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

Les halogénures d’alkyle, ou haloalcanes, sont des composés organiques composés d’un groupe alkyle et d’un atome d’halogène lié à un carbone hybridé SP3.
Chaque atome de carbone est défini par sa proximité avec l’atome d’halogène. Le carbone lié directement à l’halogène est noté « carbone alpha » ; Le carbone à côté est appelé « bêta-carbone » et ainsi de suite.
En fonction du nombre de groupes alkyle sur le carbone alpha, les halogénures d’alkyle sont en outre classés en primaires, secondaires et tertiaires.
La nomenclature des halogénures d’alkyle suit les mêmes règles de l’UICPA que pour les alcanes. Les halogènes sont considérés comme des substituants et sont nommés fluoro-, chloro-, bromo-, et iodo-.
Tout d’abord, comptez les carbones sur la chaîne la plus longue. Ici, cinq correspond au pentane. Deuxièmement, trouvez et nommez les substituants. Ici, il s’agit de bromo-, chloro-, et méthyl.
Ensuite, numérotez la chaîne mère et attribuez un locant aux substituants en leur donnant le nombre le plus bas possible. Enfin, assemblez les substituants par ordre alphabétique. Ce composé est appelé 4-bromo-1-chloro-4-méthyl pentane.
Si la chaîne contient une double liaison, la double liaison a la priorité et obtient le nombre le plus bas, ce qui donne le composé 3-fluoro-1-butène.
Si une molécule est cyclique, chirale et avec plusieurs substituants, les substituants se voient également attribuer le localisant le plus bas possible, et les groupes sont classés par ordre alphabétique. Les stéréocentres sont indiqués au début du nom, ce qui donne (1R,2 R)-3-butyl-1,2-dichloro-5-éthylcyclohexane.
Les halogénures d’alkyle sont utilisés comme éléments constitutifs dans les synthèses en raison de leur réactivité. En descendant dans le groupe 17, la taille des atomes et la longueur de la liaison de l’halogène augmentent, tandis que l’électronégativité et la basicité diminuent.
L’électronégativité plus élevée des halogènes entraîne le retrait de la densité électronique du carbone adjacent, ce qui rend le carbone alpha électrophile et, par conséquent, plus réactif.
La réactivité de l’halogénure d’alkyle est en outre influencée par la stabilité de la base conjuguée de l’halogène. L’iodure, le bromure et le chlorure sont des bases conjuguées stables mais faibles de leurs acides hydrohaliques forts. En raison de leur stabilité, ils peuvent être plus facilement remplacés par une base plus forte, ce qui en fait de bons groupes de départ.

halogénures d'alkyle

Alkyl Halides

<h4>Propriétés structurelles</h4>

Les halogénures d'alkyle sont des alcanes substitués par un halogène dans lesquels un ou plusieurs atomes d'hydrogène d'un alcane sont remplacés par un atome d'halogène tel que le fluor, le chlore, le brome ou l'iode. L'atome de carbone dans un halogénure d'alkyle est lié à l'atome d'halogène, qui est hybride sp3 et présente une forme tétraédrique.

Contrairement aux halogénures d'alkyle, les composés dans lesquels un atome d'halogène est lié à un atome de carbone hybride sp2 d'une double liaison carbone-carbone (C=C) sont appelés halogénures de vinyle. Alors que les halogénures d'aryle sont des composés dans lesquels un atome d'halogène est lié à un atome de carbone hybride sp2 d'un cycle aromatique.

De plus, en fonction du degré de substitution au niveau de l'atome de carbone contenant l'halogène, les halogénures d'alkyle sont classés en halogénures d'alkyle primaires, secondaires et tertiaires. Le carbone lié à un atome d’halogène est appelé carbone α. L’atome de carbone lié au carbone α est appelé carbone β. Les carbones adjacents sont ensuite désignés par des alphabets grecs (γ, δ, etc.).

<h4>Nomenclature</h4>

La dénomination des halogénures d'alkyle suit les règles générales de la chimie organique de l'IUPAC :

<ol>
	<li>Identifiez la chaîne carbonée la plus longue et nommez-la.</li>
	<li>Recherchez et nommez le(s) substituant(s).</li>
	<li>Numérotez la chaîne carbonée la plus longue et désignez un numéro, ou localisant, pour chaque substituant.</li>
	<li>Assemblez les substituants par ordre alphabétique.</li>
</ol>

<h4>Réactivité et applications</h4>

Les halogénures d'alkyle sont des composés polyvalents qui servent de solvants, de pesticides et d'intermédiaires dans la préparation de colorants, de médicaments et de polymères synthétiques. La réactivité chimique des halogénures d'alkyle diffère selon la classification structurelle. Les halogénures d'alkyle sont classés en halogénures d'alkyle primaires, secondaires et tertiaires en fonction du degré de substitution sur un halogène carboné.

De plus, l’électronégativité de l’atome d’halogène joue un rôle important dans la réactivité de l’halogénure d’alkyle. À l'exception de l'iode, les autres halogènes ont une électronégativité nettement supérieure à celle du carbone. En raison des différences d'électronégativité entre les atomes de carbone et d'halogène, la liaison covalente entre les atomes est polarisée ; ainsi, le carbone porte une charge partielle positive et l’atome d’halogène une charge partielle négative. Par conséquent, le carbone attaché à l’halogène est électrophile.

Dans un tableau périodique, en descendant dans la famille des halogènes, l'électronégativité diminue ; inversement, la taille des atomes d’halogène augmente. Ainsi, la longueur de la liaison carbone-halogène augmente tandis que La puissance et la polarité de la liaison diminuent du fluor à l'iode, ce qui facilite la rupture de la liaison.

Outre ces facteurs, la stabilité relative des bases conjuguées correspondantes, ou anions halogénures, influence également la nature chimique de l'halogénure d'alkyle. La stabilité des anions halogénures peut être mesurée par rapport aux acidités relatives des acides halohydriques. L'acide halohydrique le plus puissant, l'iodure d'hydrogène, a un pKa de -11. Cela signifie qu’il se dissocie complètement de la base conjuguée la plus stable, l’ion iodure et d’un proton. Ainsi, l'iode est un excellent groupe partant. À l'exception de l'acide fluorhydrique avec un pKa de 3,2, les autres acides halohydriques ont des valeurs de pKa inférieures à 0, ce qui en fait des acides forts avec des bases conjuguées faibles et stabilisées, qui sont d'excellents groupes partants.

Halogénures d'alkyle

Propriétés structurelles
Les halogénures d’alkyle sont des alcanes substitués par des halogènes dans lesquels un ou plusieurs atomes d’hydrogène d’un alcane sont remplacés par un atome d’halogène tel que ...

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Organic Chemistry

Chimie organique

Unit 1

Unité 1

Nucleophilic Substitution and Elimination Reactions of Alkyl Halides

Réactions de substitution nucléophile et d'élimination des halogénures d'alkyle

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

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

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 Chimie Organique explique les concepts de base de la stéréochimie, des réactions et des mécanismes à l'aide de leçons vidéo animées concises et faciles à comprendre. De plus, les vidéos de scientifiques à l'action présentent l'application de ces concepts grâce à des expériences de recherche classiques et originales réalisées dans des laboratoires modernes.

Alkyl Halides: Structure and Reactivity

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