Alkene

Ethylene (ethene), showing the pi bond in green

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.[8]

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.

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.[9] Following Fawcett and defining S as the total number of non-bridgehead atoms in the rings,[10] bicyclic systems require S ≥ 7 for stability[9] and tricyclic systems require S ≥ 11.[11]

Alkenes react in many addition reactions, which occur by opening up the double-bond. Most of these addition reactions follow the mechanism of electrophilic addition. Examples are hydrohalogenation, halogenation, halohydrin formation, oxymercuration, hydroboration, dichlorocarbene addition, Simmons–Smith reaction, catalytic hydrogenation, epoxidation, radical polymerization and hydroxylation.

Hydrogenation of alkenes produces the corresponding alkanes. The reaction is sometimes carried out under pressure and at elevated temperature. Metallic catalysts are almost always required. Common industrial catalysts are based on platinum, nickel, and palladium. A large scale application is the production of margarine.

Aside from the addition of H−H across the double bond, many other H−X's can be added. These processes are often of great commercial significance. One example is the addition of H-SiR3, i.e., hydrosilylation. This reaction is used to generate organosilicon compounds. Another reaction is hydrocyanation, the addition of H−CN across the double bond.

Hydration, the addition of water across the double bond of alkenes, yields alcohols. The reaction is catalyzed by phosphoric acid or sulfuric acid. This reaction is carried out on an industrial scale to produce synthetic ethanol.

${\displaystyle {\ce {CH2=CH2 + H2O -> CH3-CH2OH}}}$

Alkenes can also be converted into alcohols via the oxymercuration–demercuration reaction , the hydroboration–oxidation reaction or by Mukaiyama hydration.

In electrophilic halogenation the addition of elemental bromine or chlorine to alkenes yields vicinal dibromo- and dichloroalkanes (1,2-dihalides or ethylene dihalides), respectively. The decoloration of a solution of bromine in water is an analytical test for the presence of alkenes:

${\displaystyle {\ce {CH2=CH2 + Br2 -> BrCH2-CH2Br}}}$

Related reactions are also used as quantitative measures of unsaturation, expressed as the bromine number and iodine number of a compound or mixture.

Hydrohalogenation is the addition of hydrogen halides, such as HCl or HI, to alkenes to yield the corresponding haloalkanes:

${\displaystyle {\ce {CH3-CH=CH2}}+{\ce {HI->CH3-CH{\mathit {I}}-CH2-{\mathit {H}}}}}$

If the two carbon atoms at the double bond are linked to a different number of hydrogen atoms, the halogen is found preferentially at the carbon with fewer hydrogen substituents. This patterns is known as Markovnikov's rule. The use of radical initiators or other compounds can lead to the opposite product result. Hydrobromic acid in particular is prone to forming radicals in the presence of various impurities or even atmospheric oxygen, leading to the reversal of the Markovnikov result:[13]

${\displaystyle {\ce {CH3-CH=CH2}}+{\ce {HBr->CH3-CH{\mathit {H}}-CH2-{\mathit {Br}}}}}$

Alkenes react with water and halogens to form halohydrins by an addition reaction. Markovnikov regiochemistry and anti-stereochemistry occur.

${\displaystyle {\ce {CH2=CH2 + X2 + H2O -> XCH2-CH2OH + HX}}}$

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

${\displaystyle {\ce {RCH=CH2 + RCO3H -> RCHOCH2 + RCO2H}}}$

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

${\displaystyle {\ce {C2H4 + 1/2 O2 -> C2H4O}}}$

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

${\displaystyle {\ce {RCH=CHR' + O3 + SMe2 -> RCHO + R'CHO + O=SMe2}}}$

${\displaystyle {\ce {R2C=CHR' + O3 -> R2CHO + R'CHO + O=SMe2}}}$

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).[14] 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 a diene such as cyclopentadiene to yield an endoperoxide:

Another example is the Schenck ene reaction, in which singlet oxygen reacts with an allylic structure to give a transposed allyl peroxide:

Terminal alkenes are precursors to polymers via processes termed polymerization. Some polymerizations are of great economic significance, as they generate as 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.

Structure of bis(cyclooctadiene)nickel(0), a metal–alkene complex

Alkenes are ligands in transition metal alkene complexes. The two carbon centres bond to the metal using the 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 itself 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.

Alkenes are produced by hydrocarbon cracking. Raw materials are mostly natural gas condensate components (principally ethane and propane) in the US and Mideast and naphtha in Europe and Asia. Alkanes are broken apart at high temperatures, often in the presence of a zeolite 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).[15]

Cracking of n-octane to give pentane and propene

Related to this is catalytic dehydrogenation, where an alkane loses hydrogen at high temperatures to produce a corresponding alkene.[1] This is the reverse of the catalytic hydrogenation of alkenes.

Dehydrogenation of butane to give butadiene and isomers of butene

This process is also known as reforming. Both processes are endothermic and are driven towards the alkene at high temperatures by entropy.

Catalytic synthesis of higher α-alkenes (of the type RCH=CH₂) can also be achieved by a reaction of ethylene with the organometallic compound triethylaluminium in the presence of nickel, cobalt, or platinum.

One of the principal methods for alkene synthesis in the laboratory is the room elimination of alkyl halides, alcohols, and similar compounds. Most common is the β-elimination via the E2 or E1 mechanism,[16] but α-eliminations are also known.

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 a tosylate or triflate). When an alkyl halide is used, the reaction is called a dehydrohalogenation. For unsymmetrical products, the more substituted alkenes (those with fewer hydrogens attached to the C=C) tend to predominate (see Zaitsev'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 is anti to the leaving group, even though this leads to the less stable Z-isomer.[17]

An example of an E2 Elimination

Alkenes can be synthesized from alcohols via dehydration, in which case water is lost via the E1 mechanism. For example, the dehydration of ethanol produces ethylene:

CH₃CH₂OH → H₂C=CH₂ + H₂O

An alcohol may also be converted to a better leaving group (e.g., xanthate), so as to allow a milder syn-elimination such as the Chugaev elimination and the Grieco elimination. Related reactions include eliminations by β-haloethers (the Boord olefin synthesis) and esters (ester pyrolysis).

Alkenes can be prepared indirectly from alkyl amines. The amine or ammonia is not a suitable leaving group, so the amine is first either alkylated (as in the Hofmann elimination) or oxidized to an amine oxide (the Cope reaction) to render a smooth elimination possible. The Cope reaction is a syn-elimination that occurs at or below 150 °C, for example:[18]

Synthesis of cyclooctene via Cope elimination

The Hofmann elimination is unusual in that the less substituted (non-Zaitsev) alkene is usually the major product.

Alkenes are generated from α-halosulfones in the Ramberg–Bäcklund reaction, via a three-membered ring sulfone intermediate.

Another important method for alkene synthesis involves construction of a new carbon–carbon double bond by coupling of a carbonyl compound (such as an aldehyde or ketone) to a carbanion equivalent. Such reactions are sometimes called olefinations. The most well-known of these methods is the Wittig reaction, but other related methods are known, including the Horner–Wadsworth–Emmons reaction.

The Wittig reaction involves reaction of an aldehyde or ketone with a Wittig reagent (or phosphorane) of the type Ph₃P=CHR to produce an alkene and Ph₃P=O. The Wittig reagent is itself prepared easily from triphenylphosphine and an alkyl halide. The reaction is quite general and many functional groups are tolerated, even esters, as in this example:[19]

A typical example of the Wittig reaction

Related to the Wittig reaction is the Peterson olefination, which uses silicon-based reagents in place of the phosphorane. This reaction allows for the selection of E- or Z-products. If an E-product is desired, another alternative is the Julia olefination, which uses the carbanion generated from a phenyl sulfone. The Takai 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 be deoxygenated to generate an alkene. Symmetrical alkenes can be prepared from a single aldehyde or ketone coupling with itself, using titanium metal reduction (the McMurry reaction). If different ketones are to be coupled, a more complicated method is required, such as the Barton–Kellogg reaction.

A single ketone can also be converted to the corresponding alkene via its tosylhydrazone, using sodium methoxide (the Bamford–Stevens reaction) or an alkyllithium (the Shapiro reaction).

The formation of longer alkenes via the step-wise polymerisation of smaller ones is appealing, as ethylene (the smallest alkene) is both inexpensive and readily available, with hundreds of millions of tonnes produced annually. The Ziegler–Natta process allows for the formation of very long chains, for instance those used for polyethylene. Where shorter chains are wanted, as they for the production of surfactants, then processes incorporating a olefin metathesis step, such as the Shell higher olefin process are important.

Olefin metathesis is also used commercially for the interconversion of ethylene and 2-butene to propylene. Rhenium- and molybdenum-containing heterogeneous catalysis are used in this process:[20]

CH₂=CH₂ + CH₃CH=CHCH₃ → 2 CH₂=CHCH₃

Transition metal catalyzed hydrovinylation is another important alkene synthesis process starting from alkene itself.[21] It involves the addition of a hydrogen and a vinyl group (or an alkenyl group) across a double bond.

Reduction of alkynes is a useful method for the stereoselective synthesis of disubstituted alkenes. If the cis-alkene is desired, hydrogenation in the presence of Lindlar'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 by sodium metal in liquid ammonia gives the trans-alkene.[22]

Synthesis of cis- and trans-alkenes from alkynes

For the preparation multisubstituted alkenes, carbometalation of alkynes can give rise to a large variety of alkene derivatives.

Alkenes can be synthesized from other alkenes via rearrangement reactions. Besides olefin metathesis (described above), many pericyclic reactions can be used such as the ene reaction and the Cope rearrangement.

Cope rearrangement of divinylcyclobutane to cyclooctadiene

In the Diels–Alder reaction, a cyclohexene derivative is prepared from a diene and a reactive or electron-deficient alkene.

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 of cis–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 prefixes cis- (from Latin "on this side of") or trans- ("across", "on the other side of") before the name, respectively; as in cis-2-pentene or trans-2-butene.

The difference between cis- and trans- 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 general E–Z notation, instead of the cis and trans prefixes. This notation considers the group with highest CIP priority in each of the two carbons. If these two groups are on opposite sides of the double bond's plane, the configuration is labeled E (from the German entgegen meaning "opposite"); if they are on the same side, it is labeled Z (from German zusammen, "together"). This labeling may be taught with mnemonic "Z means 'on ze zame zide'".[23]

The difference between E and Z isomers

IUPAC recognizes two names for hydrocarbon groups containing carbon–carbon double bonds, the vinyl group and the allyl group.[5]