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

Los haluros de alquilo, o haloalcanos, son compuestos orgánicos compuestos por un grupo alquilo y un átomo de halógeno unidos a un carbono hibridado sp3.
Cada átomo de carbono se define por su proximidad al átomo de halógeno. El carbono unido directamente al halógeno se denota "carbono alfa"; el carbono que está a su lado se llama "beta-carbono" y así sucesivamente.
Dependiendo del número de grupos alquilo en el carbono alfa, los haluros de alquilo se clasifican a su vez como primarios, secundarios y terciarios.
La nomenclatura de los haluros de alquilo sigue normas de la IUPAC similares a las de los alcanos. Los halógenos se consideran sustitutos y se denominan fluoro-, cloro-, bromo- y yodo-.
Primero, cuenta los carbonos de la cadena más larga. Aquí, cinco corresponde al pentano. En segundo lugar, encuentre y nombre los sustituyentes. Aquí es bromo-, cloro- y metilo.
A continuación, numere la cadena principal y asigne un locante a los sustituyentes dándoles el número más bajo posible. Por último, ensamble los sustitutos alfabéticamente. Este compuesto se llama 4-bromo-1-cloro-4-metilpentano.
Si la cadena contiene un doble enlace, el doble enlace tiene prioridad y obtiene el número más bajo dando el compuesto 3-fluoro-1-buteno.
Si una molécula es cíclica, quiral y con múltiples sustituyentes, a los sustituyentes también se les asigna el locante más bajo posible, y los grupos se ordenan alfabéticamente. Los estereocentros se indican al principio del nombre, dando (1R,2 R)-3-butil-1,2-dicloro-5-etilciclohexano.
Los haluros de alquilo se utilizan como bloques de construcción en las síntesis debido a su reactividad. Al descender en el grupo 17, el tamaño del átomo del halógeno y la longitud del enlace aumentan, mientras que la electronegatividad y la basicidad disminuyen.
La mayor electronegatividad de los halógenos conduce a la retirada de la densidad de electrones del carbono adyacente, lo que hace que el carbono alfa sea electrófilo y, por lo tanto, más reactivo.
La reactividad del haluro de alquilo se ve influida aún más por la estabilidad de la base conjugada del halógeno. El yoduro, el bromuro y el cloruro son bases conjugadas estables pero débiles de sus ácidos hidrohalicos fuertes. Debido a su estabilidad, pueden ser sustituidos más fácilmente por una base más fuerte, lo que los convierte en buenos grupos de salida.

haluros de alquilo

Alkyl Halides

Propiedades estructurales


 Los haluros de alquilo son alcanos sustituidos con halógeno en los que uno o más átomos de hidrógeno de un alcano se reemplazan por un átomo de halógeno tal como flúor, cloro, bromo o yodo. El átomo de carbono en un haluro de alquilo está unido al átomo de halógeno, que tiene hibridación sp3 y exhibe una forma tetraédrica.


 A diferencia de los haluros de alquilo, los compuestos en los que un átomo de halógeno está unido a un átomo de carbono con hibridación sp2 de un doble enlace carbono-carbono (C=C) se denominan haluros de vinilo. Mientras que los haluros de arilo son compuestos en los que un átomo de halógeno está unido a un átomo de carbono con hibridación sp2 de un anillo aromático.


 Además, dependiendo del grado de sustitución en el átomo de carbono que contiene el halógeno, los haluros de alquilo se clasifican en haluros de alquilo primarios, secundarios y terciarios. El carbono unido a un átomo de halógeno se llama carbono α. El átomo de carbono unido al carbono α se llama carbono β. Los carbonos adyacentes se indican posteriormente mediante alfabetos griegos (γ, δ, etc.).


 Nomenclatura


 La denominación de los haluros de alquilo sigue las reglas generales de química orgánica de la IUPAC:


<ol>
 <li>Identifica la cadena de carbono más larga y nómbrala.</li>
 <li>Encuentre y nombre los sustituyentes.</li>
 <li>Numere la cadena de carbonos más larga y designe un número o localizador para cada sustituyente.</li>
 <li>Reúna los sustituyentes en orden alfabético.</li>
 </ol>
 

 Reactividad y Aplicaciones


 Los haluros de alquilo son compuestos versátiles que sirven como disolventes, pesticidas e intermediarios en la preparación de tintes, fármacos y polímeros sintéticos. La reactividad química de los haluros de alquilo difiere según la clasificación estructural. Los haluros de alquilo se clasifican en haluros de alquilo primarios, secundarios y terciarios según el grado de sustitución en un halógeno que contiene carbono.


 Además, la electronegatividad del átomo de halógeno juega un papel importante en la reactividad del haluro de alquilo. A excepción del yodo, otros halógenos tienen una electronegatividad significativamente mayor que el carbono. Debido a las diferencias de electronegatividad entre los átomos de carbono y halógeno, el enlace covalente entre los átomos está polarizado; por tanto, el carbono lleva una carga positiva parcial y el átomo de halógeno una carga negativa parcial. En consecuencia, el carbono unido al halógeno es un electrófilo.


 En una tabla periódica, bajando por la familia de los halógenos, la electronegatividad disminuye; inversamente, el tamaño del átomo de halógeno aumenta. Por lo tanto, la longitud del enlace carbono-halógeno aumenta mientras que la fuerza y ​​la polaridad del enlace disminuyen del flúor al yodo, lo que facilita la ruptura del enlace.


 Además de estos factores, la estabilidad relativa de las correspondientes bases conjugadas o aniones haluro también influye en la naturaleza química del haluro de alquilo. La estabilidad de los aniones haluro se puede medir con respecto a las acidez relativas de los ácidos halogenuros. El ácido hidrohálico más fuerte, el yoduro de hidrógeno, tiene un pKa de -11. Lo que significa que se disocia completamente en la base conjugada más estable, el ion yoduro y un protón. Por tanto, el yodo es un excelente grupo saliente. A excepción del ácido fluorhídrico con un pKa de 3,2, otros ácidos halohídricos tienen valores de pKa inferiores a 0, lo que los convierte en ácidos fuertes con bases conjugadas débiles y estabilizadas, que son excelentes grupos salientes.

Haluros de alquilo

Propiedades estructurales
Los haluros de alquilo son alcanos sustituidos por halógenos en los que uno o más átomos de hidrógeno de un alcano son reemplazados por un átomo de halógeno como ...

Education

Educación

JoVE Core

Organic Chemistry

Química Orgánica

Unit 1

Unidad 1

Nucleophilic Substitution and Elimination Reactions of Alkyl Halides

Reacciones de sustitución nucleofílica y de eliminación de haluros de alquilo

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

KEY TERMS AND CONCEPTS

TÉRMINOS Y CONCEPTOS CLAVE

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

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Alkyl Halides: Structure and Reactivity

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