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

ハロゲン化アルキル、またはハロアルカンは、アルキル基とsp3混成炭素に結合したハロゲン原子からなる有機化合物です。
各炭素原子は、ハロゲン原子への近接性によって定義されます。ハロゲンに直接結合した炭素は「アルファカーボン」と表記されます。その隣にある炭素は「ベータカーボン」などと呼ばれます。
アルファ炭素上のアルキル基の数に応じて、ハロゲン化アルキルはさらに一次、二次、および三次に分類されます。
ハロゲン化アルキルの命名法は、アルカンと同様のIUPACルールに従います。ハロゲンは置換基とされ、フルオロ、クロロ、ブロモ、ヨードと呼ばれています。
まず、最も長い鎖の炭素を数えます。ここで、5はペンタンに対応します。次に、置換基を見つけて名前を付けます。ここでは、ブロモ、クロロ、メチルです。
次に、親鎖に番号を付け、置換基にローカントを割り当てて、可能な限り低い番号を与えます。最後に、置換基をアルファベット順に組み立てます。この化合物は4-ブロモ-1-クロロ-4-メチルペンタンと呼ばれます。
鎖が二重結合を含んでいれば、二重結合が優先され、化合物3-フルオロ-1-ブテンを与える最も低い数を得る。
分子が環状でキラルで、複数の置換基を持つ場合、置換基にも同様に可能な限り低い位置が割り当てられ、グループはアルファベット順に並べられます。立体中心は名前の冒頭に示され、(1R、2R)-3-ブチル-1,2-ジクロロ-5-エチルシクロヘキサンが与えられます。
ハロゲン化アルキルは、その反応性のために合成のビルディングブロックとして使用されます。グループ17を下回ると、ハロゲンの原子サイズと結合長が増加し、電気陰性度と塩基性度は減少します。
ハロゲンの電気陰性度が高いと、隣接する炭素から電子密度が引き出され、アルファ炭素が求電子性になり、したがって反応性が向上します。
ハロゲン化アルキルの反応性は、ハロゲンの共役塩基の安定性によってさらに影響を受けます。ヨウ化物、臭化物、および塩化物は、それらの強ハロゲン酸の安定でありながら弱い共役塩基です。その安定性により、より強力なベースでより簡単に置き換えることができ、グループを離れるのに適しています。

ハロゲン化アルキル

Alkyl Halides

構造の特性

ハロゲン化アルキルは、アルカンの 1 つ以上の水素原子がフッ素、塩素、臭素、ヨウ素などのハロゲン原子で置換されたハロゲン置換アルカンです。 ハロゲン化アルキルの炭素原子はハロゲン原子に結合しており、sp3 混成して四面体の幾何学的配置を示します。

ハロゲン化アルキルとは異なり、炭素間二重結合 (C=C) の sp2 混成炭素原子にハロゲン原子が結合した化合物はハロゲン化ビニルと呼ばれます。 一方、ハロゲン化アリールは、芳香環のsp2混成炭素原子にハロゲン原子が結合した化合物です。

さらに、ハロゲンを含む炭素原子の置換度に応じて、ハロゲン化アルキルは、第一級、第二級、第三級のハロゲン化アルキルに分類される。 ハロゲン原子に結合した炭素をα-炭素といいます。 α炭素に結合した炭素原子をβ炭素といいます。 隣接する炭素は、ギリシャ文字 (γ、δ など) で表示されます。

命名法

ハロゲン化アルキルの命名は、有機化学の一般的な IUPAC 規則に基づいています。

<ol>
	<li>最も長い炭素鎖を特定し、それに名前を付けます。</li>
	<li>置換基を見つけて名前を付けます。</li>
	<li>最長の炭素鎖に番号を付け、各置換基の番号または位置を指定します。</li>
	<li>置換基をアルファベット順に組み立てます。</li>
</ol>

反応性と応用

ハロゲン化アルキルは、溶媒、殺虫剤、染料、薬剤、合成ポリマーの製造における中間体として機能する多用途の化合物です。 ハロゲン化アルキルの化学反応性は構造分類によって異なります。 ハロゲン化アルキルは、炭素含有ハロゲンの置換度に応じて、第一級、第二級、および第三級ハロゲン化アルキルに分類されます。

さらに、ハロゲン原子の電気陰性度は、ハロゲン化アルキルの反応性に重要な役割を果たします。 ヨウ素を除いて、他のハロゲンは炭素よりも大幅に大きな電気陰性度を持っています。 炭素原子とハロゲン原子の間の電気陰性度の違いにより、原子間の共有結合は分極します。 したがって、炭素は部分的に正電荷を帯び、ハロゲン原子は部分的に負電荷を帯びます。 したがって、ハロゲンに結合した炭素は求電子剤となります。

周期表では、ハロゲン族に行くほど電気陰性度は減少します。 逆に、ハロゲン原子のサイズは増加します。 したがって、炭素とハロゲンの結合長は増加しますが、結合の強度と極性はフッ素からヨウ素に減少するため、結合が切断されやすくなります。

これらの要因とは別に、対応する共役塩基またはハロゲン化物アニオンの相対的な安定性もハロゲン化アルキルの化学的性質に影響を与えます。 ハロゲン化物アニオンの安定性は、ハロゲン化水素酸の相対的な酸性度に関して測定できます。 最も強いハロゲン化水素酸であるヨウ化水素の pKa は -11 です。 これは、最も安定な共役塩基であるヨウ化物イオンとプロトンに完全に解離することを意味します。 したがって、ヨウ素は優れた脱離基です。 pKa 3.2 のフッ化水素酸を除いて、他のハロゲン化水素酸は pKa 値が 0 未満であり、優れた脱離基である弱く安定した共役塩基を持つ強酸になります。

構造特性
ハロゲン化アルキルは、アルカンの1つ以上の水素原子が次のようなハロゲン原子に置き換えられたハロゲン置換アルカンです...

Education

JoVE Core

Organic Chemistry

有機化学

Unit 1

ユニット1

Nucleophilic Substitution and Elimination Reactions of Alkyl Halides

ハロゲン化アルキルの親核置換反応および脱離反応

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

KEY TERMS AND CONCEPTS

主要用語と概念

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 有機化学では、立体化学、反応、メカニズムの基本概念を、簡潔でわかりやすいアニメーション ビデオ レッスンを使用して説明します。さらに、科学者の実際のビデオでは、世界中の今日の研究室で行われている古典的および独自の研究実験における関連概念の適用を示しています。

Alkyl Halides: Structure and Reactivity

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