What are Alkenes? (Edexcel IGCSE Chemistry (Modular))

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Alkenes

What are alkenes?

  • Alkenes are unsaturated hydrocarbons

  • The general formula of an alkene is CnH2n

  • All alkenes contain a double carbon bond, which is shown as two lines between two of the carbon atoms i.e. C=C

  • All alkenes contain a double carbon bond, which is the functional group and is what allows alkenes to react in ways that alkanes cannot

  • The names and structure of the first four alkenes are shown below:

Table of alkenes

Displayed formula

Name

Molecular formula

ethene

ethene

C2H4 

propene

propene

C3H6 

but-1-ene

but-1-ene

C4H8 

pent-1-ene

pent-1-ene

C5H10 

The first four members of the alkene homologous series

  • Compounds that have a C=C double bond are also called unsaturated compounds

  • That means they can make more bonds with other atoms by opening up the C=C bond and allowing incoming atoms to form another single bond with each carbon atom of the functional group

  • Each of these carbon atoms now forms 4 single bonds instead of 1 double and 2 single bonds

  • This makes them much more reactive than alkanes

Double Bond Opening, IGCSE & GCSE Chemistry revision notes

A carbon-carbon double can break and form a single bond, allowing more atoms to attach to the carbon atoms

Examiner Tip

The numbers in butene, pentene and hexene refer to the carbon atom in which the C=C begins, counting from the left. E.g. pent-2-ene, C5H10 has the C=C between the 2nd and 3rd carbon atoms. In pent-3-ene the C=C bond is between the 3rd and 4th carbon atoms from the left.

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