What chemical test can be used to differentiate between alkenes and alkanes. Describe and explain the results.
Alkenes decolourise bromine water (orange/brown to colourless) while alkanes cause no change in colour. An electrophilic addition reaction occurs with the alkene. In a reaction between ethene and bromine water, Br2 is shown as an electrophile, attracted to electrons due to the formation of a partial positive charge, which attacks the exposed pi bond in ethene. The two electrons in one of the ethene double bonds then goes to the partially positively charged Br atom, causing the carbon atoms to become positively charged. As both Br atoms have two spare electrons, they form bonds with either positively charged carbon atom. Therefore the decolourisation occurs because both bromine atoms have become ionised in the reaction. This addition reaction cannot occur in alkanes because of the lack of carbon - carbon double bonds.
