Industrial methods 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). Related to this is catalytic
dehydrogenation, where an alkane loses hydrogen at high temperatures to produce a corresponding alkene. A commercially significant example is the production of
vinyl 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 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. 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). A
thioketone and a
phosphite ester combined (the
Corey-Winter olefination) or
diphosphorus tetraiodide will deoxygenate
glycols to alkenes. 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: The Hofmann elimination is unusual in that the
less substituted (non-
Zaitsev) alkene is usually the major product. Alkenes are generated from α-halo
sulfones in the
Ramberg–Bäcklund reaction, via a three-membered ring sulfone intermediate.
Synthesis from carbonyl compounds 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 an
aldehyde or
ketone) to a
carbanion or its equivalent. Pre-eminent is the
aldol condensation. Knoevenagel condensations are a related class of reactions that convert carbonyls into alkenes.Well-known methods are called
olefinations. The
Wittig reaction is illustrative, 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. 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).
Synthesis from alkenes 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: :CH2=CH2 + CH3CH=CHCH3 → 2 CH2=CHCH3 Transition metal catalyzed
hydrovinylation is another important alkene synthesis process starting from alkene itself. It involves the addition of a hydrogen and a vinyl group (or an alkenyl group) across a double bond.
Synthesis from alkynes 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. For the preparation multisubstituted alkenes,
carbometalation of alkynes can give rise to a large variety of alkene derivatives.
Rearrangements and related reactions 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. In the
Diels–Alder reaction, a
cyclohexene derivative is prepared from a diene and a reactive or electron-deficient alkene. == Application ==