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

알킬 할리데스 또는 할랄카네인은 알킬 그룹과 Sp3 혼성 탄소에 결합된 할로겐 원자로 구성된 유기 화합물입니다.
각 탄소 원자는 할로겐 원자에 근접하여 정의됩니다. 할로겐에 직접 결합된 탄소는 "알파 카본"으로 표시됩니다. 그 옆에 있는 탄소는 "베타 탄소"라고 불립니다.
알파 탄소에 알킬 그룹의 수에 따라, 알킬 할라이드는 더 기본으로 분류, 보조, 및 고등.
알킬 할리데스의 명칭은 알칸과 유사한 IUPAC 규칙을 따릅니다. 할로겐은 대체체로 간주되며 불소, 클로로, 브로모 및 요오도-명명된다.
첫째, 가장 긴 사슬에서 탄소를 계산합니다. 여기서 5개는 펜탄에 해당합니다. 둘째, 대체 체를 찾아 이름을 지정합니다. 여기에 브로모, 클로로, 메틸입니다.
다음으로, 상위 체인의 번호를 매기고 대체자들에게 로캔을 할당하여 가능한 가장 낮은 숫자를 제공합니다. 마지막으로 대체체를 알파벳 순으로 조립합니다. 이 화합물은 4-브로모-1-클로로-4-메틸 펜탄이라고 합니다.
체인에 이중 결합이 포함되어 있는 경우 이중 결합이 우선하며 화합물 3-플루오로-1-부틴을 주는 가장 낮은 숫자를 얻습니다.
분자가 순환, 키랄 및 다중 대체체인 경우 대체체도 마찬가지로 가능한 가장 낮은 locant를 할당하고 그룹은 알파벳 순으로 정렬됩니다. 스테레오센터는 이름의 시작 부분에 표시되어(1R,2 R)-3-butyl-1,2-dichloro-5-에틸시클로헥산이 있습니다.
알킬 할리데스는 반응성 때문에 합성의 빌딩 블록으로 사용됩니다. 그룹 17을 내려가면 할로겐의 원자 크기와 결합 길이가 증가하고 전기성 및 기본성이 감소합니다.
할로겐의 높은 전기성성은 인접한 탄소로부터 전자 밀도의 철수로 이어지며 알파 탄소 전기필릭을 만들고, 따라서 더 반응성있게 된다.
알킬 할라이드의 반응성은 할로겐의 컨쥬게이트 기지의 안정성에 의해 더욱 영향을 받습니다. 요오드, 브로마이드 및 염화물은 강한 하이드로할산의 안정적이면서도 약한 컨쥬게이트 베이스입니다. 그들의 안정성 때문에, 그들은 더 쉽게 더 강한 기지에 의해 대체 될 수 있습니다., 그들을 좋은 떠나는 그룹 만들기.

알킬 할라이드

Alkyl Halides

구조적 특성


알킬 할라이드는 알칸의 하나 이상의 수소 원자가 불소, 염소, 브롬 또는 요오드와 같은 할로겐 원자로 대체된 할로겐 치환 알칸입니다. 할로겐화 알킬의 탄소 원자는 할로겐 원자에 결합되어 있으며, 이는 sp3-혼성화되어 사면체 모양을 나타냅니다.


알킬 할라이드와 달리 할로겐 원자가 탄소-탄소 이중 결합(C=C)의 sp2-혼성 탄소 원자에 결합된 화합물을 비닐 할라이드라고 합니다. 아릴 할라이드는 할로겐 원자가 방향족 고리의 sp2-혼성화된 탄소 원자에 결합된 화합물입니다.


또한, 할로겐을 함유한 탄소 원자의 치환 정도에 따라 알킬 할라이드는 1차, 2차, 3차 알킬 할라이드로 분류됩니다. 할로겐 원자에 결합된 탄소를 α-탄소라고 합니다. α-탄소에 연결된 탄소 원자를 β-탄소라고 합니다. 인접한 탄소는 이후 그리스 알파벳(γ, δ 등)으로 표시됩니다.


명명법

할로겐화 알킬의 명명은 유기 화학의 일반적인 IUPAC 규칙을 따릅니다.



<ol style="display: flex; flex-direction: column; gap: 10px;">
 <li>가장 긴 탄소 사슬을 식별하고 이름을 지정합니다.</li>
 <li>치환체를 찾아 이름을 지정합니다.</li>
 <li>가장 긴 탄소 사슬에 번호를 매기고 각 치환기에 대한 번호 또는 위치를 지정합니다.</li>
 <li>치환기를 알파벳순으로 조립합니다.</li>
</ol>

반응성과 응용


할로겐화 알킬은 염료, 약물, 합성 고분자 제조 시 용매, 살충제, 중간체 역할을 하는 다용도 화합물입니다. 할로겐화 알킬의 화학 반응성은 구조 분류에 따라 다릅니다. 알킬 할라이드는 탄소 함유 할로겐의 치환 정도에 따라 1차, 2차, 3차 알킬 할라이드로 분류됩니다.


또한, 할로겐 원자의 전기음성도는 할로겐화 알킬의 반응성에 중요한 역할을 합니다. 요오드를 제외한 다른 할로겐은 탄소보다 전기음성도가 훨씬 더 큽니다. 탄소와 할로겐 원자 사이의 전기 음성도 차이로 인해 원자 사이의 공유 결합은 분극화됩니다. 따라서 탄소는 부분적인 양전하를 띠고 할로겐 원자는 부분적인 음전하를 띤다. 결과적으로 할로겐에 부착된 탄소는 친전자체입니다.


주기율표에서 할로겐족으로 내려갈수록 전기음성도는 감소합니다. 반대로 할로겐 원자 크기는 증가합니다. 따라서 탄소-할로겐 결합 길이는 증가하는 반면 결합 강도와 극성은 불소에서 요오드로 감소하여 결합이 더 쉽게 끊어집니다.


이러한 요인 외에도 해당 짝염기 또는 할로겐화물 음이온의 상대적 안정성도 알킬 할로겐화물의 화학적 성질에 영향을 미칩니다. 할로겐화물 음이온의 안정성은 할로겐화수소산의 상대적 산도와 관련하여 측정할 수 있습니다. 가장 강한 할로겐화수소산인 요오드화수소의 pKa는 -11입니다. 이는 가장 안정적인 짝염기, 요오드화물 이온 및 양성자로 완전히 해리된다는 의미입니다. 따라서 요오드는 우수한 이탈기이다. pKa가 3.2인 불화수소산을 제외하고 다른 할로겐화수소산은 pKa 값이 0 미만이므로 약하고 안정화된 짝염기를 갖는 강산이 되며 이는 우수한 이탈기입니다.

구조적 특성
알킬 할라이드는 할로겐 치환 된 알칸이며, 여기서 알칸의 하나 이상의 수소 원자는 다음과 같은 할로겐 원자로 대체됩니다.

Education

교육

JoVE Core

Organic Chemistry

유기 화학

Unit 1

제1단원

Nucleophilic Substitution and Elimination Reactions of Alkyl Halides

알킬 할라이드의 친핵성 치환 및 제거 반응

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

KEY TERMS AND CONCEPTS

주요 용어 및 개념

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 Organic Chemistry explains basic concepts of stereochemistry, reactions, and mechanisms using concise and easy-to-understand animated video lessons. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

Alkyl Halides: Structure and Reactivity

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