Addition Reactions of Alkenes (AQA GCSE Chemistry)

Hydrogenation

Addition Reactions

• The chemistry of the alkenes is determined by the C=C functional group

• Since all members of the alkene homologous series contain the same functional group then they all react similarly

• Alkenes mainly undergo addition reactions in which atoms of a simple molecule add across the C=C double bond

• The carbon-carbon double bond opens up, forming a single bond between the carbons allowing for two more atoms to bond, one on each carbon

• A saturated compound is formed when alkenes undergo addition reactions

Addition of a general chemical to an alkene

The chemical adds across the C=C bond

Hydrogenation of alkenes

• Alkenes undergo addition reactions with hydrogen in which an alkane is formed

• These are hydrogenation reactions and occur at 150 ºC using a nickel catalyst

• Hydrogenation reactions are used to manufacture margarine from vegetable oils

  • Vegetable oils are polyunsaturated molecules which are partially hydrogenated to increase the Mr and turn the oils into solid fats

Hydrogenation of ethene

Hydrogen atoms add across the C=C in the hydrogenation of ethene to produce ethane

Hydration

Hydration of alkenes

• Alkenes also undergo addition reactions with steam in which an alcohol is formed. Since water is being added to the molecule it is also called a hydration reaction

• The reaction is very important industrially for the production of alcohols and it occurs using the following conditions:

  • Temperature of around 330ºC

  • Pressure of 60 – 70 atm

  • Concentrated phosphoric acid catalyst

• When the reaction is complete, the reaction chamber holds unreacted ethene, ethanol and water

• The contents are transferred to a condenser where ethene is separated easily as it has a much lower boiling point than ethanol and water:

  • Ethanol: 78 ^oC

  • Ethene: -103 ^oC

  • Water: 100 ^oC

• The ethanol and water are separated afterwards by fractional distillation

Hydration of ethene

A water molecule adds across the C=C in the hydration of ethene to produce ethanol

Halogenation

Halogenation of alkenes

• The halogens also participate in addition reactions with alkenes

• The same process works for any halogen and any alkene in which the halogen atoms always add to the carbon atoms involved in the C=C double bond

• The reaction occurs readily at room temperature

Halogenation of ethene with chlorine

Chlorine atoms add across the C=C in the addition reaction of ethene and chlorine