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9.6.7 - Electrophilic substitution

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Introduction to Electrophilic Substitution

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Teacher
Teacher

Welcome class! Today we will explore electrophilic substitution reactions in amines. Can anyone remind me why amines are considered good nucleophiles?

Student 1
Student 1

Because of the lone pair of electrons on the nitrogen atom.

Teacher
Teacher

Exactly! The nitrogen's lone pair can attack electrophiles. Let’s dive deeper. What happens during this attack?

Student 2
Student 2

The electrophile gets attached to the aromatic ring, right?

Teacher
Teacher

Correct! The amino group directs the incoming electrophiles to the ortho and para positions of the ring, enhancing reactivity.

Student 3
Student 3

Can you give an example of this reaction?

Teacher
Teacher

Sure! A classic example is the bromination of aniline, which forms 2,4,6-tribromoaniline. Let’s remember that the electropilic attack typically happens at the ortho and para positions!

Teacher
Teacher

To summarize: Amine groups make aromatic compounds highly reactive. This is essential in synthetic organic chemistry.

Controlled Reactivity in Electrophilic Substitution

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Teacher
Teacher

Now, let’s discuss the reactivity issues with amines, particularly in nitration. Why might we prefer to protect the amino group?

Student 1
Student 1

Because uncontrolled reactions can yield multiple products?

Teacher
Teacher

Yes! When nitrating aniline, we can end up with tarry products due to excess reactivity. Thus, we often use acetylation to protect the amino group. Can anyone tell me more about the process?

Student 2
Student 2

Isn't it treating the amine with acetic anhydride first?

Teacher
Teacher

Correct! After that, we can perform nitration and hydrolyze the product to yield the desired substituted amine.

Student 4
Student 4

What happens when you try to do Friedel-Crafts reactions with aniline?

Teacher
Teacher

Great question! Aniline cannot perform Friedel-Crafts reactions due to the formation of a cationic species that deactivates the ring.

Teacher
Teacher

Let's summarize: For nitration, protect the amino group; avoid Friedel-Crafts due to deactivation.

Introduction & Overview

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Quick Overview

This section covers the process of electrophilic substitution reactions involving amines, focusing on their activity as electron-rich nucleophiles in aromatic systems.

Standard

Amines undergo electrophilic substitution reactions due to the electron-donating nature of their amino group. This section details the mechanisms behind these reactions with various reagents like bromine and nitrous acid, as well as challenges in controlling the reactivity of amines during these processes.

Detailed

Electrophilic Substitution Reactions of Amines

Amines, particularly arylamines, are known for their increased reactivity in electrophilic substitution reactions due to the presence of an amino group, which is an electron-donating group. The fundamental concepts of electrophilic substitution involve the attack of an electrophile on an aromatic ring, with the amino group directing the incoming electrophile to the ortho and para positions due to the resonance structures of the amine.

  1. Bromination: Aniline, when reacted with bromine, precipitates a product called 2,4,6-tribromoaniline. This high reactivity necessitates control over the conditions of the reaction.
  2. Nitration and Sulphonation: The direct nitration of aniline often leads to side reactions forming tarry products, hence alternative methods like acetylation are used.
  3. Friedel-Crafts Reactions: Unlike typical aromatic compounds, aniline does not undergo Friedel-Crafts reactions due to salt formation with the Lewis acid catalyst, making the amino group act as a deactivating group.
  4. Coupling Reactions: Amines can engage in coupling reactions with diazonium salts used in the synthesis of azo compounds, demonstrating their utility in forming dyes. The reactivity and substitution pattern of amines during electrophilic aromatic substitution reactions highlight their role as versatile intermediates in organic synthesis.

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Audio Book

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Overview of Aniline and Electron Density

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You have read earlier that aniline is a resonance hybrid of five structures. Where do you find the maximum electron density in these structures? Ortho- and para-positions to the –NH2 group become centres of high electron density. Thus –NH2 group is ortho and para directing and a powerful activating group.

Detailed Explanation

Aniline, which is an amine with an amino group (-NH2) directly attached to a benzene ring, exhibits resonance. This means that the structure can be represented in multiple ways, allowing for the delocalization of electrons. The resonance forms show that the ortho (next to) and para (across from) positions relative to the amino group have higher electron density. Because of this, these positions become particularly reactive in electrophilic substitution reactions, allowing new groups to attach to the benzene ring at these locations.

Examples & Analogies

Think of the electron density like a crowded party. The ortho and para positions are the hottest spots at the party where everyone wants to be—there’s more energy and excitement there. Just like how guests are likely to hang out where the action is, chemical reactions are more likely to happen at these electron-rich areas on the benzene ring.

Bromination of Aniline

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Aniline reacts with bromine water at room temperature to give a white precipitate of 2,4,6-tribromoaniline.

Detailed Explanation

When aniline is treated with bromine water, it takes place readily due to the high electron density at the ortho and para positions. The amino group enhances the reactivity of the benzene ring, making it easier for bromine to add to those sites. The result is the formation of 2,4,6-tribromoaniline, which appears as a white precipitate. Bromination is a classic example of electrophilic substitution where the benzene part of aniline is substituted by bromine atoms.

Examples & Analogies

Imagine trying to add decorations to a cake (the cake being the benzene ring). The more colorful and exciting the cake is (thanks to the amino group), the easier it is to add decorations (bromine atoms) on the most attractive spots (ortho and para positions).

Controlling Reactivity in Substitution Reactions

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The main problem encountered during electrophilic substitution reactions of aromatic amines is that of their very high reactivity. Substitution tends to occur at ortho- and para-positions. If we have to prepare monosubstituted aniline derivative, how can the activating effect of –NH2 group be controlled? This can be done by protecting the -NH2 group by acetylation with acetic anhydride, then carrying out the desired substitution followed by hydrolysis of the substituted amide to the substituted amine.

Detailed Explanation

The high reactivity of aniline during electrophilic substitution can be a challenge, especially when you only want to add one group to the ring. To manage this, chemists can 'protect' the amino group. Protecting the amino group involves converting it into an acetamido group (-NHCOCH3) using acetylation, which decreases its electron-donating ability and thus reduces its reactivity. This allows a controlled substitution at the benzene ring. Once the desired substitution is accomplished at a lower reactivity, the protecting group can be removed through hydrolysis to yield the final product.

Examples & Analogies

Imagine a celebrity trying to go out without attracting too much attention. They might wear a disguise (the protection) so that they can go to a quiet restaurant (perform the substitution reaction) without being swarmed by fans. Once they’re safely in, they can take off the disguise and enjoy their meal in peace (perform hydrolysis to yield the aromatic amine).

Nitration of Aniline

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Direct nitration of aniline yields tarry oxidation products in addition to the nitro derivatives. Moreover, in the strongly acidic medium, aniline is protonated to form the anilinium ion which is meta directing. That is why besides the ortho and para derivatives, significant amounts of meta derivative are also formed.

Detailed Explanation

When attempting to nitrate aniline, the presence of both the amino group and acidic conditions creates complex reactions. Nitration typically aims to introduce a nitro group (-NO2) into the aniline. However, the strongly acidic medium can protonate the amine group, forming the anilinium ion. This protonation changes the directing influence of the amino group to a meta position rather than ortho or para, leading to a mix of products, including unwanted tarry by-products.

Examples & Analogies

It’s like trying to organize a community cleanup event. If you have excited volunteers (the amino group) but suddenly they are given confusing instructions (the acidic conditions), some might end up going in the wrong direction (producing meta nitration products), leading to a mixed and potentially messy result.

Sulphonation of Aniline

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Aniline reacts with concentrated sulphuric acid to form anilinium hydrogensulphate which on heating with sulphuric acid at 453-473K produces p-aminobenzene sulphonic acid, commonly known as sulphanilic acid, as the major product.

Detailed Explanation

In the presence of concentrated sulfuric acid, aniline forms a product called anilinium hydrogensulphate. This is a soluble salt that can be treated further. When heated, this salt converts into sulphanilic acid by replacing the hydrogen in the amino group with a sulfonic acid group (-SO3H). This reaction is significant because sulphanilic acid is an important compound in various applications, including dye production.

Examples & Analogies

Think of aniline like a sponge soaking up a new liquid (sulfonic acid). Just as a sponge can hold onto liquid until it is full, aniline adds sulfonic acid to become sulphanilic acid—an important compound that can ‘dye’ fabrics just like how a sponge can be used to clean or color things.

Challenges in Friedel-Crafts Reaction

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Aniline does not undergo Friedel-Crafts reaction (alkylation and acetylation) due to salt formation with aluminium chloride, the Lewis acid, which is used as a catalyst. Due to this, nitrogen of aniline acquires positive charge and hence acts as a strong deactivating group for further reaction.

Detailed Explanation

Friedel-Crafts reactions involve electrophilic aromatic substitution, where an alkyl or acyl group is introduced to the benzene ring. However, since aniline can form a salt with aluminum chloride (a Lewis acid), it leads to the protonation of the amino nitrogen. This protonated form acts as a deactivating group due to its positive charge, making it less reactive towards further electrophilic attack and thus unable to participate effectively in Friedel-Crafts reactions.

Examples & Analogies

Think of aniline as a club bouncer who has to keep other rowdy guests in line (the electrophiles). But if they become overwhelmed (protonated), they either can’t do their job or end up pushing everyone away (becoming a deactivating presence), letting fewer new guests enter (alkyl or acyl groups added).

Definitions & Key Concepts

Learn essential terms and foundational ideas that form the basis of the topic.

Key Concepts

  • Amines as electrophiles: Amines can act as nucleophiles due to the presence of a lone pair of electrons on nitrogen.

  • Bromination: Amines react with bromine to give substituted products.

  • Nitration challenges: The high reactivity of amines can lead to side reactions; hence protection is necessary.

  • Acetylation: A method for temporarily blocking amine reactivity during substitution reactions.

  • Coupling reactions: A means of synthesizing azo dyes through the reaction of diazonium salts with phenolic compounds.

Examples & Real-Life Applications

See how the concepts apply in real-world scenarios to understand their practical implications.

Examples

  • Bromination of aniline results in the formation of 2,4,6-tribromoaniline.

  • The acetylation of aniline can prevent multiple substitution during nitration.

  • Amines can react with diazonium salts to produce azo compounds, essential for dyes.

Memory Aids

Use mnemonics, acronyms, or visual cues to help remember key information more easily.

🎵 Rhymes Time

  • Bromine it draws, to the ortho and para's applause.

📖 Fascinating Stories

  • Imagine an amine at a party, attracting all the greetings at ortho and para positions. But watch out! Too much excitement and tar shows!

🧠 Other Memory Gems

  • BROM: 'B' for Bromination, 'RO' for reaction at ortho/para, 'M' for amines!

🎯 Super Acronyms

POET

  • Protect with Acetylation
  • Electrophiles at Ortho/Para positions.

Flash Cards

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Glossary of Terms

Review the Definitions for terms.

  • Term: Electrophilic substitution

    Definition:

    A reaction mechanism that involves the substitution of an electrophile for a hydrogen atom on an aromatic compound.

  • Term: Nucleophile

    Definition:

    A species that donates an electron pair to form a chemical bond in reaction.

  • Term: Aniline

    Definition:

    The simplest aromatic amine, chemical structure C6H5NH2.

  • Term: Bromination

    Definition:

    The introduction of bromine into a compound, often involving electrophilic aromatic substitution.

  • Term: Acetylation

    Definition:

    A chemical reaction in which an acetyl group is introduced into a molecule.