Addition Reactions (CIE IGCSE Chemistry: Co-ordinated Sciences (Double Award))

Extended tier only

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

Alkenes and bromine

• The reaction between bromine and ethene is an example of an addition reaction
• The two bromine atoms add across the carbon-carbon double bond to form a saturated compound
• Bromine water is orange but would decolourise when added to an alkene 

Bromine atoms add across the C=C in the addition reaction of ethene and bromine

 

Alkenes and hydrogen

• When alkenes undergo addition reactions with hydrogen, an alkane is formed
• This reaction requires a nickel catalyst
• The reaction between ethene and hydrogen would produce ethane, propene and hydrogen would form propane and so on 

The hydrogen adds across the carbon carbon-double bond

 Alkenes and steam

• When alkenes undergo addition reactions with steam, an alcohol is formed.
• Since water is being added to the molecule it is also called a hydration reaction 
• This reaction requires an acid catalyst
• Ethene would react with steam to produce ethanol, propene will react with steam to produce propanol and so on

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