Edexcel IGCSE Chemistry 复习笔记 4.4.2 Bromine & Alkenes

• Alkenes are a homologous series of hydrocarbon compounds with at least one double bond between two of the carbon atoms on the chain
• The double bond can be written as carbon carbon double bond or as C=C
• The general formula for alkenes is:

C_nH_2n

• Alkenes are generally more desirable than alkanes as they are more reactive due to the presence of the carbon-carbon double bond, so they can take part in reactions in which alkanes cannot, making them more useful than alkanes
• They are used to make polymers and are the starting materials for the production of many other chemicals
• Two useful reactions are the bromination of alkenes and polymerisation

Bromination of Ethene

• Alkenes undergo addition reactions in which atoms of a simple molecule add across the C=C double bond
• The reaction between bromine and ethene is an example of an addition reaction
• The same process works for any halogen and any alkene in which the halogen atoms always add to the carbon atoms across the C=C double bond

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

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

Diagram showing the result of the test using bromine water with alkanes and alkenes

• 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, hence the solution no longer contains free bromine so it loses its colour