Saturated and Unsaturated Hydrocarbons

A saturated hydrocarbon contains only single bonds between its carbon atoms. Each carbon atom is bonded to as many hydrogen atoms as possible, so no more atoms can be added to the molecule. The alkanes are saturated, with the general formula C_nH_2n+2, for example hexane, C₆H₁₄.An unsaturated hydrocarbon contains at least one carbon-carbon multiple bond, usually a C=C double bond. The alkenes are unsaturated. An open-chain alkene with one double bond has the general formula C_nH_2n, for example hex-1-ene, C₆H₁₂.Comparing formulae shows the difference: each C=C double bond means the molecule has two fewer hydrogen atoms than the alkane with the same number of carbon atoms.Worked example: an open-chain hydrocarbon has formula C₅H₁₀. The alkane with five carbons is C₅H₁₂, so C₅H₁₀ has two fewer hydrogen atoms and contains one C=C double bond. It is unsaturated.Unsaturated compounds are more reactive than saturated ones, because atoms can be added across the double bond in addition reactions. Saturated compounds cannot do this; they react mainly by substitution.Test for unsaturation: shake the hydrocarbon with bromine water. An unsaturated compound decolourises the bromine water, which changes from orange to colourless. A saturated compound leaves the bromine water orange.The words are also used for fats and oils: saturated fats have no C=C bonds in their chains, while unsaturated oils contain one or more.