Editing Alkene (section)

===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 Ph<sub>3</sub>P=CHR to produce an alkene and [[Triphenylphosphine oxide|Ph<sub>3</sub>P=O]]. The Wittig reagent is itself prepared easily from [[triphenylphosphine]] and an alkyl halide.<ref>{{cite book | last = Crowell | first = Thomas I.|chapter=Alkene-Forming Condensation Reactions  |series=PATAI'S Chemistry of Functional Groups| editor = Patai, Saul | title = The Chemistry of Alkenes| publisher = Wiley Interscience | year = 1964 | pages = 241–270|doi=10.1002/9780470771044.ch4 | isbn = 978-0-470-77104-4}}</ref>

[[Image:Wittig reaction example.svg|350px|center|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 [[deoxygenation|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]]).