In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond.^[1] The double bond may be internal or in the terminal position. Terminal alkenes are also known as α-olefins.

The International Union of Pure and Applied Chemistry (IUPAC) recommends using the name "alkene" only for acyclic hydrocarbons with just one double bond; alkadiene, alkatriene, etc., or polyene for acyclic hydrocarbons with two or more double bonds; cycloalkene, cycloalkadiene, etc. for cyclic ones; and "olefin" for the general class – cyclic or acyclic, with one or more double bonds.^[2][3][4]

Acyclic alkenes, with only one double bond and no other functional groups (also known as mono-enes) form a homologous series of hydrocarbons with the general formula C_nH_2n with n being a >1 natural number (which is two hydrogens less than the corresponding alkane). When n is four or more, isomers are possible, distinguished by the position and conformation of the double bond.

Alkenes are generally colorless non-polar compounds, somewhat similar to alkanes but more reactive. The first few members of the series are gases or liquids at room temperature. The simplest alkene, ethylene (C₂H₄) (or "ethene" in the IUPAC nomenclature) is the organic compound produced on the largest scale industrially.^[5]

Aromatic compounds are often drawn as cyclic alkenes, however their structure and properties are sufficiently distinct that they are not classified as alkenes or olefins.^[3] Hydrocarbons with two overlapping double bonds (C=C=C) are called allenes—the simplest such compound is itself called allene—and those with three or more overlapping bonds (C=C=C=C, C=C=C=C=C, etc.) are called cumulenes.

Structural isomerism

Alkenes having four or more carbon atoms can form diverse structural isomers. Most alkenes are also isomers of cycloalkanes. Acyclic alkene structural isomers with only one double bond follow:^[6]

• C₂H₄: ethylene only
• C₃H₆: propylene only
• C₄H₈: 3 isomers: 1-butene, 2-butene, and isobutylene
• C₅H₁₀: 5 isomers: 1-pentene, 2-pentene, 2-methyl-1-butene, 3-methyl-1-butene, 2-methyl-2-butene
• C₆H₁₂: 13 isomers: 1-hexene, 2-hexene, 3-hexene, 2-methyl-1-pentene, 3-methyl-1-pentene, 4-methyl-1-pentene, 2-methyl-2-pentene, 3-methyl-2-pentene, 4-methyl-2-pentene, 2,3-dimethyl-1-butene, 3,3-dimethyl-1-butene, 2,3-dimethyl-2-butene, 2-ethyl-1-butene

Many of these molecules exhibit cis–trans isomerism. There may also be chiral carbon atoms particularly within the larger molecules (from C₅). The number of potential isomers increases rapidly with additional carbon atoms.

Structure and bonding

Bonding

A carbon–carbon double bond consists of a sigma bond and a pi bond. This double bond is stronger than a single covalent bond (611 kJ/mol for C=C vs. 347 kJ/mol for C–C),^[1] but not twice as strong. Double bonds are shorter than single bonds with an average bond length of 1.33 Å (133 pm) vs 1.53 Å for a typical C-C single bond.^[7]

Each carbon atom of the double bond uses its three sp² hybrid orbitals to form sigma bonds to three atoms (the other carbon atom and two hydrogen atoms). The unhybridized 2p atomic orbitals, which lie perpendicular to the plane created by the axes of the three sp² hybrid orbitals, combine to form the pi bond. This bond lies outside the main C–C axis, with half of the bond on one side of the molecule and a half on the other. With a strength of 65 kcal/mol, the pi bond is significantly weaker than the sigma bond.

Rotation about the carbon–carbon double bond is restricted because it incurs an energetic cost to break the alignment of the p orbitals on the two carbon atoms. Consequently cis or trans isomers interconvert so slowly that they can be freely handled at ambient conditions without isomerization. More complex alkenes may be named with the E–Z notation for molecules with three or four different substituents (side groups). For example, of the isomers of butene, the two methyl groups of (Z)-but-2-ene (a.k.a. cis-2-butene) appear on the same side of the double bond, and in (E)-but-2-ene (a.k.a. trans-2-butene) the methyl groups appear on opposite sides. These two isomers of butene have distinct properties.

Shape

As predicted by the VSEPR model of electron pair repulsion, the molecular geometry of alkenes includes bond angles about each carbon atom in a double bond of about 120°. The angle may vary because of steric strain introduced by nonbonded interactions between functional groups attached to the carbon atoms of the double bond. For example, the C–C–C bond angle in propylene is 123.9°.

For bridged alkenes, Bredt's rule states that a double bond cannot occur at the bridgehead of a bridged ring system unless the rings are large enough.^[8] Following Fawcett and defining S as the total number of non-bridgehead atoms in the rings,^[9] bicyclic systems require S ≥ 7 for stability^[8] and tricyclic systems require S ≥ 11.^[10]

Isomerism

In organic chemistry,the prefixes cis- and trans- are used to describe the positions of functional groups attached to carbon atoms joined by a double bond. In Latin, cis and trans mean "on this side of" and "on the other side of" respectively. Therefore, if the functional groups are both on the same side of the carbon chain, the bond is said to have cis- configuration, otherwise (i.e. the functional groups are on the opposite side of the carbon chain), the bond is said to have trans- configuration.

• 
  structure of cis-2-butene
• 
  structure of trans-2-butene
• 
  (E)-But-2-ene
• 
  (Z)-But-2-ene

For there to be cis- and trans- configurations, there must be a carbon chain, or at least one functional group attached to each carbon is the same for both. E- and Z- configuration can be used instead in a more general case where all four functional groups attached to carbon atoms in a double bond are different. E- and Z- are abbreviations of German words zusammen (together) and entgegen (opposite). In E- and Z-isomerism, each functional group is assigned a priority based on the Cahn–Ingold–Prelog priority rules. If the two groups with higher priority are on the same side of the double bond, the bond is assigned Z- configuration, otherwise (i.e. the two groups with higher priority are on the opposite side of the double bond), the bond is assigned E- configuration. Cis- and trans- configurations do not have a fixed relationship with E- and Z-configurations.

Physical properties

Many of the physical properties of alkenes and alkanes are similar: they are colorless, nonpolar, and combustible. The physical state depends on molecular mass: like the corresponding saturated hydrocarbons, the simplest alkenes (ethylene, propylene, and butene) are gases at room temperature. Linear alkenes of approximately five to sixteen carbon atoms are liquids, and higher alkenes are waxy solids. The melting point of the solids also increases with increase in molecular mass.

Alkenes generally have stronger smells than their corresponding alkanes. Ethylene has a sweet and musty odor. Strained alkenes, in particular, like norbornene and trans-cyclooctene are known to have strong, unpleasant odors, a fact consistent with the stronger π complexes they form with metal ions including copper.^[11]

Boiling and melting points

Below is a list of the boiling and melting points of various alkenes with the corresponding alkane and alkyne analogues.^[12][13]

Infrared spectroscopy

The stretching of C=C bond will give an IR absorption peak at 1670–1600 cm⁻¹, while the bending of C=C bond absorbs between 1000 and 650 cm⁻¹ wavelength.

NMR spectroscopy

In ¹H NMR spectroscopy, the hydrogen bonded to the carbon adjacent to double bonds will give a δ_H of 4.5–6.5 ppm. The double bond will also deshield the hydrogen attached to the carbons adjacent to sp² carbons, and this generates δ_H=1.6–2. ppm peaks.^[14] Cis/trans isomers are distinguishable due to different J-coupling effect. Cis vicinal hydrogens will have coupling constants in the range of 6–14 Hz, whereas the trans will have coupling constants of 11–18 Hz.^[15]

In their ¹³C NMR spectra of alkenes, double bonds also deshield the carbons, making them have low field shift. C=C double bonds usually have chemical shift of about 100–170 ppm.^[15]

Combustion

Like most other hydrocarbons, alkenes combust to give carbon dioxide and water.

The combustion of alkenes release less energy than burning same molarity of saturated ones with same number of carbons. This trend can be clearly seen in the list of standard enthalpy of combustion of hydrocarbons.^[16]

Reactions

Alkenes are relatively stable compounds, but are more reactive than alkanes. Most reactions of alkenes involve additions to this pi bond, forming new single bonds. Alkenes serve as a feedstock for the petrochemical industry because they can participate in a wide variety of reactions, prominently polymerization and alkylation. Except for ethylene, alkenes have two sites of reactivity: the carbon–carbon pi-bond and the presence of allylic CH centers. The former dominates but the allylic sites are important too.

Addition to the unsaturated bonds

Hydrogenation involves the addition of H₂ resulting in an alkane. The equation of hydrogenation of ethylene to form ethane is:

H₂C=CH₂ + H₂→H₃C−CH₃

Hydrogenation reactions usually require catalysts to increase their reaction rate. The total number of hydrogens that can be added to an unsaturated hydrocarbon depends on its degree of unsaturation.

Similar to hydrogen, halogens added to double bonds.

H₂C=CH₂ + Br₂→H₂CBr−CH₂Br

Halonium ions are intermediates. These reactions do not require catalysts.

Bromine test is used to test the saturation of hydrocarbons.^[17] The bromine test can also be used as an indication of the degree of unsaturation for unsaturated hydrocarbons. Bromine number is defined as gram of bromine able to react with 100g of product.^[18] Similar as hydrogenation, the halogenation of bromine is also depend on the number of π bond. A higher bromine number indicates higher degree of unsaturation.

The π bonds of alkenes hydrocarbons are also susceptible to hydration. The reaction usually involves strong acid as catalyst.^[19] The first step in hydration often involves formation of a carbocation. The net result of the reaction will be an alcohol. The reaction equation for hydration of ethylene is:

H₂C=CH₂ + H₂O→H₃C-CH₂OH

Hydrohalogenation involves addition of H−X to unsaturated hydrocarbons. This reaction results in new C−H and C−X σ bonds. The formation of the intermediate carbocation is selective and follows Markovnikov's rule. The hydrohalogenation of alkene will result in haloalkane. The reaction equation of HBr addition to ethylene is:

H₂C=CH₂ + HBr → H₃C−CH₂Br

Cycloaddition

Alkenes add to dienes to give cyclohexenes. This conversion is an example of a Diels-Alder reaction. Such reaction proceed with retention of stereochemistry. The rates are sensitive to electron-withdrawing or electron-donating substituents. When irradiated by UV-light, alkenes dimerize to give cyclobutanes.^[20] Another example is the Schenck ene reaction, in which singlet oxygen reacts with an allylic structure to give a transposed allyl peroxide:

Oxidation

Alkenes react with percarboxylic acids and even hydrogen peroxide to yield epoxides:

RCH=CH₂ + RCO₃H → RCHOCH₂ + RCO₂H

For ethylene, the epoxidation is conducted on a very large scale industrially using oxygen in the presence of silver-based catalysts:

C₂H₄ + 1/₂ O₂ → C₂H₄O

Alkenes react with ozone, leading to the scission of the double bond. The process is called ozonolysis. Often the reaction procedure includes a mild reductant, such as dimethylsulfide (SMe₂):

RCH=CHR' + O₃ + SMe₂ → RCHO + R'CHO + O=SMe₂

R₂C=CHR' + O₃ → R₂CHO + R'CHO + O=SMe₂

When treated with a hot concentrated, acidified solution of KMnO₄, alkenes are cleaved to form ketones and/or carboxylic acids. The stoichiometry of the reaction is sensitive to conditions. This reaction and the ozonolysis can be used to determine the position of a double bond in an unknown alkene.

The oxidation can be stopped at the vicinal diol rather than full cleavage of the alkene by using osmium tetroxide or other oxidants:

${\displaystyle {\ce {R'CH=CR2 + 1/2 O2 + H2O -> R'CH(OH)-C(OH)R2}}}$

This reaction is called dihydroxylation.

In the presence of an appropriate photosensitiser, such as methylene blue and light, alkenes can undergo reaction with reactive oxygen species generated by the photosensitiser, such as hydroxyl radicals, singlet oxygen or superoxide ion. Reactions of the excited sensitizer can involve electron or hydrogen transfer, usually with a reducing substrate (Type I reaction) or interaction with oxygen (Type II reaction).^[21] These various alternative processes and reactions can be controlled by choice of specific reaction conditions, leading to a wide range of products. A common example is the -cycloaddition of singlet oxygen with a diene such as cyclopentadiene to yield an endoperoxide:

Polymerization

Terminal alkenes are precursors to polymers via processes termed polymerization. Some polymerizations are of great economic significance, as they generate the plastics polyethylene and polypropylene. Polymers from alkene are usually referred to as polyolefins although they contain no olefins. Polymerization can proceed via diverse mechanisms. Conjugated dienes such as buta-1,3-diene and isoprene (2-methylbuta-1,3-diene) also produce polymers, one example being natural rubber.

Allylic substitution

The presence of a C=C π bond in unsaturated hydrocarbons weakens the dissociation energy of the allylic C−H bonds. Thus, these groupings are susceptible to free radical substitution at these C-H sites as well as addition reactions at the C=C site. In the presence of radical initiators, allylic C-H bonds can be halogenated.^[22] The presence of two C=C bonds flanking one methylene, i.e., doubly allylic, results in particularly weak HC-H bonds. The high reactivity of these situations is the basis for certain free radical reactions, manifested in the chemistry of drying oils.

Metathesis

Alkenes undergo olefin metathesis, which cleaves and interchanges the substituents of the alkene. A related reaction is ethenolysis:^[23]

${\displaystyle \stackrel{\text{diisobutene}}{{({\text{CH}}_3^{})}_3^{}\text{C}-<!--\; -\; -->\text{CH}=\text{C}{({\text{CH}}_3^{})}_2^{}}+{\text{CH}}_2^{}={\text{CH}}_2^{}⟶<!--\; ⟶\; -->\stackrel{}{{({\text{CH}}_3^{}}_{}^{}}}$Zdroj:https://en.wikipedia.org?pojem=Alkene
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