Electrophilic Substitution Reactions

Introduction

An existing functional group is replaced by an incoming new functional group in a substitution reaction, which is a type of chemical reaction. An electrophile is a molecule that lacks an electron. Hence, we can define Electrophilic substitution reactions as chemical reactions where an electron-deficient substance (electrophile) displaces a functional group in a compound. The displaced functional group in this type of reaction is generally a hydrogen atom. Electrophilic substitution reactions generally proceed via a three-step mechanism that involves the following steps −

• An electrophile is generated

• A carbocation (intermediate) is formed

• A proton is removed from the intermediate

In an electrophilic substitution reaction, generally, a C—H bond breaks down and a new C—E bond is formed where E is an electrophilic species like Cl⁺, NO₂⁺, etc.

Types of Electrophilic Substitution Reactions

The two main types of electrophilic substitution reactions are

• Electrophilic aliphatic substitution reaction − the incoming electrophile will attack the aliphatic compounds e.g., ketone, diazonium salt, etc.

• Electrophilic aromatic substitution reaction− the incoming electrophile will attack the aromatic compounds e.g., benzene, arene, etc.

Electrophilic Aromatic Substitution Reaction

Electrophilic Aromatic Substitution Reaction

Aromatic rings having π-electron systems are electron rich and hence they are always ready to react with electrophilic agents. During the substitution reaction, the aromaticity of the compound remains intact. The rate of the substitution reaction is increased with the help of lewis acids such as FeCl_3 and AlCl_3. Some important electrophilic aromatic substitution reactions are −

• Halogenation of aromatic compounds

• Sulfonylation and nitration of aromatic compounds

• Friedel-Crafts alkylation reaction and Friedel-Crafts acylation reaction

Mechanism

The general mechanism of the reaction is −

Step 1 − An electrophile is generated

Fig 2: Generation of an Electrophile

Step 2 − A carbocation is formed

Fig 3: Generation of Carbocation

Step 3 − A proton is removed from the intermediate

Fig 4: Removal of a proton from the intermediate

Electrophilic Aliphatic Substitution Reactions

In aliphatic electrophilic substitution, a functional group is displaced by an electrophile. A few examples of this type of reaction are −

• Keto-enol tautomerism reaction

• The aliphatic diazonium coupling reaction

• Nitrosation reaction

• Ketone halogenation etc.

There are mainly two types of aliphatic substitution reactions −

• Substitution Electrophilic internal (SE_i)

• Substitution Electrophilic bimolecular (SE₂)

Mechanism

• For SE_i reaction – a stronger electrophile displaces a weaker electrophile. The mechanism is like the SN₂ mechanism where the electrophile attacks from the front side and leaving group also leaves from the front side.

Fig 5: SE_i Mechanism

• For SE₂reaction- a stronger electrophile displaces a weaker electrophile. Here the mechanism is also like the SN₂ mechanism, but the point of difference is both the SE₂-front side and SE₂-backside attacks are possible. Accounting for very less repulsion when the electrophile attacks, the focus is to use the vacant orbitals of the substrate by the electrophile.

Fig 6: SE₂ Mechanism

Reactions of Amines

Amines are organic compounds that are created when a cycloalkyl, alkyl, or aromatic group replaces an atom of hydrogen in ammonia (NH₃) and forms a new bond with nitrogen. Some important reactions of amines are −

• Preparation of ethanol from aliphatic primary amines by oxidation with KMnO₄

• Amine is basic so when reacts with an acid forms salt.

• The reaction between alkyl halide and ammonia is an alkylation reaction by the SN₂ mechanism.

Types of electrophilic substitution reactions of Aniline

An organic aromatic amine called aniline has an attached (-NH₂) functional group to a benzene ring. The benzene ring is activated for the electrophilic substitution process by the (-NH₂) group, an electron-donating group. The benzene ring is electron-rich due to its presence of Π‑electrons, and the (-NH₂) group moves this electron density to the ortho- and para-positions of the ring. As a result, the electrophile will bind to the benzene ring in this location. Aniline is hence referred to as O- and P- guiding in electrophilic substitution processes. The different types of electrophilic substitution reactions of aniline are-

• Halogenation reaction

• Nitration reaction

• Sulphonation reaction

• Friedel-Crafts alkylation reaction

• Friedel-Crafts acylation reaction

Nitration of aniline

In this reaction, the hydrogen atom from the aniline is substituted by a nitro group. The nitration reaction is performed by the addition of a nitrating mixture of concentrated H₂ SO₄ and HNO₃. Surprisingly m- and p-nitro aniline is formed with a higher percentage along with a very less amount of o- nitro aniline. This is a result of the reaction's acidic nature. The conjugate-acid ammonium ion is the main starting point in the strongly acidic solution. The ammonium ion is a meta-directing group because it has an electron-withdrawing polar effect and does not have an unshared electron pair on nitrogen, which prevents it from donating electrons via a resonance effect. If the nitrogen is initially protected from protonation, aniline can be regioselectively nitrated at the para position. Nitric acid oxidises the majority of the aniline to create oxidation products and very few nitrated products, making direct nitration of aniline an ineffective process.

Fig 7: Nitration Reaction of aniline

Sulphonation of aniline

When the sulphonic acid (-SO₃ H) group is substituted for the hydrogen atom in an aniline, sulphonation takes place. The sulphonation process starts when oleum or fuming sulfuric acid reacts with aniline at 453 -473K. Aniline hydrogen sulphate is produced by the strong interaction of sulfuric acid with aniline. Sulphanilic acid is produced when anilinium hydrogen sulphate is heated. The zwitterion and sulfuric acid share a resonant structural feature.

Fig 8: Sulphonation Reaction of aniline

Halogenation of aniline

Anilines are very reactive toward electrophilic substitution reactions and undergo ring substitution very easily. Halogenation is the process of changing a hydrogen atom into a halogen atom like fluorine, chlorine, and bromine. Aniline undergoes substitution reactions with iodine, a halogen that is often unreactive with benzene derivatives, because of its high nucleophilic reactivity. The product formed in this reaction is 2,4,6-trihalo aniline. All the o-, p-, and m- position is substituted because of the high activating nature of the -NH₂ group.

Fig 9: Halogenation Reaction of aniline

Fun fact

• The oxygen molecule in water is more electronegative (because it contains two lone pairs and a δ^- charge, which makes it nucleophilic), while each hydrogen molecule acts as an electrophile since it carries a δ⁺charge, making water both an electrophile and a nucleophile.

• H⁺ ion is one of the few electrophiles that only accept electrons because it doesn't have any.

Conclusion

An electrophile replaces a molecule's functional group in a chemical reaction known as an electrophilic substitution reaction. Usually, the displace functional group is composed of one hydrogen atom. A C—H bond normally dissolves during an electrophilic substitution process, resulting in the formation of a new C—E bond (where E is an electrophilic species). Electrophilic aliphatic and electrophilic aromatic substitution reactions are the two main types of electrophilic substitution reactions. Compared to aliphatic compounds, aromatic rings have a higher tendency for substitution reactions with electrophiles because they are electron rich.

FAQ

1. Which is the rate-determining step in the mechanism of the electrophilic substitution reaction?

Typically, the rate-determining step in electrophilic aromatic substitution is the second step. The intermediate is referred to as a sigma complex since during this process a new sigma bond is formed. Despite being resonance stabilized and only having four electrons, this carbocation is not aromatic.

2. Can you think about why the catalyst is there in the electrophilic substitution reaction of benzene?

The Catalyst acts like a helper. Firstly, the attacking reagent and the catalyst interact where the catalyst helps to form an electrophile.

3. Are all electrophiles positively charged?

Electrophiles are Lewis acids because they accept electrons. Most electrophiles have an atom that carries a partial positive charge, is positively charged, or carries an atom where the octet is not complete.

4. Between alkene and benzene who is more reactive towards the electrophilic substitution reaction?

Due to its delocalized Π-system, which forms a continuous Π-link and allows the six p electrons of the carbon atoms to be delocalized above and below the ring, benzene is less reactive and more stable than alkenes, in which the electrons are localized between specific atoms.

5. Why phenols are more reactive towards electrophilic substitution?

The oxygen's lone pair is donated into the ring structure, which raises the ring's electron density. The ring becomes substantially more reactive as a result than it is in benzene. More resonance stabilization exists in the intermediate carbocation. That is why phenols are more reactive towards electrophilic substitution.