Alkenes

Alkenes are a homologous series of hydrocarbons containing a carbon–carbon double bond (C=C), which is their functional group.

Alkenes are unsaturated hydrocarbons the double bond means the molecule does not contain the maximum number of hydrogen atoms, and more atoms can be added across the C=C.

. Examples: ethene C₂H₄, propene C₃H₆, butene C₄H₈. Names end in -ene with a number showing where the double bond starts, e.g. but-1-ene.

Isomers are possible from butene but-1-ene and but-2-ene differ in the position of the C=C, and 2-methylpropene is a branched isomer.

Bromine and alkenes bromine adds across the double bond in an addition reaction at room temperature, e.g. CH₂=CH₂ + Br₂ → CH₂BrCH₂Br. The orange bromine water becomes colourless.

This is the standard test for unsaturation alkenes decolourise bromine water rapidly, whereas alkanes do not because they only react with bromine by substitution in UV light.

Alkenes are much more reactive than alkanes and are valuable starting materials: they are made by cracking and used to make polymers and ethanol.

The fourth alkene listed in the series is pent-1-ene, C₅H₁₀, with the C=C between carbons 1 and 2. Check it fits C_nH_2n: n = 5 gives 10 hydrogen atoms.

Any halogen adds across the C=C in the same way as bromine. For example, ethene and chlorine form 1,2-dichloroethane: C₂H₄ + Cl₂ → C₂H₄Cl₂, with one chlorine atom on each carbon.

Bromine water is used for the test instead of pure bromine because it is safer and easier to handle. The orange colour comes from dissolved bromine; it disappears when the bromine is used up by addition.

The double bond is a region of high electron density, which attracts molecules such as bromine. When it opens, each carbon forms a new single bond, so alkenes react far more readily than alkanes.