Name: benzylic halogenation and radical selectivity
Uploaded: 2025-05-22T09:05:43.000Z
Duration: 1 min 11 s
Channel: JoVE

Side-chain halogenation at the benzylic position occurs through radical reactions in the presence of heat, light, or a radical initiator like peroxide.
For example, benzylic chlorination of toluene occurs in the presence of heat and light. An excess of chlorine drives multiple substitutions. 
Similarly, benzylic bromination is achieved by using N-bromosuccinimide, or NBS, in the presence of a peroxide catalyst.
Halogenation of larger alkyl side chains is highly regioselective and occurs exclusively at the benzylic position. 
For instance, bromination of ethylbenzene at the benzylic position gives a monobromo product. Similarly, chlorination of ethylbenzene gives 1-chloro-1-phenylethane as the primary product.
The high regioselectivity is dictated by the resonance stabilization of the benzylic radical intermediate.
Notably, halogenation helps introduce a functional group at the benzylic position, which could be exchanged for a different group.

reactions at the benzylic position: halogenation

Reactions at the Benzylic Position: Halogenation

Benzylic halogenation takes place under conditions that favor radical reactions such as heat, light, or a free radical initiator like peroxide.
<img alt="Aromatic substitution mechanism, Cl2 and NBS react with toluene; chemical reaction diagram." src="/files/ftp_upload/12873/12873_PT_Figure_1.svg" style="text-align: center; height: 165px;" data-altupdatedat="1754904545000" data-alt="Benzyl halogenation diagram, chlorine or NBS, aromatic compound reaction pathways." />
The reaction of toluene with an excess of chlorine can produce multiple benzylic chlorinations. However, the reaction of N-bromosuccinimide or NBS with toluene in the presence of a peroxide forms benzyl bromide. Halogenation of larger alkyl side chains are highly regioselective and occur primarily at the benzylic position. Bromination of ethylbenzene at the benzylic position solely gives a monobromo organic product. Whereas chlorination of ethylbenzene gives 1-chloro-1-phenylethane as the major product in the ratio of 9:1. The regioselectivity of halogenation reaction can be explained by the resonance stabilization of the benzylic radical intermediate. Benzylic halogenation is important because halogen substituted at the benzylic position can further be replaced by a different group.
<img alt="Aromatic substitution, chemical reaction, NBS bromination, sodium ethoxide, conversion diagram." src="/files/ftp_upload/12873/12873_PT_Figure_2.svg" style="text-align: center; height: 60px;" data-altupdatedat="1754904545000" data-alt="Bromination of toluene to bromoethyl benzene via NBS, followed by substitution with NaOEt in EtOH." />

Benzylic halogenation takes place under conditions that favor radical reactions such as heat, light, or a free radical initiator like peroxide.

The ...

reactions-at-the-benzylic-position-halogenation

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JoVE Core

Organic Chemistry

Unit 1

Reactions of Aromatic Compounds

KEY TERMS AND CONCEPTS

benzylic halogenation and radical selectivity

Benzylic halogenation is a radical reaction that occurs at the carbon next to a benzene ring. It is favored by heat, light, or a free radical initiator such as peroxide. Because the benzylic position forms a stable radical, the reaction often happens there first.

Toluene shows this pattern clearly. With excess chlorine, it can undergo multiple chlorinations at the benzylic position. In contrast, N-bromosuccinimide, or NBS, reacts with toluene in the presence of peroxide to form benzyl bromide.

Side chains on larger alkylbenzenes are also halogenated mainly at the benzylic position. Bromination of ethylbenzene gives only a monobromo product. Chlorination of ethylbenzene is less selective and gives 1-chloro-1-phenylethane as the major product in a 9:1 ratio.

This regioselectivity comes from resonance stabilization of the benzylic radical intermediate. Benzylic halogenation is useful because the halogen at the benzylic position can later be replaced by a different group.

Benzylic Halogenation and Radical Selectivity

benzylic-halogenation-and-radical-selectivity

nmr spectroscopy of benzene derivatives

Benzene Ring Signals in NMR Spectra

Benzene ring signals in NMR spectra show how substitution changes aromatic proton and carbon peaks. In 1 H NMR, simple benzene has six equivalent aromatic protons. It gives one singlet at about 7.3 ppm. That signal is far downfield because the aromatic ring current strongly deshields the protons. When a benzene ring has a substituent, the aromatic protons are no longer equivalent. They split one another, so the peak is no longer a singlet. The splitting pattern and the coupling constants...

reactions at the benzylic position: oxidation and reduction

reactions-at-the-benzylic-position-oxidation-and-reduction

Benzylic Oxidation and Selective Reduction

Benzylic oxidation and selective reduction describe how side chains next to a benzene ring react under specific conditions. The benzylic position is the carbon atom directly attached to the ring. Strong oxidizing agents such as KMnO4 or H2CrO4 can act on that carbon, while benzene itself does not undergo oxidation. Alkylbenzenes can be oxidized to benzoic acid, no matter what alkyl group is attached. Halogen and nitro groups on the benzene ring are left unchanged by these oxidizing agents. If...

Benzylic halogenation is a radical reaction that occurs at the carbon next to a benzene ring. It is favored by heat, light, or a free radical initiator such as peroxide. Because the benzylic position forms a stable radical, the reaction often happens there first. Toluene shows this pattern clearly. With excess chlorine, it can undergo multiple chlorinations at the benzylic position. In contrast, N-bromosuccinimide, or NBS, reacts with toluene in the presence of peroxide to form benzyl bromide.<br...

electrophilic aromatic substitution: overview

electrophilic-aromatic-substitution-overview

Arenium Ion Formation in Aromatic Reactions

Electrophilic aromatic substitution changes an aromatic compound by replacing a hydrogen with an electrophile. These reactions can add many different functional groups to an aromatic ring. They are important in organic chemistry because they let chemists modify aromatic compounds in a controlled way. The mechanism happens in two steps. First, the pi system of the aromatic ring reacts with the electrophile and forms an arenium ion, which is resonance-stabilized. This intermediate is also called...

electrophilic aromatic substitution: chlorination and bromination of benzene

electrophilic-aromatic-substitution-chlorination-bromination

Benzene Halogenation with Lewis Acid Catalysts

Benzene halogenation with Lewis acid catalysts shows how benzene reacts with chlorine or bromine to form halogenated substitution products. Chlorination uses a Lewis acid such as aluminium chloride or ferric chloride. Bromination uses ferric bromide as the catalyst. In bromination of benzene, the bromine molecule first reacts with ferric bromide. It donates a pair of electrons to the Lewis acid, which makes the Br–Br bond more polar and creates a more reactive electrophile. Benzene then...

electrophilic aromatic substitution: fluorination and iodination of benzene

electrophilic-aromatic-substitution-fluorination-iodination

Benzene Fluorination and Iodination Reactions

Benzene fluorination and iodination are examples of electrophilic aromatic substitution. These reactions add a halogen to an aromatic ring, but they are harder to carry out than bromination or chlorination. The video focuses on why fluorine and iodine behave differently in these substitutions. Fluorination is difficult because fluorine is extremely reactive. Its reaction with benzene is hard to control, so the yield of monofluoroaromatic products is poor. To solve this problem, the reaction...

electrophilic aromatic substitution: nitration of benzene

electrophilic-aromatic-substitution-nitration-of-benzene

Making Nitrobenzene from Benzene

Benzene can be converted into nitrobenzene through nitration, a type of electrophilic aromatic substitution. In this reaction, a very powerful electrophile called the nitronium ion attacks the aromatic ring. The nitronium ion is linear in shape and drives the substitution step. The nitronium ion forms when sulfuric acid and nitric acid interact. Sulfuric acid is the stronger acid, so it protonates nitric acid on the hydroxyl group. Water then leaves, and the nitronium ion is generated. Next,...

electrophilic aromatic substitution: sulfonation of benzene

electrophilic-aromatic-substitution-sulfonation-of-benzene

Benzene Sulfonation with Fuming Sulfuric Acid

Benzene sulfonation is a reaction that turns benzene into benzenesulfonic acid. It happens when benzene is treated with fuming sulfuric acid at room temperature. Fuming sulfuric acid is a mixture of sulfur trioxide and concentrated sulfuric acid. The key electrophile, or electron-seeking particle, can be neutral sulfur trioxide or protonated sulfur trioxide ion. Which form reacts depends on the reaction conditions. In one pathway, neutral sulfur trioxide attacks the π electron cloud of benzene.

electrophilic aromatic substitution: friedel&ndash;crafts alkylation of benzene

electrophilic-aromatic-substitution-friedelcrafts-alkylation

Benzene Alkylation with Friedel-Crafts Reagents

Benzene alkylation with Friedel-Crafts reagents is a form of electrophilic aromatic substitution. In this reaction, an aromatic proton is replaced by an alkyl group. A Lewis acid catalyst such as aluminum chloride helps drive the process. The reaction begins when an alkyl halide reacts with aluminum chloride. This Lewis-acid-base step forms a carbocation, which is the electrophile. The carbocation then attacks the pi electron cloud of benzene and forms a resonance-stabilized arenium ion. <br...

electrophilic aromatic substitution: friedel&ndash;crafts acylation of benzene

electrophilic-aromatic-substitution-friedelcrafts-acylation-of-benzene

Friedel-Crafts Acylation: Making Aryl Ketones

Friedel-Crafts acylation adds an acyl group to an aromatic ring. It is a type of electrophilic aromatic substitution. In this reaction, an acyl chloride and a Lewis acid catalyst, such as aluminum chloride, are used to form an aryl ketone. The mechanism begins when the Lewis acid forms a complex with the acyl chloride. This complex then breaks the carbon-chlorine bond to form an acylium ion. The acylium ion has a positive charge on carbon and is stabilized by resonance. It acts as the...

limitations of friedel&ndash;crafts reactions

Friedel-Crafts Alkylation and Acylation Limits

Friedel-Crafts alkylation and acylation have several important limits in aromatic chemistry. The alkyl halide used in Friedel-Crafts alkylation must have the halogen on an sp3-hybridized carbon. Vinyl halides and aryl halides do not react because the carbocations they would form are unstable under the reaction conditions. Carbocation rearrangement is another issue in Friedel-Crafts alkylation. The major products can have a rearranged carbon skeleton because the carbocation shifts before the...

directing effect of substituents: ortho&ndash;para-directing groups

directing-effect-of-substituents-orthopara-directing-groups

Electron-Donating Groups on Benzene Rings

Electron-donating groups on a benzene ring can steer new electrophiles to the ortho and para positions. These substituents make the ring more electron-rich, so the ring is more likely to react with an electrophile. In aromatic substitution, ortho means next to the substituent, and para means across from it. Groups such as amino, hydroxy, and alkoxy act this way because they have lone pairs on the atom next to the ring. They donate electrons through resonance, which helps spread electron...

directing effect of substituents: meta-directing groups

directing-effect-of-substituents-meta-directing-groups

Why Some Groups Favor Meta Substitution

Meta-directing groups on a benzene ring guide an incoming electrophile to the meta position. These substituents usually have a positive charge on the atom bonded to the ring, or a partial positive charge. They pull electron density away from the ring through inductive and resonance effects. Nitrobenzene shows why meta substitution is favored. When an electrophile adds at the ortho, meta, or para position, carbocation intermediates form. All three intermediates have resonance forms. The ortho...

ortho&ndash;para-directing activators: &ndash;ch3, &ndash;oh, &ndash;&nobreak;nh2, &ndash;och3

orthopara-directing-activators-ch3-oh-nh2-och3

Why Activating Groups Speed Up Ortho and Para

Activating groups on an aromatic ring can speed up electrophilic substitution and direct new groups to the ortho and para positions. Excluding halogens, these ortho-para directors are electron-donating groups. They make the ring carbons more electron-rich and more reactive. A clear example is anisole, which contains a methoxy group. Its electron-donating effect activates the ortho and para positions on the ring. The group also stabilizes the matching intermediates by resonance, which means...

ortho&ndash;para-directing deactivators: halogens

orthopara-directing-deactivators-halogens

Halogen Effects in Aromatic Substitution

Halogens affect aromatic substitution by changing both the reactivity and the position of incoming electrophiles. As ring substituents, they are more electronegative than carbon. This lets them pull electrons away from the ring through the inductive effect, which deactivates the aromatic ring toward electrophilic substitution. Halogens also have an electron-donating resonance effect on the ring. This resonance effect helps direct the incoming electrophile to the ortho and para positions. When...

meta-directing deactivators: &ndash;no2, &ndash;cn, &ndash;cho, &ndash;&nobreak;co2r, &ndash;cor, &ndash;co2h

meta-directing-deactivators-no2-cn-cho-co2r-cor-co2h

Why Meta Directors Slow Aromatic Reactions

Meta-directing groups slow down electrophilic aromatic substitution on an aromatic ring. These substituents are deactivating because they withdraw electrons from the ring. As a result, the ring becomes less reactive toward electrophiles. A clear example is nitrobenzene. Its nitration is about 100,000 times slower than the nitration of benzene. The nitro group causes this strong deactivating effect. The first step in electrophilic aromatic substitution is the addition of an electrophile to...

directing and steric effects in disubstituted benzene derivatives

directing-and-steric-effects-in-disubstituted-benzene-derivatives

How Substituents Control Benzene Reactions

Disubstituted benzenes can react by electrophilic substitution, and the two substituents help decide where the new group goes. Their directing effects may point to the same position or to different positions. Steric effects, which come from crowding between groups, also influence the product. When both substituents reinforce each other, a single product can form. In bromination of p-nitrotoluene, the bromine enters ortho to the methyl group and meta to the nitro group. Those directions lead to...

nucleophilic aromatic substitution: addition&ndash;elimination (snar)

nucleophilic-aromatic-substitution-additionelimination-snar

Meisenheimer Complex in Aromatic Substitution

Nucleophilic aromatic substitution can happen on aromatic compounds that carry strong electron-withdrawing groups. These groups must be placed ortho or para to the leaving group. The reaction follows an addition–elimination path and depends on the ring being activated for substitution. The reaction starts when the nucleophile attacks the carbon that bears the leaving group. This first step spreads the π electrons over the ring carbons. The resulting resonance helps stabilize the intermediate. <br...

nucleophilic aromatic substitution: elimination&ndash;addition

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Benzyne Route in Aromatic Substitution

Benzyne can drive nucleophilic aromatic substitution when simple aryl halides are exposed to strong base or high heat. In this pathway, the ring does not react by the usual route. Instead, it forms a very reactive intermediate called benzyne. The mechanism begins with a strong base removing a proton from the aromatic ring. This creates a carbanionic center, which is a carbon atom carrying a negative charge. The halide then leaves, and the benzyne intermediate forms through an elimination step.<br...

nucleophilic aromatic substitution of aryldiazonium salts: aromatic sn1

nucleophilic-aromatic-substitution-of-aryldiazonium-salts-aromatic-sn1

Aryldiazonium Salt Reactions in Aromatic Substitution

Aryldiazonium salts are useful starting materials in aromatic substitution reactions. They form when arylamines are treated with nitrous acid. In these salts, the diazonio group can be replaced by other nucleophiles to make many substituted benzenes. A key reason this reaction works is that the leaving group departs as nitrogen gas. That easy loss of nitrogen helps drive the substitution forward. In the Sandmeyer reaction, the diazonio group is replaced by chloro, bromo, or cyano groups. The...

reduction of benzene to cyclohexane: catalytic hydrogenation

reduction-of-benzene-to-cyclohexane-catalytic-hydrogenation

Why Benzene Resists Hydrogenation

Benzene resists catalytic hydrogenation much more than an alkene does. When stilbene is reduced, the alkene bond is hydrogenated while the benzene ring stays unchanged. Alkene hydrogenation is exothermic and happens easily under mild conditions. Benzene behaves differently because its ring is resonance stabilized. That extra stability makes the first step of hydrogenation endothermic, so energy must be added. As a result, benzene does not react well with regular hydrogenation conditions. To...

benzene to 1,4-cyclohexadiene: birch reduction mechanism

benzene-to-14-cyclohexadiene-birch-reduction-mechanism

Birch Reduction of Benzene Explained

Birch reduction converts benzene into 1,4-cyclohexadiene using solvated electrons as the reducing agent. The reaction changes the aromatic ring by adding electrons and protons in a stepwise process. It is a key example of how an aromatic compound can be partially reduced. The mechanism begins when a single electron transfers to the ring and forms a benzene radical anion. A radical anion is a species with both a radical and a negative charge. This intermediate is very basic, so it removes a...

hydrolysis of chlorobenzene to phenol: dow process

hydrolysis-of-chlorobenzene-to-phenol-dow-process

Dow Process for Making Phenol from Chlorobenzene

Dow process chemistry shows how chlorobenzene can be converted to phenol under harsh reaction conditions. Simple aryl halides do not react with nucleophiles under normal conditions. Aryl halides are halogen-containing benzene compounds, and nucleophiles are electron-rich reactants that attack positive or partially positive sites. In this reaction, aqueous sodium hydroxide is used at 350 °C under high pressure. These drastic conditions allow chlorobenzene to give the substituted product phenol.

benzene to phenol via cumene: hock process

benzene-to-phenol-via-cumene-hock-process

Phenol Production from Cumene Pathway

Phenol production from the cumene pathway begins with benzene and propene. A Friedel-Crafts alkylation reaction joins them to form cumene. From there, cumene is converted into cumene hydroperoxide through a radical chain reaction. The radical chain starts with initiation. A benzylic hydrogen is abstracted, which creates a benzylic radical. In the propagation step, that radical reacts with an oxygen diradical to form a cumene hydroperoxide radical. The radical then reacts with another cumene...

oxidation of phenols to quinones

Quinones in Phenol Redox Reactions

Phenols can be oxidized to quinones when an oxidizing agent is present. Quinones can also be reduced back to phenols with mild reducing agents. This reversible redox change is an important part of phenol chemistry. The hydroxyl group on a phenol donates electrons and makes the aromatic ring more reactive. Because of this effect, the ring can be oxidized even when there is no alpha hydrogen. Ortho-hydroxy phenols form o-quinones, and para-hydroxy phenols form p-quinones. These reactions...

key terms and concepts

Learn how heat, light, or peroxide drive benzylic halogenation, why toluene and ethylbenzene react there, and how radical stability controls selectivity.

Benzylic Halogenation and Radical Selectivity | JoVE K12

organic chemistry

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Benzylic Halogenation and Radical Selectivity

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