Reactions of Alkenes

A mixture of hydrogen gas and a gaseous alkene is passed over a catalyst of nickel at a temperature of 150c. The hydrogen adds across the double bond and an alkane is formed. This reaction is called hydrogenation and is an example of reduction.

Alkenes react rapidly with halogens such as chlorine, bromine and iodine at room temperature. The halogen adds across the double bond to give a di-substituted halogenoalkane. This is known as halogenation.

When bromine is added to a sample containing an alkene, the colour changes from orange to colourless. The colour change indicates that the bromine has reacted with the double bond. This reaction is used as a test for unsaturation and shows the presence of a double carbon bond.

A hydrogen halide adds across the double bond of an alkene to form a halogenoalkane. Hydrogen halides include HCl, HBr and HI. Hydrogen halides are gases at room temperature and are bubbled into liquid alkanes.

The addition of steam to alkanes is one method of preparing alcohols. This reaction is used widely in industry. Steam and the gaseous alkene are heated to a high temperature and pressure in the presence of a phosphoric acid catalyst. This process is known as the hydration of an alkene.

Unsymmetrical alkenes can also take part in addition reactions, forming a mixture of organic products. Propene reacts this way with hydrogen bromide. Note that more of one organic product is formed than the other.

Hydrogen Bromide is a polar molecule. Bromine is more electronegative than hydrogen, which causes a dipole. The electron pair in the pi bond is attracted to the slightly positive hydrogen atom, causing the double bond to break. A new bond forms between one of the carbon atoms and the hydrogen atom. The H-Br bond breaks by heterolytic fission, with the electron pair going to the bromine. A bromide ion and a carbocation are formed. The positively charged carbocation is unstable and quickly reacts with the bromide ion. The product is a halogenoalkane. In this reaction, the hydrogen halide is the electrophile.

Halogens are non-polar molecules. When a halogen approaches an alkene, the electrons in the pi bond repel the electrons in the halogen bond. This induces a dipole in the halogen molecule. The halogen is now slightly polar. The electron pair in the pi bond is attracted slightly to the slightly positive halogen atom, causing the double bond to break. A new bond forms between one of the carbon atoms and a halogen atom. The halogen bond breaks by heterolytic fission, with the electron pair going to the slightly negative atom. A halide and a carbocation are formed. Thee carbocation is unstable and quickly reacts with the halide.