Electrophilic Aromatic Substitution

Electrophilic aromatic substitution reactions are organic reactions wherein an electrophile replaces an atom which is attached to an aromatic ring. Commonly, these reactions involve the replacement of a hydrogen atom belonging to a benzene ring with an electrophile.

The aromaticity of the aromatic system is preserved in an electrophilic aromatic substitution reaction. For example, when bromobenzene is formed from the reaction between benzene and bromine, the stability of the aromatic ring is not lost. This reaction can be illustrated as follows.

Electrophilic Aromatic Substitution

Aryl halides or haloarenes can be prepared via electrophilic aromatic halogenation reactions of aromatic rings with iodine, chlorine, or bromine. These reactions generally involve the use of aluminum trihalides as catalysts.

Electrophilic Aromatic Substitution Reaction

There exist many types of electrophilic aromatic substitution reactions, the most important of which include:

  • Aromatic nitration reactions
  • Electrophilic aromatic halogenation reactions
  • Aromatic sulfonation reactions
  • Friedel-Crafts alkylation reaction
  • Friedel-Crafts acylation reaction

The aromatic system used as a reactant in these reactions is usually benzene. The details of these reactions are listed in this subsection.

Aromatic Halogenation

  • Halogenation is a type of substitution reaction where hydrogen is replaced with bromine or chlorine.
  • The process (chlorination or bromination) makes use of a Lewis acid, which takes a pair of electrons to form a permanent bond dipole in the Cl-Cl bond or the Br-Br bond.
  • The Bromine or chlorine has a formal positive charge due to the dipole making the group electrophilic enough to easily overcome the activation energy which is generated due to the loss of aromaticity of the benzene ring.

Aromatic Nitration

  • Nitration reaction involves the replacement of a hydrogen with a nitro (NO2) group.
  • Sulfuric acid (H2SO4) is used as a catalyst in this process.
  • The acid is used to protonate the nitric acid which leads to the formation of a nitronium ion.
  • The nitronium ion can then be processed as per the mechanism of electrophilic aromatic substitution reaction.

Aromatic Sulfonation

  • Sulfonation occurs when hydrogen is replaced with the help of sulfonic acid (SO3).
  • This reaction is also quite similar to nitration where an electrophile is generated by protonation of SO3 with H2SO4.
  • This helps to generate a strong electrophile. Once the item is obtained, the reaction follows the electrophilic aromatic substitution mechanism.

Friedel-Crafts Acylation

  • In the Friedel-Crafts Acylation process, hydrogen is normally replaced with an acyl group (RC=O).
  • Reagents commonly used in this type of reaction are carboxylic acid halides or acyl chlorides. Additionally, Lewis acid catalysts are used.
  • There is a formation of the electrophile (generally acylium ion) by using a single pair from the chlorine of the H3C(C=O)Cl which is also used in filling the open octet of the aluminum belonging to AlCl3.
  • In the end, chlorine carbon bond becomes weak and Cl+-AlCl3 The product that is formed is normally an aryl ketone.

Friedel-Crafts Alkylation

  • In Friedel-Crafts Alkylation, the hydrogen is mostly replaced with an alkyl group (R).
  • In this reaction, alkyl halide such as CH3CH2Cl is used along with Lewis acid like AlCl3 or FeCl3 amongst others.
  • The acid helps to accelerate the reaction by coordinating to the halogen. They further weaken the C–Cl bond and make it a better leaving group.
  • However, one disadvantage of this reaction is that the product is more nucleophilic than the reactant. Moreover, overalkylation can also occur.

Electrophilic Aromatic Substitution Mechanism

An electrophilic aromatic substitution consists of three main fundamental components:

  • During the reaction, a new σ bond is formed from a C=C in the arene nucleophile.
  • Proton is removed by the breaking of C-H σ bond.
  • The C=C is reformed which restores the aromaticity.

As for the mechanism of the reaction, it usually includes two main steps.

Step 1

  • The reaction begins by the electrophile attacking the pi electrons present in the aromatic benzene ring.
  • This results in the formation of positively charged and delocalized cyclohexadienyl cation or a resonance-stabilized carbocation known as an arenium ion.
  • This ion basically contains three resonance contributors. The electrophilic attacking the aromatic ring generally takes time and is a slow process.
  • It is further endergonic and there is a presence of high activation energy due to the loss of aromaticity.
  • Some of the main factors that are used to determine the attack of the electrophile are resonance, probability, and steric hindrance.

Step 2

  • This step involves the deprotonation of the arenium ion by a weak base.
  • The carbocation intermediate that is formed is attacked by a base that results in the loss of a proton.
  • The electrons are then used to reform a pi bond and aromaticity is yet again restored.
  • This is a very fast process and usually exergonic. An important thing to remember here is that the carbocation loses a proton as a result of the electrophile attacking the benzene ring.

Electrophilic Aromatic Substitution Mechanism

Thus, the different types of electrophilic aromatic substitution reactions are discussed along with their general mechanism. To learn more about this topic and other related topics, such as aromatic hydrocarbons, register with BYJU’S and download the mobile application on your smartphone.

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