Editing Alkene (section)

===Elimination reactions===
One of the principal methods for alkene synthesis in the laboratory is the [[elimination reaction]] of alkyl halides, alcohols, and similar compounds. Most common is the β-elimination via the E2 or E1 mechanism.<ref name="PataiBook1964">{{cite book | last = Saunders | first = W. H.  | editor = Patai, Saul | title = The Chemistry of Alkenes| chapter=Elimination Reactions in Solution|publisher = Wiley Interscience |series=PATAI'S Chemistry of Functional Groups | year = 1964 | pages = 149–201|doi=10.1002/9780470771044 | isbn = 978-0-470-77104-4 }}</ref>  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.<ref name=Cram1956>{{cite journal
 | last1 = Cram |first1 = D.J.
 | year = 1956
 | title = Studies in Stereochemistry. XXV. Eclipsing Effects in the E2 Reaction1
 | journal = Journal of the American Chemical Society
 | volume = 78
 | issue = 4
 | pages = 790–6
 | doi = 10.1021/ja01585a024
 | last2 = Greene
 | first2 = Frederick D.
 | last3 = Depuy
 | first3 = C. H.
}}</ref>

[[Image:E2EliminationExample.png|500px|center|An example of an E2 Elimination]]

Alkenes can be synthesized from alcohols via [[Dehydration reaction|dehydration]], in which case water is lost via the E1 mechanism. For example, the dehydration of [[ethanol]] produces ethylene:
:CH<sub>3</sub>CH<sub>2</sub>OH   →  H<sub>2</sub>C=CH<sub>2</sub> +  H<sub>2</sub>O

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 [[glycol]]s to alkenes.  

Alkenes can be prepared indirectly from alkyl [[amine]]s. The amine or ammonia is not a suitable leaving group, so the amine is first either [[alkylation|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&nbsp;°C, for example:<ref name="CopeElimination1973">{{cite journal | last1 = Bach |first1=R.D. | title=Mechanism of the Cope elimination | journal=J. Org. Chem. | year=1973| volume=38| pages=1742–3 | doi=10.1021/jo00949a029 | last2 = Andrzejewski | first2 = Denis | last3 = Dusold | first3 = Laurence R. | issue = 9 }}</ref>

[[Image:CopeEliminationExample.svg|300px|center|Synthesis of cyclooctene via Cope elimination]]

The Hofmann elimination is unusual in that the ''less'' substituted (non-[[Zaitsev's rule|Zaitsev]]) alkene is usually the major product.

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