Alkenes and alkynes are considered unsaturated hydrocarbons because they contain double or triple bonds, respectively, which can undergo addition reactions to add more atoms or groups to the molecule. In contrast, alkanes have only single bonds and are therefore saturated with hydrogen atoms.
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Alkenes and alkynes are considered unsaturated hydrocarbons because they contain carbon-carbon double or triple bonds, respectively. These multiple bonds introduce a degree of unsaturation, as each double bond involves the sharing of four electrons (two sigma bonds and one pi bond), and each triple bond involves the sharing of six electrons (one sigma bond and two pi bonds). The presence of these multiple bonds reduces the number of hydrogen atoms that can be bonded to carbon, making the hydrocarbon “unsaturated” with respect to the maximum number of hydrogen atoms possible in a saturated hydrocarbon.