In organic chemistry, alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond. The simplest alkene is ethylene (C2H4), which is named by the International Union of Pure and Applied Chemistry (IUPAC), is the organic compounds produced on the largest scale industrially. The physical properties of alkenes depend on the molecular mass. The simple alkenes such as ethylene, propylene and butene are gases at room temperature, linear alkenes containing five to sixteen carbon atoms are liquid, and higher alkenes are usually waxy solids.
Alkenes are non-polar compounds, in which the only intermolecular force is the dispersion force, so the alkenes are not soluble in water but soluble in carbon tetrachloride and other organic solvents. The chemical property of alkenes is relatively stable, but more active than alkanes. Considering that the carbon-carbon double bonds in alkenes are stronger than the carbon-carbon single bonds in alkanes, most of the alkenes react by breaking the double bonds and forming two new single bonds.
Alkenes are known as the raw materials in petrochemical industry because they can be applied in a wide range of reactions including addition reaction, polymerization reaction and metal complexation reaction.
Addition reaction: Hydrogenation of alkenes produces the corresponding alkanes. The reaction is carried out at a temperature of 200 °C in the presence of a metal catalyst. Common industrial catalysts are platinum, nickel or palladium catalysts. In the presence of a suitable photosensitizer, the alkene can react with reactive oxygen species such as hydroxyl radicals, singlet oxygen or superoxide ion produced by the photosensitizer. A common example is the [4+2]-cyclicaddition of singlet oxygen to a diene to yield an endoperoxide.
Polymerization reaction: Polymerization of alkenes produces high-value polymers such as plastic polyethylene and polypropylene. Polymers derived from alkene monomers are generally referred to as polyolefins. A polymer from alpha-olefins is called a polyalphaolefin (PAO). Polymerization can proceed via free radical or ionic mechanisms, converting double bonds into single bonds and forming single bonds to link other monomers.
Metal complexation: Alkenes are important ligands in transition metal alkene complexes. Two carbon centers bonded to the metal are using C-C pi- and pi*-orbitals. Mono- and diolefins are often used as ligands in stable complexes. Cyclooctadiene and norbornadiene are popular chelating agents, and even ethylene is sometimes used as a ligand, for example in Zeise's salt. In addition, metal-alkene complexes are intermediates in many metal-catalyzed reactions, including hydrogenation, hydroformylation and polymerization.