Isomerism in alkenes

· It is possible for alkenes with more than 3 carbon atoms to possess isomers

· C₄H₈ has 3 isomers from same homologous series

i) The carbon chain can be branched, as in alkanes

ii) The double bond can be at different positions along the carbon chain.

Manufacture of alkenes

1) Industrial method- by the cracking process

Cracking is the process of breaking down long-chain hydrocarbon molecules into small molecules

- Conditions of high temperature

- Pressure

- Catalyst

Importance of cracking process

i. To provide more of the smaller alkane molecules which are in higher demand, like those molecules found in petrol.

ii. As a source of alkenes and hydrogen

- E.g. Cracking of long-chain alkane molecules found in the naptha fraction can produce short-chain alkenes such as ethane and propene.

- Chemicals that are obtained from petrol are known as petrochemicals.

2) Laboratory preparation of ethane – involving dehydration of alcohols

a) Use concentrated sulfuric acid

b) Using a catalyst like aluminium oxide or unglazed porcelain

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