Orgo 2 electrophilic aromatic substitution

Define Electrophilic Aromatic Substitution (EAS).

A reaction where an electrophile replaces a hydrogen atom on an aromatic ring.

What is the role of the electrophile in EAS?

The electrophile attacks the electron-rich aromatic ring, leading to substitution.

True or False: EAS always results in a loss of aromaticity.

False. Aromaticity is restored after the reaction through subsequent steps.

List the general steps in EAS.

- Formation of the electrophile - Electrophilic attack - Proton transfer - Regeneration of aromaticity

What is the first step in EAS?

Formation of a strong electrophile, often using a catalyst.

Fill in the blank: The intermediate formed in EAS is called a __________.

Sigma complex or arenium ion.

Compare EAS and nucleophilic aromatic substitution.

EAS involves electrophiles attacking aromatic rings, while nucleophilic substitution involves nucleophiles.

What happens during the electrophilic attack in EAS?

The electrophile bonds to the aromatic ring, disrupting aromaticity temporarily.

Describe the role of a catalyst in EAS.

Catalysts increase the rate of electrophile formation and enhance reaction efficiency.

What is the significance of the proton transfer step?

It restores aromaticity by removing a proton, completing the substitution.

Give an example of a common electrophile in EAS.

Bromine ($\displaystyle Br_2$) in the presence of a catalyst like FeBr3.

True or False: All aromatic compounds undergo EAS the same way.

False. Substituents can influence the reactivity and orientation of the reaction.

What is a sigma complex?

A resonance-stabilized carbocation intermediate formed during EAS.

Identify the final product of an EAS reaction.

An aromatic compound with a new substituent in place of a hydrogen atom.

Cause → Effect: Why is the restoration of aromaticity important?

It stabilizes the final product, making it favorable and thermodynamically stable.

What is the importance of resonance in the sigma complex?

Resonance delocalizes the positive charge, stabilizing the intermediate.

Bromine (Br2) in the presence of FeBr3 →

This reagent combination generates a bromonium ion, allowing bromination of the aromatic ring.

What is the role of AlCl3 in Friedel-Crafts reactions?

AlCl3 acts as a Lewis acid to generate a more powerful electrophile from alkyl or acyl halides.

Sulfuric acid (H2SO4) →

Used for sulfonation, it introduces a sulfonyl group (-SO3H) onto the aromatic ring.

True or False: Nitronium ion (NO2+) is an electrophile.

True. The nitronium ion is a strong electrophile used in nitration reactions.

Identify the electrophile in chlorination: Cl2 + FeCl3.

The chloronium ion (Cl+) generated is the electrophile attacking the aromatic ring.

Fill in the blank: _______ is a common reagent for iodination.

Iodine (I2) in the presence of a Lewis acid like CuI.

What is the electrophile formed from acetic anhydride with AlCl3?

The electrophile is an acylium ion, which can react with the aromatic ring.

Hydrogenation of aromatic rings requires:

A strong reducing agent, usually in the presence of a catalyst like Pd/C.

What does $\displaystyle HNO3$ do in nitration?

It provides the nitronium ion (NO2+), the active electrophile in electrophilic aromatic substitution.

Comparing halides, which is the strongest electrophile?

The bromonium ion (Br+) is stronger than Cl+ due to better stabilization.

What reagent is used for the formylation of aromatic compounds?

Formyl chloride (CHOCl) in the presence of a Lewis acid like AlCl3.

What is the function of a Lewis acid in these reactions?

It activates halides or other reagents to form stronger electrophiles for substitution.

True or False: Acyl chlorides can undergo Friedel-Crafts acylation.

True. They react with Lewis acids to form acylium ions, which can substitute on aromatic rings.

What does $\displaystyle SO3$ do in sulfonation?

It introduces a sulfonyl group (-SO3H) onto the aromatic ring, creating an electrophilic site.

Identify a common reagent for methylation.

Dimethyl sulfate (DMS) is a common alkylating agent used in methylation.

What is the electrophile in the reaction with $\displaystyle CF3COCl$?

The trifluoroacetyl cation ($\displaystyle CF3C^+$) is the electrophile that can react with the aromatic system.

What is the result of using NaNO2 and HCl?

It generates the nitronium ion (NO2+), which can nitrify aromatic compounds.

Phenol is more reactive than benzene due to:

The hydroxyl group (–OH) increases electron density, enhancing electrophilic substitution.

Which reagent can lead to chlorination of benzene?

Chlorine ($\displaystyle Cl2$) in the presence of a Lewis acid like $\displaystyle FeCl3$.

Identify a common reagent for nitration.

Concentrated nitric acid ($\displaystyle HNO3$) is commonly used. It generates the nitronium ion ($\displaystyle NO2^+$) as the electrophile in the nitration of aromatic compounds.

Activating groups

Substituents that increase the rate of electrophilic aromatic substitution. They typically donate electron density to the aromatic ring.

Deactivating groups

Substituents that decrease the rate of electrophilic aromatic substitution. They withdraw electron density, making the ring less nucleophilic.

Ortho-, para- directing groups

Activating groups that direct incoming electrophiles to the ortho and para positions relative to themselves.

Meta-directing groups

Deactivating groups that direct incoming electrophiles to the meta position relative to themselves.

Example of an activating group

- Methoxy (-OCH₃) - Amino (-NH₂) - Hydroxy (-OH) All increase reactivity.

True or False: Halogens are activating groups.

False. Halogens are deactivating groups due to their electronegative nature, but they direct substitution to ortho and para positions.

Comparison: activating vs deactivating groups

Activating groups increase reactivity and direct to ortho/para; deactivating groups decrease reactivity and direct to meta.

Fill in the blank: The ______ position is favored for substitution by activating groups.

Ortho and para.

Effect of -NO₂ group

The nitro group is a strong deactivating group and meta-directing due to its electron-withdrawing effect.

Example of a meta-directing deactivating group

- Nitro (-NO₂) - Carbonyl (-C=O) These groups reduce reactivity.

True or False: Electron-donating groups increase nucleophilicity of the ring.

True. They increase electron density, making the ring more reactive towards electrophiles.

Ortho/para vs meta substitution

Ortho/para substitution occurs with activating groups, while meta substitution occurs with deactivating groups.

Mechanism overview

Electrophilic aromatic substitution involves forming a sigma complex and losing a proton to restore aromaticity.

Aromatic compound example

Toluene is an aromatic compound with a methyl group that is an activating and ortho/para-directing substituent.

Role of resonance in substitution

Resonance stabilization of the sigma complex stabilizes the intermediate formed during electrophilic aromatic substitution.

What does ortho- mean?

Refers to the 1,2-positions on the aromatic ring relative to a substituent.

What does para- mean?

Refers to the 1,4-positions on the aromatic ring relative to a substituent.

Reactivity order: ortho, para, meta

Ortho and para positions are more reactive than meta due to greater electron density at those positions.

Substituent effect on electrophile attack

Strongly activating groups enhance electrophile attack at ortho/para positions, while deactivating groups favor meta attack.

Worked example: Bromination of anisole

Anisole (C₆H₅OCH₃) reacts with Br₂ in presence of FeBr₃, predominantly at ortho and para positions.

Nitration of benzene

Benzene reacts with nitric acid and sulfuric acid to form nitrobenzene, an important intermediate in dye synthesis.

Sulfonation example

Benzene reacts with sulfuric acid to form benzenesulfonic acid, useful in making detergents.

Friedel-Crafts alkylation

This substitution introduces alkyl groups onto aromatic rings, used in the synthesis of alkylated aromatic compounds.

True or False: Electrophilic aromatic substitution only works with electron-rich aromatic compounds.

False. It can occur with both electron-rich and electron-poor aromatic compounds.

Comparison: Nitration vs. Sulfonation

Nitration adds a nitro group, while sulfonation adds a sulfonic acid group. Both modify aromatic rings.

Fill in the blank: Friedel-Crafts acylation introduces a ____ group onto the aromatic ring.

Acyl group (RCO-). This reaction uses acyl chlorides.

Uses of nitrobenzene

- Explosives production - Synthesis of aniline dyes - Intermediate for pharmaceuticals.

What is toluene sulfonation used for?

Toluene sulfonation yields toluenesulfonic acid, a key intermediate in organic synthesis.

Effect of alkyl groups on reactivity

Alkyl groups activate the benzene ring, increasing electrophilic substitution rates.

True or False: Electrophilic aromatic substitution can lead to multi-substitution on the ring.

True. Multiple substitutions can occur, leading to complex products.

Worked example: Bromination of toluene

Toluene reacts with bromine in the presence of FeBr3 to yield bromotoluene, with ortho/para products.

Benzene derivatives in pharmaceuticals

Many drugs feature aromatic rings, commonly synthesized via electrophilic aromatic substitution reactions.

What is the significance of ortho/para directing groups?

They influence the position of subsequent substitutions on the aromatic ring, guiding synthesis.

Impact of electron-withdrawing groups

Electron-withdrawing groups deactivated the ring, slowing down electrophilic substitution reactions.

Key example of electrophilic aromatic substitution application

Synthesis of aspirin involves acetylation of salicylic acid, showcasing practical use in medicinal chemistry.

Role of catalysts in electrophilic aromatic substitution

Catalysts like AlCl3 or FeCl3 enhance reaction rates, making processes more efficient.