What are Alkenes? (Edexcel IGCSE Chemistry)

The Alkenes

Spec point — 4.24

Alkenes

What are alkenes?

• Alkenes are unsaturated hydrocarbons
• The general formula of an alkene is C_nH_2n
• 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

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

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

Exam Hint 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 2^nd and 3^rd carbon atoms. In pent-3-ene the C=C bond is between the 3^rd and 4^th carbon atoms from the left.