Alkenes (Edexcel IGCSE Chemistry)

The Alkenes

Spec point — know that alkenes contain the functional group >C=C< know the general formula for alkenes explain why alkenes are classified as unsaturated hydrocarbons understand how to draw the structural and displayed formulae for alkenes with up to four carbon atoms in the molecule, and name the unbranched-chain isomers knowledge of cis/trans or E/Z notation is not required

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.