Alkenes are hydrocarbons containing at least one carbon carbon double bond. They are classified as unsaturated hydrocarbons because the presence of a double bond means that the molecule contains fewer hydrogen atoms than the corresponding saturated alkane with the same number of carbon atoms. For a simple acyclic alkene containing one double bond and no rings, the general molecular formula is CnH2n. The simplest alkene is ethene, C2H4, followed by propene, butenes and larger members.
The carbon carbon double bond consists of one sigma bond and one pi bond. The sigma bond results from direct orbital overlap along the line between the carbon nuclei, while the pi bond forms through sideways overlap of p orbitals above and below the molecular plane. The pi bond is more readily disrupted during many chemical reactions than a carbon carbon sigma bond, which explains much of the characteristic reactivity of alkenes. Alkenes are therefore fundamental intermediates and feedstocks in organic chemistry and the petrochemical industry.
The geometry of an alkene differs substantially from that of an alkane because the atoms around the double bond are approximately trigonal planar. The two carbon atoms forming a typical carbon carbon double bond are generally sp2 hybridised, with three sigma bonding directions arranged approximately 120 degrees apart. The unhybridised p orbitals overlap sideways to form the pi bond. Rotation around a carbon carbon double bond is strongly restricted because rotation would disrupt the pi overlap.
This restricted rotation allows alkenes to exhibit geometric stereoisomerism when each carbon of the double bond has two different substituents. Cis and trans terminology is sometimes used for simple cases, while the E and Z system provides a more general stereochemical description based on priority rules. Different alkene stereoisomers can have different physical properties and sometimes different biological or chemical behaviour. The inability of double bonds to rotate freely is therefore not merely a structural detail but an important source of molecular diversity and stereochemical complexity.
Alkenes are considerably more reactive than alkanes in many common chemical reactions because of the electron rich pi bond. One of their most characteristic reactions is electrophilic addition. An electrophile can interact with the pi electron density, producing an intermediate or transition state that ultimately forms new sigma bonds. Hydrogenation adds hydrogen across the double bond and converts an alkene into an alkane, usually with the assistance of a metal catalyst such as nickel, palladium or platinum.
Halogenation can add chlorine or bromine across the double bond, while hydration can add water to produce an alcohol under suitable catalytic conditions. Hydrohalogenation adds hydrogen halides such as hydrogen chloride or hydrogen bromide. The exact product depends on molecular structure, reaction mechanism and conditions. Oxidation can also transform alkenes into alcohols, carbonyl compounds, epoxides, diols or cleavage products depending on the oxidising system. These reactions make alkenes particularly valuable starting materials for organic synthesis because the double bond provides a convenient site for introducing new functional groups.
Polymerisation is one of the most important industrial uses of alkenes. In addition polymerisation, alkene molecules containing carbon carbon double bonds join together to form long chains while converting the double bonds into carbon carbon single bonds within the polymer backbone. Ethene polymerisation produces polyethylene, one of the most widely manufactured plastics. Propene polymerisation produces polypropylene, which is used in packaging, fibres, automotive components and numerous other products. Other substituted alkenes can produce polymers with specialised properties.
Polymer properties depend on molecular structure, chain length, branching, crystallinity, stereochemistry and processing conditions. Catalysts can control polymer architecture and stereochemistry, with systems such as Ziegler Natta catalysts and metallocene catalysts enabling precise control over polymerisation. The ability to transform small gaseous or liquid alkene feedstocks into solid materials illustrates the industrial importance of alkene chemistry. Polymerisation also demonstrates how the same carbon carbon double bond that makes an alkene reactive can be transformed into a stable structural unit within a large macromolecule.
Alkenes are produced on a large scale through petroleum refining and petrochemical processing. Thermal and catalytic cracking of larger hydrocarbons produces smaller alkenes such as ethene and propene. Steam cracking is a major industrial process in which hydrocarbon feedstocks are heated to very high temperatures for a short time, causing carbon carbon bonds to break and generating mixtures rich in light olefins. The products are rapidly cooled and separated by distillation and other processes.
Ethene and propene are among the most important petrochemical building blocks because they can be converted into polymers, alcohols, aldehydes, acids and numerous other intermediates. The term olefin is commonly used as another name for an alkene, particularly in industrial chemistry. Natural biological processes also produce alkenes and related unsaturated compounds, although industrial quantities are dominated by petrochemical pathways. Renewable chemical technologies increasingly investigate routes for producing olefins from biomass, alcohols or other nonfossil feedstocks.
Alkenes have important roles in biological chemistry. Many natural lipids contain carbon carbon double bonds in fatty acid chains, and the presence and geometry of these double bonds influence membrane properties. Naturally occurring unsaturated fatty acids often contain cis double bonds, which introduce bends into hydrocarbon chains and affect how efficiently the chains pack together. This influences membrane fluidity and the physical properties of fats and oils.
Plants also produce volatile alkene derivatives and related compounds involved in signalling and defence. Ethene itself acts as a plant hormone, influencing processes such as fruit ripening, leaf abscission and responses to environmental stress. The biological importance of alkene groups illustrates how a relatively small structural feature can produce major changes in molecular shape and reactivity. In medicinal and natural product chemistry, alkene geometry can affect molecular recognition and biological activity. Consequently, chemists must often control not only whether a double bond is present but also its stereochemical configuration.
Alkenes can also undergo reactions that reveal their structural features analytically. Bromine and related reagents have historically been used as qualitative tests for unsaturation because alkenes can rapidly react with bromine under suitable conditions, causing the characteristic bromine colour to disappear. Modern analytical chemistry uses more sophisticated techniques such as infrared spectroscopy, nuclear magnetic resonance spectroscopy, mass spectrometry and chromatography to identify and characterise alkenes.
Infrared spectroscopy can reveal characteristic carbon carbon double bond and carbon hydrogen stretching features, while nuclear magnetic resonance spectroscopy provides detailed information about alkene proton and carbon environments. The chemical shift and coupling patterns can help establish substitution and stereochemistry. Gas chromatography is particularly useful for volatile alkenes and can separate mixtures containing multiple hydrocarbons. These methods demonstrate that the structural feature defining an alkene can be identified through several independent physical and chemical measurements rather than relying solely on a simple reaction test.
Alkenes are important in environmental and atmospheric chemistry as well as industrial chemistry. Volatile alkenes released into the atmosphere can undergo reactions with hydroxyl radicals, ozone and other oxidising species, producing secondary compounds that contribute to atmospheric aerosol formation and photochemical pollution. Biogenic alkenes such as isoprene are emitted in very large quantities by vegetation and participate in atmospheric chemistry. Isoprene reacts with oxidants and can influence the formation of secondary organic aerosol and ozone under appropriate conditions.
Industrial emissions of volatile hydrocarbons are therefore regulated partly because their atmospheric reactions can affect air quality. In aquatic and soil environments, alkene containing hydrocarbons can undergo biodegradation and chemical transformation, with rates depending on molecular structure and environmental conditions. Their environmental behaviour differs from that of saturated hydrocarbons because the double bond introduces additional chemical reactivity.
