Name: e2 vs e1 in alkyl halide elimination
Uploaded: 2023-04-30T18:28:36.000Z
Duration: 2 min 25 s
Channel: JoVE

当卤代烷与亲核试剂反应时，亲核试剂可以取代卤素生成取代产物，也可以夺取相邻的氢原子，通过消除反应形成烯烃。
在消除反应中，亲核试剂作为路易斯碱，通过向质子提供一对电子发挥作用。常用于促进消除反应的碱包括氢氧化物（如氢氧化钠）和醇盐（如钾） 叔丁基叔丁醇盐，或乙醇等醇类。
消除反应通常涉及从底物中脱去小分子片段，从而形成至少一个π键。在卤代烷中，消除反应伴随着一个氢原子和一个卤素原子的脱去，因此也称为脱卤化氢反应。
由于与离去基团相连的碳是α碳，而相邻碳上的氢是β氢，因此这类反应通常被称为β-消除反应或1,2-消除反应。
大多数消除反应通过E2或E1机理进行。
对于E2反应，需使用乙醇钠等强碱。该协同机理首先由β碳原子发生去质子化，随后卤素离去基团离去，从而在α与β位之间形成一个π键。
相比之下，E1 反应分两步进行。第一步是离去基团离去，形成一个碳正离子中间体，随后碱对碳正离子进行去质子化，从而形成一个 π 键。
对于含有两个不同β碳的卤代烷，消除反应可以生成多种烯烃。其中，取代基较多的烯烃最为稳定，被称为扎伊采夫产物，而取代基较少的烯烃则被称为霍夫曼产物。因此，消除反应具有区域选择性。
此外，消除反应倾向于生成 trans-烯烃而非 cis-异构体，因此具有立体选择性。

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	<li>所罗门斯，G.，& 弗莱尔，C. & 斯奈德，S. （2015）。有机化学.新泽西州新泽西州:Wiley，299。</li>
	<li>Loudon， M. 和 Parise， J. （2016）。有机化学.纽约州纽约市:麦克米伦出版社，412。</li>
	<li>克莱因，D.（2017 年）。有机化学.新泽西州新泽西州:Wiley，271。</li>
	<li>Clayden， J.， & Greeves， N.， & Warren， S. （2012）。有机化学.牛津:牛津大学出版社，382。</li>
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消除反应

Elimination Reactions

亲核试剂可以与卤代烷反应，通过置换卤素得到取代产物。 或者它可以作为碱，通过相邻碳的去质子化形成烯烃来产生消除产物。 在消除反应中，底物失去来自相邻碳的两个基团，形成至少一个π键。 与卤素相连的碳称为α碳，而相邻的碳称为β碳; 因此，这些反应称为β消除或1,2-消除反应。


 亲核试剂通过向质子提供一对电子而充当路易斯碱。 用于促进消除反应的常见碱包括氢氧化物 (OH−)、醇盐 (OR−) 和酰胺 (NH2−)。 在强碱存在下，卤代烷从 β 碳失去质子，从 α 碳失去卤素，从而在两个碳原子之间形成 π 键。


 <h4>消除反应机制</h4>


 消除反应通常通过 E2 或 E1 机制发生。 E2 机制发生在一个协调的步骤中:碱基对 β 氢的夺取伴随着 α-碳-卤素键的断裂。 因此，E2 反应通过一种过渡态进行。



 E1反应分两步发生。 首先，烷基卤化物发生电离，形成碳阳离子中间体和卤化物离子。 接下来，碳阳离子被碱基去质子化，形成 π 键。 因此，在E1反应中，碳阳离子中间体是通过一个过渡态形成的，并且存在用于去质子化步骤的第二过渡态。


 <h4>区域选择性和立体选择性</h4>


 当卤代烷具有两个不同的β碳时，消除反应可以产生多于一种的烯烃。 在这种情况下，通常会观察到取代度更高（且最稳定）的烯烃，称为扎伊采夫产物。 然而，在某些情况下，获得取代度较低的烯烃（霍夫曼产物）。 碱基的选择在决定形成哪种区域选择性产物方面起着重要作用。 消除反应也有利于反式烯烃的形成而不是顺式异构体，使它们具有立体选择性。

亲核试剂可与卤代烷反应，通过取代卤素生成取代产物；或者作为碱反应生成……

Education

教育

JoVE Core

Organic Chemistry

有机化学

Unit 1

第一单元

Nucleophilic Substitution and Elimination Reactions of Alkyl Halides

卤代烷烃的亲核取代与消除反应

nucleophilic-substitution-and-elimination-reactions-of-alkyl-halides

KEY TERMS AND CONCEPTS

关键术语与概念

e2 vs e1 in alkyl halide elimination

Elimination reactions of alkyl halides form alkenes by removing groups from adjacent carbons. A base can act on the substrate instead of causing substitution. It deprotonates the neighboring carbon, and the reaction creates at least one pi bond. The carbon bonded to the halogen is the alpha carbon, and the adjacent carbon is the beta carbon. These are also called beta elimination or 1,2-elimination reactions.

Strong bases help drive this process. Common examples include hydroxides, alkoxides, and amides. The base donates an electron pair to a proton, so it behaves as a Lewis base. In the presence of a strong base, the alkyl halide loses a proton from the beta carbon and the halogen from the alpha carbon. This lets a pi bond form between the two carbon atoms.

Elimination reactions often follow either the E2 or E1 mechanism. The E2 pathway is a single concerted step. The base removes the beta hydrogen at the same time that the alpha carbon-halogen bond breaks. This means E2 has one transition state.

The E1 pathway happens in two steps. First, the alkyl halide ionizes to form a carbocation intermediate and a halide ion. Then the base removes a proton from the carbocation, which forms the pi bond. In E1 reactions, the first step has one transition state, and the deprotonation step has a second transition state.

When an alkyl halide has two different beta carbons, more than one alkene can form. The more substituted and usually more stable alkene is called the Zaitsev product. In some cases, the less substituted Hofmann product forms instead, and the base helps decide which product is favored. Elimination reactions also tend to give trans-alkenes over cis isomers, so they are stereoselective.

E2 vs E1 in Alkyl Halide Elimination

alkyl halides

Alkyl Halides: Structure and Reactivity

Alkyl halides are halogen-substituted alkanes. In these compounds, one or more hydrogen atoms in an alkane is replaced by a halogen atom such as fluorine, chlorine, bromine, or iodine. The carbon bonded to the halogen is sp3-hybridized and has a tetrahedral shape. Not every carbon-halogen compound is an alkyl halide. If the halogen is attached to an sp2-hybridized carbon in a carbon-carbon double bond, the compound is a vinyl halide. If the halogen is attached to an sp2-hybridized carbon in an...

nucleophilic substitution reactions

SN1 and SN2 in Organic Substitution

Nucleophilic substitution reactions replace one group in a molecule with another. In these reactions, a nucleophile, which is a Lewis base, reacts with an electrophile, which is a Lewis acid. The nucleophile takes the place of a halogen atom bonded to carbon and releases a stable ion called the leaving group. This reaction pattern is closely related to Lewis acid-base chemistry. The electron-rich species acts as the nucleophile. The electron-deficient species acts as the electrophile. A key...

nucleophiles

Nucleophilic Strength in Substitution Reactions

Nucleophiles are electron-pair donors that take part in substitution reactions. The word comes from Greek and means nucleus-loving. Because they donate electrons, nucleophiles are also Lewis bases. Many nucleophiles are negatively charged species, such as hydroxide, chloride, and hydrosulfide. Neutral molecules can also act as nucleophiles when they have lone pairs of electrons. Examples include ammonia, amines, water, and alcohols. Some molecules without a lone pair can still behave as...

electrophiles

Electrophile Reactivity in Organic Reactions

Electrophile reactivity in organic reactions depends on how a species attracts electrons. Electrophiles are electron-poor and are key partners in nucleophilic substitution reactions. Their charge and orbital structure help explain why they react the way they do. Electrophiles can be positive or neutral species. A positive electrophile such as a proton reacts because it has a vacant, low-energy 1s orbital. Other positive electrophiles, such as carbocations, are reactive because they have an...

leaving groups

How Leaving Groups Affect Substitution

Leaving groups affect how nucleophilic substitution reactions happen. In this type of reaction, a nucleophile replaces a functional group on a substrate. The group that leaves is called the leaving group, and it departs when the bond breaks by heterolytic cleavage, taking both electrons with it. A good leaving group is usually a stable weak base. It can leave as an anion or as a neutral molecule. The easier it is for the leaving group to leave, the more it can influence the reaction rate. <br...

carbocations

Carbocation Structure and Stability

Carbocations are reactive carbon species formed during some nucleophilic substitution and elimination reactions. They are electron-deficient intermediates, with the central carbon holding only six electrons and three bonded atoms. Because they are short-lived and highly reactive, they play an important role in many organic reaction pathways. The central carbon in a carbocation is sp2 hybridized and has a trigonal planar shape. It also contains an empty p orbital that sits perpendicular to the...

sn2 reaction: kinetics

SN2 Rate Law and Reaction Mechanism

SN2 reaction kinetics links reaction rate to the concentration of reactants. In a chemical reaction, kinetic studies measure how fast reactants disappear or products appear under fixed conditions such as temperature, pressure, and solvent. These studies help chemists find the rate law, which can give clues about the species involved in the rate-determining step and the reaction mechanism. The rate of a reaction is found experimentally. Scientists withdraw small samples, called aliquots, at...

sn2 reaction: mechanism

SN2 Backside Attack and Inversion

SN2 reactions happen in one step and end with inversion of configuration. The nucleophile and the substrate both take part in the rate-determining step, and no intermediate forms. The substrate contains a polarized carbon-halide bond. The more electronegative halogen pulls the electron cloud toward itself. That leaves the carbon atom electrophilic, or electron-poor, with a partial positive charge. The halide has a partial negative charge. The electrophilic carbon attracts the nucleophile,...

sn2 reaction: transition state

SN2 Reaction: Reactivity and Steric Effects

An SN2 reaction of an alkyl halide happens in one step. The nucleophile forms a bond with the electrophilic carbon while the halide leaves at the same time. No intermediate forms during this process. As the nucleophile approaches with its lone pairs, the leaving group moves away with the electron pair that was bonded to carbon. The transition state is shown with dotted partial bonds and square brackets. This state is very unstable, so it quickly changes into the more favored product state. <br...

sn2 reaction: stereochemistry

SN2 Inversion of Configuration

SN2 reactions cause inversion of configuration at the carbon being substituted. The nucleophile attacks from the back side, while the leaving group departs at the same time. This happens through a transition state, where the carbon changes from tetrahedral to trigonal bipyramidal and then back to tetrahedral. The change in shape helps explain the product stereochemistry. If the substrate is achiral at the alpha-carbon, the inversion is not seen. When the alpha-carbon is chiral, the reversal is...

sn1 reaction: kinetics

SN1 Reaction Rate and Unimolecular Step

SN1 reaction rate depends on the substrate, not the nucleophile. This pattern was identified by Sir Christopher Ingold and Edward D. Hughes while they studied nucleophilic substitution reactions of tertiary alkyl halides. A tertiary alkyl halide can still undergo substitution even when the nucleophile is weak. In their kinetic studies, the rate of product formation stayed the same when the nucleophile concentration changed, as long as the substrate concentration remained constant. That means...

sn1 reaction: mechanism

SN1 Reaction Steps and Carbocation Formation

The SN1 reaction follows a step-by-step pathway that begins with carbocation formation. In a polar protic solvent, a tertiary haloalkane can ionize so that only the substrate takes part in the rate-determining step, which is the slow step. First, the haloalkane undergoes heterolytic cleavage to form a carbocation intermediate and a halide ion. This bond break is highly endothermic and has a large activation energy. The polar protic solvent helps stabilize the ions by solvation, and that makes...

sn1 reaction: stereochemistry

SN1 Pathway and Product Chirality

SN1 reactions can change the chirality of a product because the reaction goes through a carbocation intermediate. First, the bond between the electrophilic carbon and the leaving group ionizes. Then the nucleophile attacks the carbocation. The carbocation formed in an SN1 reaction is sp2 hybridized and has a trigonal planar shape. Its three substituents lie in the same plane, which creates a plane of symmetry. That makes the carbocation achiral, so the nucleophile can approach from either side...

predicting products: sn1 vs. sn2

SN1 and SN2 Reaction Factors

Nucleophilic substitution of alkyl halides can follow an SN1 or SN2 pathway. In an SN2 reaction, the nucleophile attacks as the leaving group leaves at the same time. In an SN1 reaction, the substrate first splits to form a carbocation, which is a positively charged intermediate. The structure of the alkyl halide helps decide which mechanism is more likely. As alkyl substitution increases, steric hindrance also increases, which makes backside attack harder for SN2. More substituted alkyl...

elimination reactions

Elimination reactions of alkyl halides form alkenes by removing groups from adjacent carbons. A base can act on the substrate instead of causing substitution. It deprotonates the neighboring carbon, and the reaction creates at least one pi bond. The carbon bonded to the halogen is the alpha carbon, and the adjacent carbon is the beta carbon. These are also called beta elimination or 1,2-elimination reactions. Strong bases help drive this process. Common examples include hydroxides, alkoxides,...

e2 reaction: kinetics and mechanism

E2 Elimination: Rate, Base, and Solvent

E2 elimination is a concerted reaction of alkyl halides that forms an alkene. In this mechanism, the base removes a beta hydrogen as the halide leaves from the alpha carbon. The two bonds change at the same time, so the products are formed in one step. The rate-limiting transition state has partially broken carbon-hydrogen and carbon-halogen bonds. It also has a partially formed pi bond between the alpha and beta carbons. Because the beta hydrogen and the halide are eliminated together, the...

e2 reaction: stereochemistry and regiochemistry

e2-reaction-stereochemistry-and-regiochemistry

E2 Elimination: Major and Minor Products

E2 elimination reactions of alkyl halides can form one or more alkenes, and the product mix depends on regiochemistry and stereochemistry. Regiochemistry controls where the double bond appears. Stereochemistry controls the shape of the alkene that forms. When a substrate has two different beta hydrogens, a strong base can remove either one. That can give two regioisomeric alkenes. The more substituted alkene is the major product and is called the Zaitsev product. The less substituted alkene is...

e1 reaction: kinetics and mechanism

E1 Elimination: Carbocation Stability and Rates

E1 elimination depends on carbocation stability and reaction rate. It follows a two-step path. The first step is the rate-limiting loss of the leaving group. The second step is the abstraction of the beta hydrogen by a weak base. Experimental support for the E1 mechanism comes from kinetic studies and isotope studies. Kinetic studies show first-order kinetics, which means the rate depends only on substrate concentration. Isotope studies show that hydrogen is removed only in the second step. <br...

e1 reaction: stereochemistry and regiochemistry

e1-reaction-stereochemistry-and-regiochemistry

E1 Elimination: Zaitsev Product and Carbocation

E1 elimination often gives the most substituted alkene, called the Zaitsev product. In the example, water acts as a weak base and favors elimination over substitution, so two alkene products can form. The Zaitsev alkene is usually more stable than the Hofmann product because it has more alkyl groups attached to the double bond. The lower-energy pathway also helps explain why the Zaitsev product is favored. Its transition state has lower energy, so this product is both thermodynamically stable...

predicting products: substitution vs. elimination

predicting-products-substitution-vs-elimination

Choosing Products in Substitution and Elimination

Substitution and beta-elimination compete when a nucleophile reacts with an alkyl halide. The major product depends on the substrate, the nucleophile, the temperature, and the solvent. Because more than one pathway can happen at once, a reaction often gives a product mixture. The structure of the alkyl halide helps predict which mechanism is more likely. Methyl halides and 1° halides usually favor bimolecular reactions, especially SN2. SN1 and E1 are disfavored for 1° halides because the 1°...

key terms and concepts

Explore how alkyl halides form alkenes through E2 and E1 elimination, including beta hydrogen removal, Zaitsev and Hofmann products, and trans selectivity.

E2 vs E1 in Alkyl Halide Elimination | JoVE K12

organic chemistry

JoVE Core 有机化学（Organic Chemistry）使用简明易懂的动画视频课程解释立体化学、反应和机制的基本概念。 此外，科学家在行动视频展示了相关概念在当今世界各地实验室进行的经典和原创研究实验中的应用。

E2 vs E1 in Alkyl Halide Elimination

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