Chemical Reactions of Alkenes

Combustion reaction- Similar to combustion reaction of alkanes.

Alkenes burn completely in excess oxygen to produce carbon dioxide and water.

(Refer to alkanes' combustion for a clearer picture of how combustion works)

However, alkenes burn with sootier flames as compared to alkanes. This is because alkenes have more % of carbon in their molecules than alkanes. Hence, alkanes are cleaner than alkenes.

Addition Reaction- Is a reaction in which atoms or groups of atoms are added to each carbon atom of a carbon-carbon multiple bond to form a single product. The reason why alkenes are able to do this is due to their reactivity due to their double bond.

Addition of hydrogen- Ethene reacts with hydrogen at 180 degrees in the presence of nickel or platinum catalyst to produce ethane. This process is called catalytic hydrogenation.

CH2=CH2 + H2 -> CH3-CH3

This hydrogenation reaction is used to convert an unsaturated compound to a saturated compound. It is used in the industry to change vegetables oils which are unsaturated into saturated fats for making margarine.

Addition of water(Hydration)- Alkenes do not react with cold water under normal conditions. When a mixture of an alkene and steam is passed over a catalyst, a molecule of water is added to the carbon-carbon double bond to produce an alcohol. For example, ethane reacts with steam at 300 degrees and with phosphoric acid as a catalyst to form ethanol.

Addition of halogens- Addition of halogens to alkenes. Chlorine, bromine, iodine react readily with alkenes. No catalyst or uv light is needed. This is different from alkanes.

Testing for unsaturation- Alkenes react with aqueous bromine (bromine water) turning it from red to colourless. A colourless liquid, 1,2-dibromoethane is formed. Hence, bromine water is a reagent used as a test for the presence of a carbon-carbon double bond in organic molecules.

Addition of hydrogen halide- Hydrogen halides react readily with alkenes. No catalyst is required. The hydrogen halides that react with alkenes include hydrogen chlorides (HCI), hydrogen bromide (HBr) and hydrogen iodide. (HI)