Electrophilic Aromatic Substitution Mechanism
Electrophilic aromatic substitution mechanism is the method of substitution reaction in aromatic hydrocarbons or compounds. Aromatic compounds hydrocarbons or organic compounds tend to go through this reaction. In which an atom of a compound such as benzene reacts with an electrophile. And it replaces that atom (i.e. attaches to the aromatic ring). In some common reactions such as benzene, the electrophile replaces the hydrogen atom from the aromatic ring. This aromatic reaction helps preserve the aromaticity of an aromatic compound. Now let us discuss some electrophilic aromatic substitution examples. One such case of aromatic stability is the reaction of a benzene ring with chlorine to form iron chloride and hydrochloride. Similarly, sulphur trioxide reacts with benzene to form sulphuric acid. Here sulphur trioxide is the electrophile.
Different Types of Electrophilic Aromatic Substitution Reaction
Although there are multiple types of electrophilic aromatic substitution reaction, Let us discuss a few of them. Also, we will go through some example of electrophilic substitution reaction. They are Nitration, halogenation, sulfonation, Friedel crafts alkylation and acylation. All of them are aromatic reactions, but they are very different from each other. The only thing common between them is the benzene ring. Some electrophilic aromatic substitution examples are:
Nitration
Aromatic Nitration reactions involve nitro (NO₂) group. Nitro group acts as an electrophile to replace the hydrogen atom. This process also involves the use of a catalyst in the form of sulfuric acid (H₂SO₄). There is another acid used as well called nitric acid that loses a proton to form nitronium ion. By the application of Electrophilic aromatic substitution mechanism, we can process this nitronium ion. A great example of Electrophilic substitution reaction involving the nitro group is TNT or high explosives. Toluene also is known as methylbenzene that goes through this process to create trinitrotoluene.
Halogenation
Aromatic halogenation reactions involve halogen group elements, mainly bromine and chlorine. Benzene goes through a substitution reaction to replace its hydrogen atoms with chlorine or bromine. Since they do not have the strength to complete the reaction on their own, we use acids such as lewis acids as a catalyst to speed up or complete the process. These acids, such as aluminium bromide or iron bromide, transfer a pair of electrons so that their atoms can form permanent bonds (cl-cl or Br-Br). In this reaction, the benzene ring loses its aromaticity and generates activation energy. To overcome that energy Br or cl uses their electrophilic strength due to their positive charge.
Sulfonation
As the name suggests, the aromatic sulfonation reaction involves sulfonic acid (SO₃). We use sulfuric acid (lewis acid) as a catalyst in this reaction. That makes it possible for sulfonic acid to gain a proton and generate a strong electrophile. Subsequently, this electrophile reacts with benzene and replaces its hydrogen atom. Then we use the electrophilic aromatic substitution mechanism to further complete the process. This reaction is quite similar to the aromatic nitration reaction.
Friedel Crafts Alkylation
Friedel Crafts alkylation reaction involves the use of alkyl group (R). In the previous reactions, we saw the reaction of different molecules with the carbon of benzene, but it is also possible to form a carbon-carbon bond. It requires alkyl halides to react with benzene in the presence of a catalyst such as lewis acids. An example of Electrophilic substitution reaction can be, chloromethane reacts with benzene in the presence of aluminium chloride or iron chloride. The lewis acids make it easy for the chlorine atom to leave the bond by weakening the bond. Although, the product of this reaction has high nucleophilic strength.
Friedel Crafts Acylation
This reaction is similar to the Friedel Crafts alkylation only it involves the use of acyl group (RC=O) instead of the alkyl group. Presence of lewis acids speeds up the process. For instance, acyl chlorides gain a proton in the presence of Lewis acids to become acyl ions. This ion acts as an electrophile and weakens the carbon chlorine bond. It uses one pair of chlorine while the other fills with the aluminium octet. Generally, aryl ketone comes out as a product of this reaction. Let us go through the steps involving this mechanism.
What is the Mechanism for Electrophilic Aromatic Substitution
This mechanism mainly involves three fundamentals. There is a formation of a new pi bond from carbon double bond, removal of a proton from the carbon-hydrogen bond, and reformation of carbon double bond. You must understand these two main steps involving electrophilic aromatic substitution reaction mechanism. The first step initiates the attack of an electrophile on the benzene ring. After that, initial attack helps the formation of arenium ion by gaining positive charge or protons. Subsequently, the entire process is slow due to electrophile taking its time attacking the aromatic ring.
Since the aromatic ring loses its aromaticity, it results in the release of high activation energy. Several factors, such as steric hindrance, probability, and resonance, play a crucial role in the electrophilic attack. And the second step involves the removal of a proton from the ion by a weak base. This removal occurs due to the attack of a weak base on the formed carbocation. Then the aromaticity is stored again by the formation of the pi bond via electrons. The entire process is relatively fast. One key thing to remember is that due to the attack of the electrophile, carbocation loses a proton in the process.
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FAQs on Electrophilic Aromatic Substitution
1. What is an electrophilic aromatic substitution (EAS) reaction?
An electrophilic aromatic substitution (EAS) reaction is a fundamental organic reaction where an atom, usually hydrogen, attached to an aromatic ring is replaced by an electrophile. The aromatic ring, such as benzene, acts as a nucleophile due to its electron-rich pi system, attacking the electron-deficient electrophile. The overall process preserves the stability of the aromatic system.
2. What are the key steps in the general mechanism of electrophilic aromatic substitution?
The mechanism for electrophilic aromatic substitution generally proceeds in two main steps as per the CBSE curriculum:
- Step 1: Formation of the Arenium Ion: The electrophile (E⁺) attacks the pi electron cloud of the benzene ring, forming a resonance-stabilised carbocation known as an arenium ion or sigma complex. This step is typically slow and rate-determining because it temporarily disrupts the ring's aromaticity.
- Step 2: Deprotonation to restore Aromaticity: A weak base removes a proton (H⁺) from the carbon atom bearing the electrophile. The electrons from the C-H bond move back into the ring, restoring the stable aromatic system and forming the final substituted product. This step is very fast.
3. How do substituents already on a benzene ring affect its reactivity towards further substitution?
Substituents significantly influence the rate of electrophilic aromatic substitution by either donating or withdrawing electron density from the ring. They are classified into two types:
- Activating Groups: These are electron-donating groups (e.g., -OH, -NH₂, -OCH₃, -CH₃) that increase the electron density in the benzene ring. This makes the ring more nucleophilic and more reactive towards electrophiles than benzene itself.
- Deactivating Groups: These are electron-withdrawing groups (e.g., -NO₂, -CN, -SO₃H, -COOH, -CHO) that decrease the electron density in the ring. This makes the ring less nucleophilic and less reactive towards electrophiles than benzene.
4. Can you provide a common example of an electrophilic aromatic substitution reaction?
A classic example is the nitration of benzene. In this reaction, benzene is treated with a mixture of concentrated nitric acid (HNO₃) and concentrated sulfuric acid (H₂SO₄) at around 50-60°C. The sulfuric acid acts as a catalyst to generate the powerful electrophile, the nitronium ion (NO₂⁺). This ion then substitutes a hydrogen atom on the benzene ring to form nitrobenzene.
5. Why do some substituents direct incoming groups to ortho/para positions, while others direct to the meta position?
This directive influence is determined by the stability of the intermediate arenium ion. Activating groups (-OH, -NH₂, -R) are ortho-para directing because they can donate electron density through resonance, which specifically stabilises the positive charge when the attack occurs at the ortho and para positions. In contrast, deactivating groups (-NO₂, -CHO) are meta directing because they withdraw electron density, destabilising the ortho and para intermediates more than the meta intermediate. Therefore, the meta position becomes the least unfavorable site for attack.
6. Why are halogens like chlorine considered deactivating yet ortho-para directing?
This is a special case due to the interplay of two opposing electronic effects. The halogen atom is highly electronegative, so it withdraws electron density from the ring through the inductive effect (-I effect), making the ring less reactive overall (deactivating). However, it also possesses lone pairs of electrons that can be donated to the ring through the resonance effect (+R effect). This resonance donation specifically stabilises the carbocation intermediates formed during ortho and para attack, making these positions more favorable than the meta position.
7. What is the key difference between electrophilic and nucleophilic aromatic substitution?
The primary difference lies in the attacking species. In electrophilic aromatic substitution, an electron-deficient electrophile attacks the electron-rich aromatic ring. This is characteristic of typical benzene reactions. In nucleophilic aromatic substitution, an electron-rich nucleophile attacks an electron-deficient aromatic ring. This requires the ring to have strong electron-withdrawing groups (like -NO₂) and usually a leaving group other than hydrogen, such as a halogen.
8. Why does the Friedel-Crafts alkylation of benzene often result in multiple substitutions and rearrangements?
The Friedel-Crafts alkylation reaction has two major limitations:
- Polyalkylation: The initial product, an alkylbenzene, has an activating alkyl group (-R). This makes the product more reactive than the original benzene, leading to further alkylation and a mixture of products.
- Rearrangement: The reaction proceeds via a carbocation intermediate. Primary carbocations, if formed, will readily rearrange to more stable secondary or tertiary carbocations, leading to an isomeric mixture of products. To avoid these issues, Friedel-Crafts acylation is often used instead, as the acyl group is deactivating and does not rearrange.
9. What are some important industrial applications of electrophilic aromatic substitution?
Electrophilic aromatic substitution is crucial for synthesising many important industrial chemicals. Key applications include:
- The production of styrene from benzene (via alkylation and dehydrogenation), which is used to make polystyrene plastic.
- The synthesis of cumene, a precursor for producing phenol and acetone.
- The creation of synthetic detergents through the sulfonation of alkylbenzenes.
- The manufacturing of explosives like TNT (trinitrotoluene) and dyes from nitrated aromatic compounds.
