Name: alkyl halides: structure and reactivity
Uploaded: 2023-04-30T18:28:35.000Z
Duration: 2 min 45 s
Channel: JoVE

卤代烷，或称烷基卤化物，是一类由烷基与卤素组成的有机化合物&#160;以及一个与卤素原子成键的 sp3 杂化碳
每个碳原子根据其与卤素原子的相对位置来定义。直接与卤素相连的碳称为“α-碳”；其相邻的碳称为“β-碳”，依此类推。
根据α碳原子上烷基的数量，卤代烷可进一步分为伯、仲和叔卤代烷。
卤代烷的命名遵循与烷烃类似的IUPAC规则。卤素被视为取代基，分别命名为氟代（fluoro-）、氯代（chloro-）、溴代（bromo-）和碘代（iodo-）。
首先，数出最长碳链上的碳原子数。此处五个碳对应戊烷。其次，找出并命名取代基。此处为溴代、氯代和甲基。
接下来，对母链进行编号，并为取代基分配位次，使其位次尽可能小。最后，将取代基按字母顺序排列。该化合物称为4-溴-1-氯-4-甲基戊烷。
如果碳链中含有双键，则双键具有优先权，并应获得最小编号，使该化合物命名为3-氟-1-丁烯。
如果一个分子是环状的、手性的，并且带有多个取代基，则这些取代基同样应被赋予尽可能低的位次编号，并按字母顺序排列。立体中心在名称开头标明，例如 (1R,2R)-3-丁基-1,2-二氯-5-乙基环己烷。
卤代烷因其反应活性而被用作合成中的构建单元。沿元素周期表第17族自上而下，卤素原子的原子半径和键长增加，而电负性和碱性则降低。
卤素较高的电负性导致其从相邻碳原子中吸引电子密度，使α-碳原子具有亲电性，因而更具反应活性。
卤代烃的反应活性还受到卤素共轭碱稳定性的影响。碘离子、溴离子和氯离子分别是其强氢卤酸的稳定但弱的共轭碱。由于这些离子具有较高的稳定性，它们更容易被更强的碱取代，因此是良好的离去基团。

烷基卤化物

Alkyl Halides

<h4>结构特性</h4>


 烷基卤化物是卤素取代的烷烃，其中烷烃的一个或多个氢原子被卤素原子如氟、氯、溴或碘取代。 卤代烷中的碳原子与卤原子键合，为 sp3 杂化，呈四面体形状。



 与卤代烷不同，卤原子与碳-碳双键（C=C）的 sp2 杂化碳原子键合的化合物称为卤乙烯。 而芳基卤化物是其中卤素原子键合至芳环的 sp2 杂化碳原子的化合物。



 此外，根据含有卤素的碳原子的取代度，烷基卤化物分为伯烷基卤化物、仲烷基卤化物和叔烷基卤化物。 与卤素原子键合的碳称为α-碳。 与α-碳相连的碳原子称为β-碳。 相邻的碳随后用希腊字母表示（γ、δ 等）。


 <h4>命名法</h4>


 卤代烷的命名遵循有机化学的一般 IUPAC 规则:


 <ol>
 <li>找出最长的碳链并命名。</li>
 <li>找到并命名取代基。</li>
 <li>给最长的碳链编号并为每个取代基指定一个数字或位置。</li>
 <li>按字母顺序组装取代基。</li>
 </ol>

 <h4>活性和应用</h4>


 卤代烷是用途广泛的化合物，可用作溶剂、杀虫剂以及制备染料、药物和合成聚合物的中间体。 卤代烷的化学反应性因结构分类而异。 根据含碳卤素的取代度，烷基卤分为伯烷基卤、仲烷基卤和叔烷基卤。



 此外，卤素原子的电负性对卤代烷的活性起着重要作用。 除碘外，其他卤素的电负性明显大于碳。 由于碳原子和卤素原子之间的电负性差异，原子间的共价键发生极化; 因此，碳带有部分正电荷，卤素原子带有部分负电荷。 因此，连接到卤素上的碳是亲电子试剂。



 在元素周期表中，沿着卤素族向下，电负性降低; 相反，卤素原子尺寸增加。 因此，从氟到碘，碳-卤键长度增加，而键强度和极性降低，从而更容易断裂键。



 除了这些因素之外，相应的共轭碱或卤化物阴离子的相对稳定性也会影响烷基卤化物的化学性质。 卤化物阴离子的稳定性可以根据氢卤酸的相对酸度来测量。 最强的氢卤酸是碘化氢，pKa 为-11。 这意味着它完全解离成最稳定的共轭碱、碘离子和质子。 因此，碘是一种极好的离去基团。 除氢氟酸的 pKa 为 3.2 外，其他氢卤酸的 pKa 值均小于 0，这使得它们成为具有弱且稳定的共轭碱的强酸，是优异的离去基团。

结构特性  
卤代烷是氢原子被卤素原子（如……）取代的烷烃，其中一个或多个氢原子被卤素原子取代

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

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 aromatic ring, the compound is an aryl halide. Alkyl halides are also grouped as primary, secondary, or tertiary based on how substituted the carbon bearing the halogen is.

The carbon attached to the halogen is called the alpha-carbon. The carbon next to it is the beta-carbon. The nearby carbons are then labeled with Greek letters such as gamma and delta. Alkyl halides are named using standard IUPAC rules. First, identify the longest carbon chain and name it. Then name the substituents, assign locants, and list the substituents in alphabetical order.

Alkyl halides are useful as solvents, pesticides, and starting materials for dyes, drugs, and synthetic polymers. Their reactivity depends on their structure and on the halogen atom. Because halogens are usually more electronegative than carbon, the carbon-halogen bond is polarized. This gives the carbon a partial positive charge, so it acts as an electrophile.

Halogen trends also matter. Moving down the halogen family, electronegativity decreases and atomic size increases. As a result, the carbon-halogen bond gets longer, weaker, and less polar from fluorine to iodine, so it is easier to break. The stability of the matching halide anion also affects reactivity. Hydroiodic acid is the strongest hydrohalic acid, with a pKa of -11, so iodide is a very stable conjugate base and an excellent leaving group. With the exception of hydrofluoric acid, which has a pKa of 3.2, the other hydrohalic acids have pKa values below 0 and form weak, stabilized conjugate bases that leave easily.

Alkyl Halides: Structure and Reactivity

alkyl halides

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

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,...

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

Learn how alkyl halides are built, named, and classified, and why bond polarity and leaving-group strength affect their reactivity in organic chemistry.

Alkyl Halides: Structure and Reactivity | 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。

Alkyl Halides: Structure and Reactivity

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