Alkenes are hydrocarbons containing at least one carbon carbon double bond, with acyclic monoalkenes commonly represented by the general molecular formula CnH2n. They are classified as unsaturated hydrocarbons because a carbon carbon double bond contains a pi bond in addition to a sigma bond and therefore reduces the number of hydrogen atoms that can be attached to the carbon framework compared with the corresponding alkane.
Ethene is the simplest alkene and consists of two carbon atoms connected by a double bond, while propene, butenes and larger structures demonstrate increasing possibilities for substitution, branching and stereochemistry. The double bonded carbon atoms are approximately trigonal planar because each uses three orbital directions to form sigma bonds while retaining an orbital for pi bonding. The pi bond restricts rotation around the double bond, which has important consequences for molecular geometry and stereoisomerism.
Alkenes are central to organic chemistry because their electron rich pi bonds undergo numerous reactions, particularly addition reactions. They are also major industrial feedstocks, especially ethene and propene, which are used to manufacture polyethylene, polypropylene and many other chemical products. The term alkene identifies a structural class rather than a single substance, and individual alkenes can differ greatly in physical properties, reactivity, toxicity and industrial application.
The carbon carbon double bond gives alkenes characteristic electronic and geometric properties. It consists of one sigma bond formed by direct orbital overlap and one pi bond formed by lateral overlap of appropriate orbitals. The pi component is more exposed and generally more reactive than the sigma framework, which explains why many alkene reactions involve addition across the double bond.
The trigonal planar arrangement around the double bonded carbon atoms means that substituents occupy approximately 120 degree bond angles in an idealised local geometry. Rotation around the double bond is restricted because twisting disrupts effective pi orbital overlap. This restriction allows certain alkenes to exist as geometric stereoisomers. When appropriate substituents are attached to each double bonded carbon, compounds can exhibit cis and trans relationships or, more generally, E and Z configurations determined by priority rules.
Alkene nomenclature requires identification of the longest chain containing the double bond and assignment of the lowest possible locant to the multiple bond under applicable systematic rules. The suffix ene identifies an alkene, while multiple double bonds are represented through terms such as diene and triene. Structural isomerism can arise through different positions of the double bond, different carbon skeletons and different substituent arrangements. Conjugated, isolated and cumulated double bonds have different electronic structures and reactivity because the relative arrangement of pi systems changes electron delocalisation and orbital interactions.
Addition reactions are the defining chemical behaviour of many alkenes. In hydrogenation, hydrogen adds across the carbon carbon double bond to form an alkane, usually with a suitable catalyst. Halogenation can add bromine or chlorine across the double bond, and the reaction is commonly used as a qualitative test for unsaturation in appropriate laboratory contexts. Addition of hydrogen halides produces haloalkanes, with regiochemical outcomes influenced by substrate structure and reaction conditions.
Hydration adds the elements of water across the double bond to produce an alcohol, and industrial or laboratory methods can use different catalysts and mechanisms. Alkene oxidation can produce epoxides, diols, carbonyl compounds or cleavage products depending on the oxidising system and reaction conditions. Polymerisation is especially important because many alkenes undergo addition polymerisation in which the double bonds are converted into carbon carbon single bonds while long chains form.
Ethene produces polyethylene, propene produces polypropylene and numerous substituted alkenes produce specialised polymers. The precise reaction pathway depends on catalysts, substituents, solvent, temperature, pressure and other factors. Alkene chemistry therefore connects fundamental organic reaction mechanisms with large scale industrial manufacture of plastics, intermediates, coatings and other materials.
Industrial alkene production is closely linked to petroleum refining and petrochemical processing. Ethene is produced on a large scale by processes such as steam cracking of hydrocarbon feedstocks, while propene can be obtained through several refinery and petrochemical routes. Steam cracking involves high temperature thermal decomposition of suitable hydrocarbon feedstocks followed by rapid cooling and extensive separation. The resulting olefin rich streams contain multiple compounds that must be separated and purified for downstream chemical processes.
Ethene is used to manufacture polyethylene, ethylene oxide, ethanol through particular industrial routes, ethylene dichloride and numerous other intermediates. Propene is a major feedstock for polypropylene, propylene oxide, acrylonitrile and other products. Butenes have applications in fuels, alkylation processes, polymer production and chemical synthesis. The historical term olefin is often used as a synonym or near synonym for alkene, particularly in industrial contexts, although exact terminology can vary. Alkene production and consumption form a major part of the global chemical industry because carbon carbon double bonds provide reactive handles from which many different functionalised compounds can be manufactured.
The physical properties of alkenes depend on molecular size, branching, double bond position and stereochemistry. Small alkenes are generally gases at ordinary temperatures, while larger members become liquids and eventually higher molecular mass materials. Their boiling points generally increase with molecular size, although structural effects modify simple trends. Alkenes are usually poorly soluble in water because their hydrocarbon frameworks are predominantly nonpolar.
The presence of a pi bond can slightly alter polarizability and intermolecular interactions relative to corresponding alkanes, but it does not make ordinary alkenes strongly water soluble. The geometry of the double bond can influence melting point and packing efficiency, particularly when comparing stereoisomers. Cis and trans or E and Z isomers can therefore exhibit different physical properties even though they have identical molecular formulae and connectivity.
Alkene vapours can be flammable, and appropriate precautions are necessary when handling volatile members. Some industrial alkene monomers require controlled storage because polymerisation can be highly exothermic under unsuitable conditions. The physical properties of alkenes are therefore important not only for theoretical classification but also for distillation, storage, transport, polymerisation and process safety.
Alkenes occur naturally as well as being produced industrially. Some plants, microorganisms and animals produce volatile unsaturated hydrocarbons as signalling compounds, pheromone components or metabolic products. Natural products can contain isolated, conjugated or otherwise substituted carbon carbon double bonds that strongly influence biological activity and physical properties. Unsaturated hydrocarbon structures are also found within larger biological molecules, although many such substances contain additional functional groups and therefore are not classified as simple alkenes.
In environmental systems, volatile alkenes can enter the atmosphere through industrial emissions, fuel related sources and biological processes. Their atmospheric chemistry can involve reactions with hydroxyl radicals, ozone and other reactive species, producing oxidised compounds and contributing to secondary atmospheric pollution under suitable conditions. Alkene degradation and atmospheric lifetime depend strongly on molecular structure and environmental conditions. In synthetic chemistry, the double bond is frequently used as a site for functionalisation, allowing chemists to convert relatively simple hydrocarbon structures into more complex molecules. This makes alkenes important intermediates in pharmaceuticals, agrochemicals, polymers, solvents, surfactants and fine chemicals.
Several distinctions are important when classifying alkenes. A molecule containing a double bond is not automatically a simple alkene if the double bond occurs within a different functional system or if additional structural features require more specific classification. Alkynes contain triple bonds and therefore have different geometries and reaction patterns. Aromatic compounds can contain multiple carbon carbon bonds but are not simply treated as ordinary alkenes because aromatic delocalisation produces distinctive stability and reactivity.
Alkenes can be monosubstituted, disubstituted, trisubstituted or tetrasubstituted according to the number of carbon substituents attached to the double bonded carbon atoms, and substitution can affect stability. More highly substituted alkenes are often thermodynamically more stable than less substituted isomers because of electronic and hyperconjugative effects, although individual systems can behave differently. Conjugation can further stabilise certain alkenes through electron delocalisation.
A common misconception is that every alkene necessarily reacts rapidly with every reagent; actual reaction rates depend on mechanism, substitution, steric effects, electronic effects, catalysts and conditions. Another misconception is that all alkenes have identical formulae, whereas only particular subclasses such as acyclic monoalkenes share the simple CnH2n relationship.
Alkenes are extensively studied using spectroscopy and instrumental analysis. Infrared spectroscopy can provide evidence for carbon carbon double bonds through characteristic absorptions, although interpretation must consider substitution and other functional groups. Nuclear magnetic resonance spectroscopy is particularly informative because alkene hydrogen and carbon signals occur in characteristic regions and coupling patterns can provide information about substitution and stereochemistry. Mass spectrometry can determine molecular mass and produce fragmentation patterns useful for identification.
Gas chromatography can separate volatile alkenes, while high performance liquid chromatography and other techniques can be used for less volatile derivatives. Mechanistically, alkene reactions provide important examples of electrophilic addition, radical addition, catalytic surface reactions and concerted processes. The stability of carbocation intermediates, radicals, transition states and organometallic intermediates can influence regioselectivity and stereoselectivity. Alkene chemistry is therefore a central part of understanding reaction mechanisms, stereochemistry, polymer chemistry and industrial synthesis. The combination of structural diversity, accessible reactivity and industrial importance makes alkenes one of the most significant hydrocarbon classes in both theoretical and applied chemistry.
