E1cB elimination reaction
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[[Image:E1cB.gif|right|frame|The E1cB reaction mechanism.]]The '''E1cB elimination reaction''' is a special type of [[elimination reaction]] in [[organic chemistry]]. This [[reaction mechanism]] explains the formation of [[alkene]]s from (mostly) [[alkyl halide]]s through a [[carbanion]] intermediate given specified reaction conditions and specified substrates. The abbreviation stands for '''E''' limination '''c'''onjugate '''B'''ase. The reaction takes place around a [[orbital hybridization|sp<sup>3</sup>]] - sp<sup>3</sup> carbon to carbon [[covalent bond]] with an α-acidic [[hydrogen]] [[atom]] [[substituent]] and a β-[[leaving group]]. This leaving group can be a [[halide]] or a [[sulfonic acid]] ester such as a [[tosyl]] group. A strong [[base (chemistry)|base]] abstracts the α proton generating a [[carbanion]]. The electron pair then expels the leaving group and the double bond is formed. When the first step to the carbanion is slow and the second step fast the reaction is irreversible and named (E1cB)<sub>i</sub>. When the first step is fast and the [[deprotonation]] reversible then the reaction mechanism is (E1cB)<sub>r</sub>. In the (E1cB)<sub>anion</sub> variation the carbanion is especially stable with a rapid first step and a slow second step.
[[Image:E1cBexample.gif|right|frame|The E1cB reaction mechanism. Dehydration of 1-methyl-2-(2-fluoroethyl)pyridinium iodide).]]
A named reaction displaying E1cB elimination mechanism is the [[Boord olefin synthesis]].
Being able to distinguish between different elimination reaction mechanisms (E2, E1, E1cB) is often difficult and involves the study of [[chemical kinetics]], [[kinetic isotope effect]]s, substituent effects and [[computational chemistry]]. The [[dehydration reaction|dehydration]] of 1-methyl-2-(2-fluoroethyl)pyridinium iodide by the [[hydroxide]] ion in water is an example of a 2-step E1cB reaction because the carbanion is stabilized by the [[resonance effect]] with the [[enamine]].<ref>{{cite journal | title = Evidence of a Borderline Region between E1cb and E2 Elimination Reaction Mechanisms: A Combined Experimental and Theoretical Study of Systems Activated by the Pyridine Ring | author = Sergio Alunni, Filippo De Angelis, Laura Ottavi, Magdalini Papavasileiou, and Francesco Tarantelli | journal = [[J. Am. Chem. Soc.]] | volume = 127 | pages = 15151 | year = 2005 | doi = 10.1021/ja0539138}}</ref> [[Computational chemistry]] predicts that the carbanion in this example is stabilized by only 2.8 [[calorie|kcal]]/[[mole (unit)|mol]] (11.7 [[kilojoule|kJ]]/mol) and only when [[solvation]] of all the chemical species with water is taken into account. The same fluoride substrate but as a free [[pyridine]] does not have this stabilization and the reaction proceeds by a [[concerted reaction|concerted]] one-step [[elimination reaction|E2]] [[reaction mechanism]]. When the leaving group in the methylated pyridine compound is [[chlorine]] instead of fluoride the reaction mechanism is also E2 because now the carbon to chloride bond is too weak.
==References==
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[[Category:Elimination reactions]]
[[Category:Reaction mechanisms]]