[[파일:Example_of_E1cB_Mechanism_Hemiacetal2.jpg|오른쪽|프레임|의 예 E1cB 반응 메커니즘에 저하의 [[헤미아세탈|hemiacetal]] 에서 기본 조건입니다.]]
The '''E1cB elimination reaction''' is a type of [[제거 반응|elimination reaction]] which occurs under basic conditions, where a particularly poor [[이탈기|leaving group]] (such as -OH or -OR) and an acidic hydrogen eliminate to form an additional bond. E1cB is a two-step process. First, a [[염기|base]] abstracts the most acidic proton to generate a stabilized [[이온|anion]]. The lone pair of electrons on the anion then moves to the neighboring atom, thus expelling the leaving group and forming [[이중 결합|double]] or [[삼중 결합|triple bond]].<ref><cite class="citation book">Grossman, R.B. (2008). </cite></ref> The name of the mechanism - '''E1cB''' - stands for '''E'''limination '''U'''nimolecular '''c'''onjugate '''B'''ase. ''Elimination'' refers to the fact that the mechanism is an [[제거 반응|elimination reaction]] and will lose two substituents. ''Unimolecular'' refers to the fact that the rate-determining step of this reaction only involves one molecular entity. Finally, ''conjugate base'' refers to the formation of the carbanion [[반응 중간체|intermediate]], which is the conjugate base of the starting material.
== 메커니즘 ==
α and β assignments in a molecule with leaving group, LG.]]
There are two main requirements to have a reaction proceed down an E1cB mechanistic pathway. The compound must have an [[산 (화학)|acidic]] hydrogen on its β-carbon and a relatively poor [[이탈기|leaving group]] on the α- carbon.
The first step of an E1cB mechanism is the deprotonation of the β-carbon, resulting in the formation of an [[이온|anionic]] transition state, such as a carbanion. The greater the stability of this transition state, the more the mechanism will favor an E1cB mechanism. This transition state can be stabilized through induction or delocalization of the [[전자|electron]] [[고립 전자쌍|lone pair]] through resonance. An example of an E1cB mechanism that has a stable transition state can be seen in the degradation of ethiofencarb - a carbamate [[살충제|insecticide]] that has a relatively short [[반감기|half-life]] in earth's atmosphere. Upon deprotonation of the [[아민|amine]], the resulting [[아마이드|amide]] is relatively stable because it is [[공액계|conjugated]] with the neighboring [[카보닐기|carbonyl]].
In addition to containing an acidic hydrogen on the β-carbon, a relatively poor leaving group is also necessary. A bad [[이탈기|leaving group]] is necessary because a good leaving group will leave before the [[전리|ionization]] of the molecule. As a result, the compound will likely proceed through an [[제거 반응|E2]] pathway. Some examples of compounds that contain poor leaving groups and can undergo the E1cB mechanism are [[알코올|alcohols]] and fluoroalkanes.
It has also been suggested that the E1cB mechanism is more common among [[알켄|alkenes]] eliminating to [[알카인|alkynes]] than from an [[알케인|alkane]] to alkene.<ref><cite class="citation book">Smith, Michael (2007). </cite></ref> One possible explanation for this is that the [[혼성 궤도|sp<sup>2</sup>]] hybridization creates slightly more acidic protons. Although it should be noted that this mechanism is not limited to [[탄소|carbon-based]] eliminations. It has been observed with other [[헤테로원자|heteroatoms]], such as [[질소|nitrogen]] in the elimination of a [[페놀|phenol]] derivative from ethiofencarb.<ref name="OuertaniEl Atrache2013"><cite class="citation journal">Ouertani, Randa; El Atrache, Latifa Latrous; Hamida, Nejib Ben (2013). </cite></ref>
[[파일:E1cB_Mechanism_Ethiofencarb2.jpg|가운데|프레임|933x933픽셀|Degradation of ethiofencarb illustrating the presence of a stable anion due to resonance between the amide functional group and the carbonyl group.]]
== Distinguishing E1cB-elimination reactions from E1 and E2-elimination reactions ==
=== Rate Law ===
== 메커니즘 ==
The rate law that governs E1cB mechanisms is relatively simple to determine. Consider the following reaction scheme.
[[파일:E1cB_reaction_mechanism_with_leaving_group_LG.jpg|가운데|프레임|An example of an E1cB-elimination mechanism with a generic leaving group, LG, and ethoxide as the base.]]
== Aldol reactions ==
The most well known reaction that undergoes E1cB elimination is the aldol condensation reaction under basic conditions. This involves the deprotonation of a compound containing a [[카보닐기|carbonyl]] group that results in the formation of an [[엔올|enolate]]. The enolate is very stable conjugate base of the starting material, and is one of the intermediates in the reaction. This enolate then acts as a nucleophile and can attack an electrophilic aldehyde. The Aldol product is then deprotonated forming another enolate followed by the elimination of water in an E1cB [[탈수 반응|dehydration reaction]]. Aldol reactions are a key reaction in organic chemistry because they provide a means of forming carbon-carbon bonds, allowing for the synthesis of more complex molecules.<ref><cite class="citation book">Wade, L.G. (2005). </cite></ref>
[[파일:Aldol_reaction_and_E1cB_mechanism.jpg|가운데|프레임|522x522픽셀|An aldol condensation reaction is one of the most common examples of an E1cB mechanism.]]