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Unsaturated hydrocarbons are hydrocarbons that have double or triple covalent bonds between adjacent carbon atoms. Those with at least one carbon-to-carbon double bond are called alkenes and those with at least one carbon-to-carbon triple bond are called alkynes. The position of the double or triple bond is shown by a number written either at the start of the name, or just before the -ene or -yne suffix (e.g. pent-2-ene and 2-butyne). The number represents the position of the first of the two carbons making the bond, in the longest carbon chain. Alkenes and alkynes with more than one double or triple bond respectively are named with a prefix preceding the -ene or -yne (e.g. 2,4-pentadiene).
Unsaturated hydrocarbons with both double and triple bonds have the suffix -enyne and are handled in a similar manner.
The physical properties of unsaturated hydrocarbons are very similar to those of the corresponding saturated compounds. They are slightly soluble in water.
Except for aromatic compounds, unsaturated hydrocarbons are highly reactive and undergo addition reactions to their multiple bonds. Typical reagents added are hydrogen halides, water, sulfuric acid, elemental halogens and alcohols.
To test whether the hydrocarbon is unsaturated one would typically use the bromine test, which involves the addition of bromine water to the hydrocarbon in question; if the bromine water is decolourised by the hydrocarbon it can then be concluded that the hydrocarbon is unsaturated.