Alkenes (WJEC GCSE Chemistry: Combined Science)

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

• Alkenes undergo addition reactions with hydrogen in which an alkane is formed
  • These are called hydrogenation reactions 
• 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

The addition of hydrogen to an alkene

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

Bromine Water Test

• Alkanes and alkenes have different molecular structures
• All alkanes are saturated and alkenes are unsaturated
• The presence of the C=C double bond allows alkenes to react in ways that alkanes cannot
• This allows us to tell alkenes apart from alkanes using a simple chemical test called the bromine water test
  • Bromine water is used in the test for alkenes as it is safer and easier to handle than bromine

• Bromine water is an orange coloured solution
• When bromine water is added to an alkane, it will remain as an orange solution as alkanes do not have double carbon bonds (C=C) so the bromine remains in solution
• But when bromine water is added to an alkene:
  • The bromine atoms add across the C=C bond
  • The solution no longer contains free bromine so it loses its colour / decolourises

The bromine water test

Bromine water decolourises in the presence of an alkene

• The alkene reacts with bromine to form a colourless 'bromoalkane'

The addition of bromine to an alkene

Bromine atoms add across the C=C bond of an alkene to produce a bromoalkane