TheInternational Union of Pure and Applied Chemistry (IUPAC)recommends using the name "alkene" only foracyclic hydrocarbons with just one double bond;alkadiene,alkatriene, etc., orpolyene for acyclic hydrocarbons with two or more double bonds;cycloalkene,cycloalkadiene, etc. forcyclic 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 otherfunctional groups (also known asmono-enes) form ahomologous series ofhydrocarbons with the general formulaCnH2n withn being a >1 natural number (which is twohydrogens less than the correspondingalkane). Whenn is four or more,isomers are possible, distinguished by the position andconformation of the double bond.
Alkenes are generally colorlessnon-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 theIUPAC nomenclature) is theorganic 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 calledallenes—the simplest such compound is itself calledallene—and those with three or more overlapping bonds (C=C=C=C,C=C=C=C=C, etc.) are calledcumulenes.
Alkenes having four or morecarbon atoms can form diversestructural isomers. Most alkenes are also isomers ofcycloalkanes. Acyclic alkene structural isomers with only one double bond follow:[6]
Many of these molecules exhibitcis–trans isomerism. There may also bechiral carbon atoms particularly within the larger molecules (fromC5). The number of potential isomers increases rapidly with additional carbon atoms.
A carbon–carbon double bond consists of asigma bond and api bond. This double bond is stronger than a singlecovalent 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 averagebond length of 1.33Å (133pm) vs 1.53 Å for a typical C-C single bond.[7]
Each carbon atom of the double bond uses its three sp2hybrid 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 thep orbitals on the two carbon atoms. Consequentlycis ortrans isomers interconvert so slowly that they can be freely handled at ambient conditions without isomerization. More complex alkenes may be named with theE–Z notation for molecules with three or four differentsubstituents (side groups). For example, of theisomers 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.
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 definingS as the total number of non-bridgehead atoms in the rings,[9] bicyclic systems requireS ≥ 7 for stability[8] and tricyclic systems requireS ≥ 11.[10]
Inorganic chemistry, theprefixescis- and trans- are used to describe the positions of functional groups attached tocarbon atoms joined by a double bond. In Latin,cis andtrans 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 havecis- configuration, otherwise (i.e. the functional groups are on the opposite side of the carbon chain), the bond is said to havetrans- 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 onefunctional 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 wordszusammen (together) andentgegen (opposite). In E- and Z-isomerism, each functional group is assigned a priority based on theCahn–Ingold–Prelog priority rules. If the two groups with higher priority are on the same side of the double bond, the bond is assignedZ- configuration, otherwise (i.e. the two groups with higher priority are on the opposite side of the double bond), the bond is assignedE- configuration. Cis- and trans- configurations do not have a fixed relationship betweenE- andZ-configurations.
Many of the physical properties of alkenes andalkanes are similar: they are colorless, nonpolar, and combustible. Thephysical state depends onmolecular mass: like the corresponding saturated hydrocarbons, the simplest alkenes (ethylene,propylene, andbutene) 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 andtrans-cyclooctene are known to have strong, unpleasant odors, a fact consistent with the stronger π complexes they form with metal ions including copper.[11]
In theIR spectrum, the stretching/compression of C=C bond gives a peak at 1670–1600 cm−1. The band is weak in symmetrical alkenes. The bending of C=C bond absorbs between 1000 and 650 cm−1 wavelength
In1HNMR spectroscopy, thehydrogen bonded to the carbon adjacent to double bonds will give aδH of 4.5–6.5 ppm. The double bond will alsodeshield the hydrogen attached to the carbons adjacent to sp2 carbons, and this generates δH=1.6–2. ppm peaks.[14] Cis/trans isomers are distinguishable due to differentJ-coupling effect. Cisvicinal 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 their13C 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]
Like most otherhydrocarbons, alkenescombust to give carbon dioxide and water.
The combustion of alkenes release less energy than burning samemolarity of saturated ones with same number of carbons. This trend can be clearly seen in the list ofstandard enthalpy of combustion of hydrocarbons.[16]
Alkenes are relatively stable compounds, but are more reactive thanalkanes. Most reactions of alkenes involve additions to this pi bond, forming newsingle bonds. Alkenes serve as a feedstock for thepetrochemical 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 ofallylic CH centers. The former dominates but the allylic sites are important too.
Hydrogenation involves the addition ofH2, resulting in an alkane. The equation of hydrogenation ofethylene to formethane is:
H2C=CH2 + H2→H3C−CH3
Hydrogenation reactions usually requirecatalysts to increase theirreaction rate. The total number of hydrogens that can be added to an unsaturated hydrocarbon depends on itsdegree of unsaturation.
Similarly,halogenation involves the addition of a halogen molecule, such asBr2, resulting in a dihaloalkane. The equation of bromination of ethylene to form ethane is:
H2C=CH2 + Br2→H2CBr−CH2Br
Unlike hydrogenation, these halogenation reactions do not require catalysts. The reaction occurs in two steps, with ahalonium ion as an intermediate.
Bromine test is used to test the saturation of hydrocarbons.[17] The bromine test can also be used as an indication of thedegree 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 tohydration. The reaction usually involvesstrong acid ascatalyst.[19] The first step in hydration often involves formation of acarbocation. The net result of the reaction will be analcohol. The reaction equation for hydration of ethylene is:
Example of hydrohalogenation: addition ofHBr to an alkene
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 followsMarkovnikov's rule. The hydrohalogenation of alkene will result inhaloalkane. The reaction equation of HBr addition to ethylene is:
Alkenes add todienes to givecyclohexenes. This conversion is an example of aDiels-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 givecyclobutanes.[20] Another example is theSchenck ene reaction, in which singlet oxygen reacts with anallylic structure to give a transposed allylperoxide:
For ethylene, theepoxidation 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 calledozonolysis. 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 ofKMnO4, alkenes are cleaved to formketones and/orcarboxylic 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 thevicinaldiol rather than full cleavage of the alkene by usingosmium tetroxide or other oxidants:
In the presence of an appropriatephotosensitiser, such asmethylene blue and light, alkenes can undergo reaction with reactive oxygen species generated by the photosensitiser, such ashydroxyl radicals,singlet oxygen orsuperoxide 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 [4+2]-cycloaddition of singlet oxygen with adiene such ascyclopentadiene to yield anendoperoxide:
Terminal alkenes are precursors topolymers via processes termedpolymerization. Some polymerizations are of great economic significance, as they generate the plasticspolyethylene andpolypropylene. Polymers from alkene are usually referred to aspolyolefins although they contain no olefins. Polymerization can proceed via diverse mechanisms.Conjugateddienes such asbuta-1,3-diene andisoprene (2-methylbuta-1,3-diene) also produce polymers, one example being natural rubber.
The presence of a C=C π bond in unsaturated hydrocarbons weakens the dissociation energy of theallylic C−H bonds. Thus, these groupings are susceptible tofree radical substitution at these C-H sites as well as addition reactions at the C=C site. In the presence ofradical 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 ofdrying oils.
Intransition metal alkene complexes, alkenes serve as ligands for metals.[24] In this case, the π electron density is donated[clarification needed] to the metal d orbitals. The stronger the donation is, the stronger theback bonding from the metal d orbital to π* anti-bonding orbital of the alkene. This effect lowers the bond order of the alkene and increases the C-Cbond length. One example is the complexPtCl3(C2H4)]−. These complexes are related to the mechanisms of metal-catalyzed reactions of unsaturated hydrocarbons.[23]
Alkenes are produced by hydrocarboncracking. Raw materials are mostlynatural-gas condensate components (principally ethane and propane) in the US and Mideast andnaphtha in Europe and Asia. Alkanes are broken apart at high temperatures, often in the presence of azeolite catalyst, to produce a mixture of primarily aliphatic alkenes and lower molecular weight alkanes. The mixture is feedstock and temperature dependent, and separated by fractional distillation. This is mainly used for the manufacture of small alkenes (up to six carbons).[25]
Cracking ofn-octane to give pentane and propene
Related to this is catalyticdehydrogenation, where an alkane loses hydrogen at high temperatures to produce a corresponding alkene.[1] This is the reverse of thecatalytic hydrogenation of alkenes.
Dehydrogenation of butane to give butadiene and isomers of butene
This process is also known asreforming. Both processes are endothermic and are driven towards the alkene at high temperatures byentropy.
One of the principal methods for alkene synthesis in the laboratory is theelimination reaction of alkyl halides, alcohols, and similar compounds. Most common is the β-elimination via the E2 or E1 mechanism.[26] A commercially significant example is the production ofvinyl chloride.
The E2 mechanism provides a more reliable β-elimination method than E1 for most alkene syntheses. Most E2 eliminations start with an alkyl halide or alkyl sulfonate ester (such as atosylate ortriflate). When an alkyl halide is used, the reaction is called adehydrohalogenation. For unsymmetrical products, the more substituted alkenes (those with fewer hydrogens attached to the C=C) tend to predominate (seeZaitsev's rule). Two common methods of elimination reactions are dehydrohalogenation of alkyl halides and dehydration of alcohols. A typical example is shown below; note that if possible, the H isanti to the leaving group, even though this leads to the less stableZ-isomer.[27]
An example of an E2 Elimination
Alkenes can be synthesized from alcohols viadehydration, in which case water is lost via the E1 mechanism. For example, the dehydration ofethanol produces ethylene:
Alkenes can be prepared indirectly from alkylamines. The amine or ammonia is not a suitable leaving group, so the amine is first eitheralkylated (as in theHofmann elimination) or oxidized to anamine oxide (theCope reaction) to render a smooth elimination possible. The Cope reaction is asyn-elimination that occurs at or below 150 °C, for example:[28]
Synthesis of cyclooctene via Cope elimination
The Hofmann elimination is unusual in that theless substituted (non-Zaitsev) alkene is usually the major product.
Another important class of methods for alkene synthesis involves construction of a new carbon–carbon double bond by coupling or condensation of a carbonyl compound (such as analdehyde orketone) to acarbanion or its equivalent. Pre-eminent is thealdol condensation. Knoevenagel condensations are a related class of reactions that convert carbonyls into alkenes.Well-known methods are calledolefinations. TheWittig reaction is illustrative, but other related methods are known, including theHorner–Wadsworth–Emmons reaction.
The Wittig reaction involves reaction of an aldehyde or ketone with aWittig reagent (or phosphorane) of the type Ph3P=CHR to produce an alkene andPh3P=O. The Wittig reagent is itself prepared easily fromtriphenylphosphine and an alkyl halide.[29]
A typical example of the Wittig reaction
Related to the Wittig reaction is thePeterson olefination, which uses silicon-based reagents in place of the phosphorane. This reaction allows for the selection ofE- orZ-products. If anE-product is desired, another alternative is theJulia olefination, which uses the carbanion generated from aphenylsulfone. TheTakai olefination based on an organochromium intermediate also delivers E-products. A titanium compound,Tebbe's reagent, is useful for the synthesis of methylene compounds; in this case, even esters and amides react.
A pair of ketones or aldehydes can bedeoxygenated to generate an alkene. Symmetrical alkenes can be prepared from a single aldehyde or ketone coupling with itself, usingtitanium metal reduction (theMcMurry reaction). If different ketones are to be coupled, a more complicated method is required, such as theBarton–Kellogg reaction.
The formation of longer alkenes via the step-wise polymerisation of smaller ones is appealing, asethylene (the smallest alkene) is both inexpensive and readily available, with hundreds of millions of tonnes produced annually. TheZiegler–Natta process allows for the formation of very long chains, for instance those used forpolyethylene. Where shorter chains are wanted, as they for the production ofsurfactants, then processes incorporating aolefin metathesis step, such as theShell higher olefin process are important.
Olefin metathesis is also used commercially for the interconversion of ethylene and 2-butene to propylene. Rhenium- and molybdenum-containingheterogeneous catalysis are used in this process:[30]
CH2=CH2 + CH3CH=CHCH3 → 2 CH2=CHCH3
Transition metal catalyzedhydrovinylation is another important alkene synthesis process starting from alkene itself.[31] It involves the addition of a hydrogen and a vinyl group (or an alkenyl group) across a double bond.
Reduction ofalkynes is a useful method for thestereoselective synthesis of disubstituted alkenes. If thecis-alkene is desired,hydrogenation in the presence ofLindlar's catalyst (a heterogeneous catalyst that consists of palladium deposited on calcium carbonate and treated with various forms of lead) is commonly used, though hydroboration followed by hydrolysis provides an alternative approach. Reduction of the alkyne bysodium metal in liquidammonia gives thetrans-alkene.[32]
Synthesis ofcis- andtrans-alkenes from alkynes
For the preparation multisubstituted alkenes,carbometalation of alkynes can give rise to a large variety of alkene derivatives.
Alkenes are prevalent in nature.Plants are the main natural source of alkenes in the form ofterpenes.[33] Many of the most vivid natural pigments are terpenes; e.g.lycopene (red in tomatoes),carotene (orange in carrots), andxanthophylls (yellow in egg yolk). The simplest of all alkenes,ethylene is asignaling molecule that influences the ripening of plants.
TheCuriosity rover discovered on Mars long chain alkanes with up to 12 consecutive carbon atoms. They could be derived from either abiotic or biological sources.[34]
Although the nomenclature is not followed widely, according to IUPAC, an alkene is an acyclic hydrocarbon with just one double bond between carbon atoms.[2] Olefins comprise a larger collection of cyclic and acyclic alkenes as well as dienes and polyenes.[3]
To form the root of theIUPAC names for straight-chain alkenes, change the-an- infix of the parent to-en-. For example,CH3-CH3 is thealkaneethANe. The name ofCH2=CH2 is thereforeethENe.
For straight-chain alkenes with 4 or more carbon atoms, that name does not completely identify the compound. For those cases, and for branched acyclic alkenes, the following rules apply:
Find the longest carbon chain in the molecule. If that chain does not contain the double bond, name the compound according to the alkane naming rules. Otherwise:
Number the carbons in that chain starting from the end that is closest to the double bond.
Define the locationk of the double bond as being the number of its first carbon.
Name the side groups (other than hydrogen) according to the appropriate rules.
Define the position of each side group as the number of the chain carbon it is attached to.
Write the position and name of each side group.
Write the names of the alkane with the same chain, replacing the "-ane" suffix by "k-ene".
The position of the double bond is often inserted before the name of the chain (e.g. "2-pentene"), rather than before the suffix ("pent-2-ene").
The positions need not be indicated if they are unique. Note that the double bond may imply a different chain numbering than that used for the corresponding alkane:(H 3C) 3C–CH 2–CH 3 is "2,2-dimethyl pentane", whereas(H 3C) 3C–CH=CH 2 is "3,3-dimethyl 1-pentene".
If the double bond of an acyclic mono-ene is not the first bond of the chain, the name as constructed above still does not completely identify the compound, because ofcis–trans isomerism. Then one must specify whether the two single C–C bonds adjacent to the double bond are on the same side of its plane, or on opposite sides. For monoalkenes, the configuration is often indicated by the prefixescis- (fromLatin "on this side of") ortrans- ("across", "on the other side of") before the name, respectively; as incis-2-pentene ortrans-2-butene.
The difference betweencis- andtrans- isomers
More generally,cis–trans isomerism will exist if each of the two carbons of in the double bond has two different atoms or groups attached to it. Accounting for these cases, the IUPAC recommends the more generalE–Z notation, instead of thecis andtrans prefixes. This notation considers the group with highestCIP priority in each of the two carbons. If these two groups are on opposite sides of the double bond's plane, the configuration is labeledE (from theGermanentgegen meaning "opposite"); if they are on the same side, it is labeledZ (from Germanzusammen, "together"). This labeling may be taught with mnemonic "Z means 'on ze zame zide'".[35]
^abHartwig, John (2010).Organotransition Metal Chemistry: From Bonding to Catalysis. New York: University Science Books. p. 1160.ISBN978-1-938787-15-7.
^Toreki, Rob (31 March 2015)."Alkene Complexes".Organometallic HyperTextbook. Retrieved29 May 2019.
^Saunders, W. H. (1964). "Elimination Reactions in Solution". In Patai, Saul (ed.).The Chemistry of Alkenes. PATAI'S Chemistry of Functional Groups. Wiley Interscience. pp. 149–201.doi:10.1002/9780470771044.ISBN978-0-470-77104-4.{{cite book}}:ISBN / Date incompatibility (help)
^Cram, D.J.; Greene, Frederick D.; Depuy, C. H. (1956). "Studies in Stereochemistry. XXV. Eclipsing Effects in the E2 Reaction1".Journal of the American Chemical Society.78 (4):790–6.Bibcode:1956JAChS..78..790C.doi:10.1021/ja01585a024.
^Bach, R.D.; Andrzejewski, Denis; Dusold, Laurence R. (1973). "Mechanism of the Cope elimination".J. Org. Chem.38 (9):1742–3.doi:10.1021/jo00949a029.
^Crowell, Thomas I. (1964). "Alkene-Forming Condensation Reactions". In Patai, Saul (ed.).The Chemistry of Alkenes. PATAI'S Chemistry of Functional Groups. Wiley Interscience. pp. 241–270.doi:10.1002/9780470771044.ch4.ISBN978-0-470-77104-4.{{cite book}}:ISBN / Date incompatibility (help)
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