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

Gli alogenuri alchilici, o aloalcani, sono composti organici costituiti da un gruppo alchilico e da un atomo di alogeno legato a un carbonio ibridato sp3.
Ogni atomo di carbonio è definito dalla sua vicinanza all'atomo alogeno. Il carbonio legato direttamente all'alogeno è indicato con "alfa-carbonio"; Il carbonio accanto ad esso è chiamato "beta-carbonio" e così via.
A seconda del numero di gruppi alchilici sul carbonio alfa, gli alogenuri alchilici sono ulteriormente classificati come primari, secondari e terziari.
La nomenclatura degli alogenuri alchilici segue regole IUPAC simili a quelle degli alcani. Gli alogeni sono considerati sostituenti e denominati fluoro-, cloro-, bromo- e iodo-.
Per prima cosa, conta i carboni sulla catena più lunga. Qui, cinque corrisponde al pentano. In secondo luogo, trova e nomina i sostituenti. Qui è bromo-, cloro- e metile.
Successivamente, numera la catena genitore e assegna un locante ai sostituenti dando loro il numero più basso possibile. Infine, assembla i sostituenti in ordine alfabetico. Questo composto è chiamato 4-bromo-1-cloro-4-metil pentano.
Se la catena contiene un doppio legame, il doppio legame ha la precedenza e ottiene il numero più basso dando il composto 3-fluoro-1-butene.
Se una molecola è ciclica, chirale e con più sostituenti, ai sostituenti viene assegnato il locante più basso possibile e i gruppi sono ordinati alfabeticamente. Gli stereocentri sono indicati all'inizio del nome, dando (1R,2 R)-3-butil-1,2-dicloro-5-etilcicloesano.
Gli alogenuri alchilici sono usati come elementi costitutivi nelle sintesi a causa della loro reattività. Scendendo nel gruppo 17, le dimensioni dell'atomo dell'alogeno e la lunghezza del legame aumentano, mentre l'elettronegatività e la basicità diminuiscono.
La maggiore elettronegatività degli alogeni porta al ritiro della densità elettronica dal carbonio adiacente rendendo il carbonio alfa elettrofilo e, quindi, più reattivo.
La reattività dell'alogenuro alchilico è ulteriormente influenzata dalla stabilità della base coniugata dell'alogeno. Lo ioduro, il bromuro e il cloruro sono basi coniugate stabili ma deboli dei loro forti acidi idroalici. A causa della loro stabilità, possono essere più facilmente sostituiti da una base più forte, rendendoli buoni gruppi uscenti.

alogenuri alchilici

Alkyl Halides

Proprietà strutturali

Gli alogenuri alchilici sono alcani alogeno-sostituiti in cui uno o più atomi di idrogeno di un alcano sono sostituiti da un atomo di alogeno come fluoro, cloro, bromo o iodio. L'atomo di carbonio in un alogenuro alchilico è legato all'atomo di alogeno, che è ibridato sp3 e presenta una forma tetraedrica.

A differenza degli alogenuri alchilici, i composti in cui un atomo di alogeno è legato ad un atomo di carbonio ibridato sp2 con un doppio legame carbonio-carbonio (C=C) sono chiamati alogenuri vinilici. Mentre gli alogenuri arilici sono composti in cui un atomo di alogeno è legato ad un atomo di carbonio ibridato sp2 di un anello aromatico.

Inoltre, a seconda del grado di sostituzione nell'atomo di carbonio contenente l'alogeno, gli alogenuri alchilici sono classificati in alogenuri alchilici primari, secondari e terziari. Il carbonio legato ad un atomo di alogeno è chiamato carbonio α. L'atomo di carbonio legato al carbonio α è chiamato carbonio β. I carboni adiacenti vengono successivamente indicati con lettere greche (γ, δ, e così via).

Nomenclatura

La denominazione degli alogenuri alchilici segue le regole generali IUPAC della chimica organica:

<ol>
	<li>Identifica la catena di carbonio più lunga e darle un nome.</li>
	<li>Trova e dai un nome ai sostituenti.</li>
	<li>Numera la catena di carbonio più lunga e designa un numero, o locante, per ciascun sostituente.</li>
	<li>Assembla i sostituenti in ordine alfabetico.</li>
</ol>

Reattività e applicazioni

Gli alogenuri alchilici sono composti versatili che fungono da solventi, pesticidi e intermedi nella preparazione di coloranti, farmaci e polimeri sintetici. La reattività chimica degli alogenuri alchilici varia a seconda della classificazione strutturale. Gli alogenuri alchilici sono classificati in alogenuri alchilici primari, secondari e terziari a seconda del grado di sostituzione di un alogeno contenente carbonio.

Inoltre, l’elettronegatività dell’atomo di alogeno gioca un ruolo significativo nella reattività dell’alogenuro alchilico. Ad eccezione dello iodio, gli altri alogeni hanno un'elettronegatività significativamente maggiore del carbonio. A causa delle differenze di elettronegatività tra gli atomi di carbonio e di alogeno, il legame covalente tra gli atomi è polarizzato; quindi, il carbonio porta una carica parziale positiva e l'atomo di alogeno una carica parziale negativa. Di conseguenza, il carbonio attaccato all'alogeno è un elettrofilo.

In una tavola periodica, scendendo nella famiglia degli alogeni, l'elettronegatività diminuisce; inversamente, la dimensione dell'atomo di alogeno aumenta. Pertanto, la lunghezza del legame carbonio-alogeno aumenta mentre la forza del legame e la polarità diminuiscono dal fluoro allo iodio, rendendo più facile la rottura del legame.

Oltre a questi fattori, anche la stabilità relativa delle corrispondenti basi coniugate, o anioni alogenuro, influenza la natura chimica dell'alogenuro alchilico. La stabilità degli anioni alogenuro può essere misurata rispetto alle acidità relative degli acidi idroalici. L'acido idroalico più forte, l'acido iodidrico, ha un pKa di -11. Ciò significa che si dissocia completamente dalla base coniugata più stabile, lo ione ioduro e un protone. Pertanto, lo iodio è un eccellente gruppo uscente. Ad eccezione dell'acido fluoridrico con un pKa di 3,2, altri acidi idroalici hanno valori di pKa inferiori a 0, rendendoli acidi forti con basi coniugate deboli e stabilizzate, che sono eccellenti gruppi uscenti.

Alogenuri alchilici

Proprietà strutturali
Gli alogenuri alchilici sono alcani alogenati sostituiti in cui uno o più atomi di idrogeno di un alcano sono sostituiti da un atomo di alogeno come ...

Education

Didattica

JoVE Core

Organic Chemistry

Chimica Organica

Unit 1

Unità 1

Nucleophilic Substitution and Elimination Reactions of Alkyl Halides

Reazioni di sostituzione nucleofila ed eliminazione degli alogenuri alchilici

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

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

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 spiega i concetti di base della stereochimica, delle reazioni e dei meccanismi usando video-lezioni concise e facili da capire. Inoltre, i video scientist-in-action dimostrano l'applicazione di tali concetti in esperimenti di ricerca classici e originali eseguiti al giorno d'oggi nei laboratori in tutto il mondo.

Alkyl Halides: Structure and Reactivity

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