By Claire Gillespie, Updated Mar 24, 2022
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An alkene is an unsaturated hydrocarbon containing one or more carbon–carbon double bonds, whereas an alkane is a saturated hydrocarbon with only single bonds. Transforming an alkane into an alkene requires the removal of hydrogen atoms—a process known as dehydrogenation. This endothermic reaction is typically carried out at very high temperatures and is a cornerstone of modern petrochemical production.
Converting an alkane to an alkene involves dehydrogenation, an endothermic reaction that removes hydrogen from the alkane at temperatures above 500 °C.
Alkanes are simple hydrocarbons composed solely of carbon and hydrogen atoms, with all carbon–carbon bonds being single. Their saturated nature renders them relatively inert, reacting mainly with oxygen during combustion to produce water and carbon dioxide. The physical properties of alkanes—such as boiling point and viscosity—increase predictably with chain length, making them valuable as fuels and solvents. Common examples include methane, ethane, propane, butane, and pentane.
Alkenes, in contrast, contain one or more carbon–carbon double bonds, which confer greater reactivity. This unsaturation allows alkenes to serve as key intermediates in the synthesis of aldehydes, alcohols, polymers, aromatics, and more. For instance, reacting an alkene with steam in a catalytic process yields the corresponding alcohol.
Hydrogenation is the reverse of dehydrogenation: an alkene is converted into an alkane by adding hydrogen across the double bond. The reaction typically employs a nickel catalyst and is conducted at around 150 °C (302 °F). This process is widely used to produce saturated fuels and various industrial chemicals.
Dehydrogenation removes hydrogen from alkanes—such as propane or isobutane—to form alkenes like propylene or isobutylene. In the petrochemical industry, this step is essential for producing aromatics, styrene, and other specialty chemicals. The reaction is highly endothermic and requires temperatures exceeding 500 °C (932 °F). Typical dehydrogenation pathways include aromatization of cyclohexene in the presence of hydrogen acceptors (often sulfur or selenium) and the conversion of amines to nitriles using reagents such as iodine pentafluoride.
Beyond industrial chemistry, dehydrogenation principles are applied in food technology, where saturated fats are converted to unsaturated fats during margarine production. The high‑temperature environment facilitates the release of hydrogen gas, driving the equilibrium toward the unsaturated product.
