AlkeneEdit
Alkenes are hydrocarbons defined by the presence of at least one carbon–carbon double bond, a feature that sets them apart from the saturated alkanes. The simplest acyclic alkenes follow the general formula CnH2n, with ethene ethene serving as the foundational member. The C=C unit brings unsaturation into the molecule, making alkenes more reactive than alkanes and giving rise to a broad set of addition reactions that transform the carbon skeleton in predictable ways. In addition to straight‑chain alkenes, cyclic alkenes and substituted variants expand the chemistry even further, with cycloalkenes having the formula CnH2n−2 due to the ring and the double bond. Natural occurrence, industrial production, and a wide range of transformations keep alkenes central to modern chemistry and materials science.
Alkenes exhibit distinctive structural features rooted in the C=C bond. The carbons of the double bond are sp2 hybridized, leading to a planar arrangement around the double bond and a characteristic bond length and strength that influence reactivity. The pi bond, formed by sideways overlap of p orbitals, lies above and below the plane of the sigma framework, contributing to the bond’s reactivity while restricting rotation around the C=C axis. These geometric features enable geometric isomerism, described as E/Z isomerism, in many alkenes when substituents on each carbon differ. The combination of unsaturation, planarity, and substituent effects makes alkenes versatile building blocks in organic synthesis and polymer chemistry double bond sp2 hybridization pi bond geometric isomerism.
Structure and properties
The core feature of an alkene is the carbon–carbon double bond, which consists of a sigma component and a pi component. The sigma bond provides the primary connectivities, while the pi bond confers part of the bonding energy and enables reactions that add across the double bond. The pi component also makes the double bond more electron-rich than a single bond, attracting electrophiles in a predictable fashion. Substitution patterns around the double bond, as well as conjugation with adjacent π systems, influence both stability and reactivity. For example, conjugated alkenes stabilize reactive intermediates and alter the outcome of many addition reactions double bond conjugation.
Geometric isomerism is a defining property of many alkenes. When both carbons of the C=C bond carry two different substituents, the relative positions of these groups give rise to cis/trans isomers in a straightforward, now widely used naming convention. The modern E/Z terminology describes the higher-priority substituents’ positions and provides a more general framework for descriptions across all substrates with opposed or same-side relationships. The arrangement around the double bond can dramatically affect physical properties such as boiling point and reactivity in chemical transformations. For a discussion of rules governing these relationships, see E/Z isomerism.
Reactivity of alkenes is dominated by the electrophilic addition paradigm: the nucleophilic π bond readily attacks electrophiles, enabling a range of additions that saturate the molecule. Common additions include hydrogenation, halogenation, hydrohalogenation, and hydration, each giving distinct products and selectivities. The choice of reagents and conditions often hinges on practical considerations such as regioselectivity and stereoselectivity, with Markovnikov and anti‑Markovnikov outcomes guiding many synthetic routes. See electrophilic addition and Markovnikov's rule for foundational descriptions of these patterns electrophilic addition Markovnikov's rule.
Nomenclature and structure‑property relationships play a central role in how chemists communicate about alkenes. The presence of the C=C bond allows for straightforward naming by chain length, substitution pattern, and, when relevant, geometric isomerism. For more on naming conventions and how they reflect molecular structure, see alkene naming and geometric isomerism.
Reactions and transformations
Alkenes are among the most reactive hydrocarbon classes because of the discharge of pi electron density into reactions with various reagents. Electrophilic additions across the C=C bond are the broadest category of transformations and include several well‑established sequential steps:
Hydrogenation: addition of hydrogen across the double bond to furnish alkanes, typically catalyzed by metals such as palladium, platinum, or nickel. This reaction is widely used to saturate unsaturated feedstocks in both laboratory and industrial settings hydrogenation.
Halogenation and hydrohalogenation: addition of halogens (X2) or hydrogen halides (HX) across the double bond, often with regioselectivity governed by the stability of intermediates and by substituent effects. Markovnikov’s rule—where the hydrogen adds to the carbon with more hydrogens and the halogen adds to the more substituted carbon—hints at predictable product outcomes in many cases. See hydrohalogenation and Markovnikov's rule.
Hydration and anti‑Markovnikov additions: hydration (water addition) converts alkenes into alcohols under acid catalysis, typically yielding Markovnikov products. In anti‑Markovnikov hydration, accomplished via hydroboration–oxidation, the alcohol ends up on the less substituted carbon. These routes are central to synthesizing alcohols with precise carbon skeletons hydration hydroboration-oxidation.
Ozonolysis and related oxidative cleavages: oxidative cleavage of alkenes breaks the C=C bond to yield carbonyl compounds, often providing a route to aldehydes and ketones that can feed into further transformations ozonolysis.
Polymerization: most industrially important reactions of alkenes involve polymerization, where many alkene molecules couple in chain growth to form polymers like polyethylene and polypropylene. This process underlies the production of plastics and materials with tailored properties, often mediated by specialized catalysts such as Ziegler–Natta systems polymerization polyethylene.
The reactivity profile of alkenes is heavily exploited in synthesis, materials science, and energy‑related chemistry. Conjugation with adjacent double bonds or aromatic systems can alter both rate and selectivity, enabling sophisticated sequences in complex molecule construction. The versatility of addition and oxidation processes makes alkenes a central theme in industrial chemistry and academic research alike conjugation.
Occurrence, production, and applications
Alkenes occur both in nature and as intermediates in industry. Ethene ethene is the simplest and most important representative, serving as a major feedstock for the production of polymers such as polyethylene and various vinyl‑based materials. In industry, alkenes are commonly generated by cracking and reforming processes that break larger hydrocarbons into smaller, more reactive fragments. Catalytic cracking and steam cracking are standard routes used to produce light alkenes from crude oil fractions, while selective methods can generate specific isomers for downstream chemistry. The economic significance of alkenes rests on their role as monomers and as reactive intermediates in a wide array of products, including fuels, solvents, and specialty chemicals ethylene polyethylene propylene.
Beyond polymers, alkenes participate in the synthesis of agrochemicals, natural products, and fine chemicals, where controlled addition or oxidation steps allow precise construction of carbon skeletons. Their study intersects with catalysis, materials science, and sustainability concerns, as researchers seek more efficient, selective, and environmentally friendly routes to value‑added products. For example, the use of alternative catalysts and greener oxidation strategies aims to reduce energy input and byproducts in large‑scale processes, while preserving the versatility that alkenes provide in synthetic planning catalysis polymerization.