a reaction of ethylene with the organometallic compound triethylaluminium in the presence of nickel, cobalt, or platinum. Elimination reactions 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, 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. 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: 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. Synthesis from carbonyl compounds 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