"E1cB-제거 반응"의 두 판 사이의 차이

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미번역 제거. 정리.
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잔글 (미번역 제거. 정리.)
{{번역 필요|날짜=2017-06-18}}
[[파일:Example_of_E1cB_Mechanism_Hemiacetal2.jpg|프레임|E1cB 반응 메커니즘의 예 - 염기 조건에서 헤미 아세탈 분해]]
'''E1cB 제거 반응'''은 나쁜 이탈기 (예 : -OH 또는 -OR) 및 산성 수소가 제거되어 추가 결합을 형성하는 염기성 조건 하에서 발생하는 일종의 제거 반응이다. E1cB는 두 단계 과정이다. 첫 단계, 염기는 가장 산성인 양성자를 떼어내어 안정화 된 음이온을 생성한다. 그 뒤 음이온 비공유전자쌍 중 하나는 인접한 원자로 이동하므로 이탈기를 내보내고, 이중 또는 삼중 결합을 형성한다. 메커니즘의 이름 - '''E1cB''' - Elimination Unimolecular conjugate Base를 의미합니다의미한다. 제거란 메카니즘이 제거 반응이고 두 개의 치환기를 잃을 것이라는 사실을 의미한다. 일분자는 이 반응의 속도 결정 단계가 하나의 분자만을 포함한다는 사실을 의미한다. 마지막으로, 짝염기는 출발 물질의 짝염기인 카보닐 음이온 중간체의 형성을 의미한다.
 
== 메커니즘 ==
 
== E1 및 E2-제거 반응과 E1cB-제거 반응의 구별 ==
모든 제거 반응은 화합물 내의 인접한 원자 쌍으로부터 2 개의 치환기를 제거하는 것을 포함한다. 알켄, 알카인 또는 유사한 헤테로 원자 변형(예 : 카보닐 및 사이아노)이 형성 될 것이다. E1cB 메커니즘은 세 가지 유형의 제거 반응 중 하나일 뿐이다. 다른 두 가지 제거 반응은 E1 및 E2 반응입니다반응이다. 메커니즘은 유사하지만 α-carbon의 탈 양성화와 이탈기의 이탈의 시간적인 차이가 있다. E1은 일분자 제거를 나타내고, E2는 이분자 제거를 나타낸다. E1 메커니즘에서, 분자는 α-탄소의 탈 양성자화 이전에 이탈되는 좋은 이탈기를 갖고 있다. 이것은 탄소 양이온 중간체를 형성하게된다. 탄소 양이온 형성 뒤 탈양성자화를 통해 새로운 파이 결합이 형성된다. 관련된 분자는 브롬이나 염소와 같은 아주 좋은 이탈 그룹을 가져야하며 상대적으로 산성이 적은 α- 탄소가 있어야 한다.
[[파일:Preferential_elimination_of_fluorine_in_an_E1cB_mechanism.jpg|오른쪽|프레임|Example of the preferential elimination of fluorine in an E1cB-elimination reaction.]]
In an E2-elimination reaction, both the deprotonation of the α-carbon and the loss of the leaving group occur simultaneously in one concerted step. Molecules that undergo E2-elimination mechanisms have more acidic α-carbons that undergo E1 mechanisms, but their α-carbons are not as acidic as those of molecules that undergo E1cB mechanisms. The key difference between the [[제거 반응|E2]] vs E1cb pathways is a distinct carbanion [[반응 중간체|intermediate]] as opposed to one concerted mechanism. Studies have been shown that the pathways differ by using different [[할로젠|halogen]] [[이탈기|leaving groups]]. One example uses [[염소 (원소)|chlorine]] as a better stabilizing [[할로젠|halogen]] for the [[이온|anion]] than [[플루오린|fluorine]],<ref name="HineBurske1957"><cite class="citation journal">Hine, Jack; Burske, Norbert W.; Hine, Mildred; Langford, Paul B. (1957). </cite></ref> which makes [[플루오린|fluorine]] the [[이탈기|leaving group]] even though chlorine is a much better leaving group.<ref><cite class="citation journal">Baciocchi, Enrico; Ruzziconi, Renzo; Sebastiani, Giovanni Vittorio (1 August 1982). </cite></ref> This provides evidence that the carbanion is formed because the products are not possible through the most stable concerted [[제거 반응|E2]] mechanism.
The following table summarizes the key differences between the three elimination reactions; however, the best way to identify which mechanism is playing a key role in a particular reaction involves the application of [[화학반응속도론|chemical kinetics]].
{| class="wikitable"
! E1
! E2
! E1cB
|-
| Stepwise reaction
| Concerted reaction
| Stepwise reaction
|-
| Carbocation intermediate
| Simultaneous removal of proton, formation of double bond, and loss of leaving group
| Carbanion intermediate
|-
| no kind of conclusion
| No preference
| no kind of conclusion
|-
| Good leaving groups
| Leaving group
| Poor leaving groups
|-
| Less acidic α-carbon
| Acidic α-carbon
| More acidic α-carbon
|}
 
== Aldol reactions 반응==
== Chemical kinetics of E1cB-elimination mechanisms ==
E1cB 제거를 거치는 가장 잘 알려진 반응은 기본 조건 하에서의 알돌 축합 반응이다. 이것은 enolate의 형성을 초래하는 카보닐기를 갖고 있는 화합물의 탈 양성자화를 포함한다. enolate는 출발 물질의 매우 안정한 짝염기이며, 반응의 중간체 중 하나입니다하나이다. 이 enolate는 친핵체 역할을 하고 친전자성 알데하이드를 공격 할 수 있습니다. 이어서 Aldol 생성물을 탈 양성자화하여 다른 enolate를 형성시킨 다음 E1cB 탈수 반응에서 물을 제거한다. 알돌 반응은 유기화학에서 중요한 반응이며, 이는 탄소 - 탄소 결합을 형성하여 보다 복잡한 분자의 합성을 가능하게하기 때문입니다때문이다.
When trying to determine whether or not a reaction follows the E1cB mechanism, [[화학반응속도론|chemical kinetics]] are essential. The best way to identify the E1cB mechanism involves the use of rate laws and the kinetic isotope effect. These techniques can also help further differentiate between E1cB, E1, and E2-elimination reactions.
[[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.]]
 
=== Rate Law ===
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.]]
Assuming that there is a steady-state carbanion concentration in the mechanism, the rate law for an E1cB mechanism.
: <math chem="" title="The rate law and observed rate for the E1cB-elimination mechanism in the previous example where the base is ethoxide.">
\begin{align}
\frac\ce{d[P]}{\ce d t} &= \frac{k_1 k_2 \ce{[substrate][base]}}{k_{-1}[\ce{conjugate\ acid}]+k_2} \\
\ce{rate} &= k_\ce{obs}\ce{[substrate][base]}
\end{align}
</math>
From this equation, it is clear the second [[반응차수|order kinetics]] will be exhibited.<ref name="McLennan1967"><cite class="citation journal">McLennan, D. J. (1967). </cite></ref>
E1cB mechanisms kinetics can vary slightly based on the rate of each step. As a result, the E1cB mechanism can be broken down into three categories:<ref><cite class="citation book">Smith, Michael (2007). </cite></ref>
# '''E1cB<sub>anion</sub>''' is when the anion is stable resulting in a rapid first step, followed by the slow formation of products (k<sub>1</sub>≫k<sub>2</sub>).
# '''E1cB<sub>rev</sub>''' is when the first step is reversible but the formation of product is slower than reforming the starting material, this again results from a slow second step (k<sub>-1</sub>≫k<sub>2</sub>).
# '''E1cB<sub>irr</sub>''' is when the first step is slow but once formed the product quickly follows (k<sub>2</sub>≫k<sub>1</sub>,k<sub>-1</sub>). This leads to an irreversible first step.
 
=== Kinetic Isotope Effect ===
 
==== Deuterium ====
The kinetic isotope effect can help distinguish between '''E1cB<sub>rev</sub>''', '''E1cB<sub>anion</sub>''', and '''E1cB<sub>irr</sub>'''. If [[중수소|deuterium]] is present in the base in place of hydrogen, then the exchange of protons can be monitored. If the reaction occurs in deuterated base and starting material is recovered that contains deuterium, then the reaction is most likely undergoing an '''E1cB<sub>rev</sub>''' type mechanism. Recall, in this mechanism k<sub>-1</sub> is faster than the k<sub>2</sub>. This means after the carbanion is formed, it will quickly remove a proton from the base to form the starting material. So if the starting material ends up with deuterium in place of its original hydrogen, then the compound was deprotonated, and then took a deuterium off of the base.
An additional kinetic isotope experiment would be to replace the solvent with deuterated solvent. So if the reaction is run in water, it can be run in [[중수|deuterium oxide]]. If the reaction takes place faster in the deuterium oxide than water, then the proton transfer step is not rate determining. Since deuterium is larger than hydrogen, the proton transfer step should be slower if the '''E1cB<sub>rev</sub>''' was dictating the reaction. A faster reaction would suggest that the reaction is either '''E1cB<sub>anion</sub>''' or '''E1cB<sub>irr</sub>'''.
 
In an E2-elimination reaction, both the deprotonation of the α-carbon and the loss of the leaving group occur simultaneously in one concerted step. Molecules that undergo E2-elimination mechanisms have more acidic α-carbons that undergo E1 mechanisms, but their α-carbons are not as acidic as those of molecules that undergo E1cB mechanisms. The key difference between the [[제거 반응|E2]] vs E1cb pathways is a distinct carbanion [[반응 중간체|intermediate]] as opposed to one concerted mechanism. Studies have been shown that the pathways differ by using different [[할로젠|halogen]] [[이탈기|leaving groups]]. One example uses [[염소 (원소)|chlorine]] as a better stabilizing [[할로젠|halogen]] for the [[이온|anion]] than [[플루오린|fluorine]],<ref name="HineBurske1957"><cite class="citation journal">Hine, Jack; Burske, Norbert W.; Hine, Mildred; Langford, Paul B. (1957). </cite></ref> which makes [[플루오린|fluorine]] the [[이탈기|leaving group]] even though chlorine is a much better leaving group.<ref><cite class="citation journal">Baciocchi, Enrico; Ruzziconi, Renzo; Sebastiani, Giovanni Vittorio (1 August 1982). </cite></ref> This provides evidence that the carbanion is formed because the products are not possible through the most stable concerted [[제거 반응|E2]] mechanism.
The following table summarizes the key differences between the three elimination reactions; however, the best way to identify which mechanism is playing a key role in a particular reaction involves the application of [[화학반응속도론|chemical kinetics]].
 
Another way that the kinetic isotope effect can help distinguish E1cB mechanisms involves the use of <sup>19</sup>F. [[플루오린|Fluorine]] is a relatively poor leaving group, and it is often employed in E1cB mechanisms. Fluorine kinetic isotope effects are also applied in the labeling of Radiopharmaceuticals and other compounds in medical research. This experiment is very useful in determining whether or not the loss of the leaving group is the rate-determining step in the mechanism and can help distinguish between '''E1cB<sub>irr</sub>''' and E2 mechanisms. <sup>11</sup>C can also be used to probe the nature of the transition state structure. The use of <sup>11</sup>C can be used to study the formation of the carbanion as well as study its lifetime which can not only show that the reaction is a two-step E1cB mechanism (as opposed to the concerted E2 mechanism), but it can also address the lifetime and stability of the transition state structure which can further distinguish between the three different types of E1cB mechanisms.<ref><cite class="citation journal">Matsson, Olle; MacMillar, Susanna (September 2007). </cite></ref>
 
== Aldol reactions ==
E1cB 제거를 거치는 가장 잘 알려진 반응은 기본 조건 하에서의 알돌 축합 반응이다. 이것은 enolate의 형성을 초래하는 카보닐기를 갖고 있는 화합물의 탈 양성자화를 포함한다. enolate는 출발 물질의 매우 안정한 짝염기이며, 반응의 중간체 중 하나입니다. 이 enolate는 친핵체 역할을 하고 친전자성 알데하이드를 공격 할 수 있습니다. 이어서 Aldol 생성물을 탈 양성자화하여 다른 enolate를 형성시킨 다음 E1cB 탈수 반응에서 물을 제거한다. 알돌 반응은 유기화학에서 중요한 반응이며, 이는 탄소 - 탄소 결합을 형성하여 보다 복잡한 분자의 합성을 가능하게하기 때문입니다.
[[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.]]
 
== Photo-induced E1cB ==
A photochemical version of E1cB has been reported by Lukeman ''et al.''<ref name="LukemanScaiano2005"><cite class="citation journal">Lukeman, Matthew; Scaiano, Juan C. (2005). </cite></ref> In this report, a photochemically induced decarboxylation reaction generates a carbanion intermediate, which subsequently eliminates the leaving group. The reaction is unique from other forms of E1cB since it does not require a base to generate the carbanion. The carbanion formation step is irreversible, and should thus be classified as '''E1cB<sub>irr</sub>'''.
[[파일:Photo-induced_E1cB_reaction_mechanism..jpg|가운데|프레임|720x720픽셀|E1cB reaction mechanism through photo-induced decarboxylation.]]
 
== 같이 보기 ==