_4{\mathrm{O}}_{10})$$(\mathrm{P}_{4}\mathrm{O}_{10})$ can also be used as the catalyst for the dehydration of alcohols.

Concentrated sulphuric acid is also used for dehydration.

Note that in a dehydration reaction the - OH group is removed from one carbon and the H atom from the adjacent carbon atom, resulting in the formation of a double bond between such carbon atoms. Such a reaction is called an elimination reaction.

3. By dehydrohalogenation of halogenoalkanes

Dehydrohalogenation means the loss of hydrogen halide. Halogenoalkanes on heating with alcoholic potassium hydroxide undergo dehydrohalogenation.

Note that removal of H and halogen takes place from two adjacent carbon atoms. Is dehydrohalogenation an elimination reaction?