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Substitution Reaction

In a substitution reaction one atom or group of atoms in a molecule is replaced by another. Substitution reactions are important reactions in organic chemistry. In these reactions, alkl groups are transferred to the nucleophiles. Organic electrophiles of this type are referred to as alkylating agents. Through the use of these reactions, readily available alkyl halides can be converted into a wide variety of other compounds.

"A reaction in which an atom or group of atoms in a compound is replaced by another atom or group of atoms."

For example, a hydrogen atom in cyclohexane is replaced by a bromine atom when the alkane is exposed to Br2 in the presence of light or heat. Electrophilic substitution of benzene is an electrophilic aromatic substitution. In this reaction, an electrophile replaces an atom, normally hydrogen, appended to an aromatic system.

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Substitution Reaction Mechanism

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Purposes of Electrophilic Substitution Mechanism
  • C6H6 (benzene) is planar molecule which have a six carbon atom ring. Each carbon is attached with a hydrogen atom.
  • De localized electrons are present above and below the plane of ring.

Substitution Reaction Mechanism

  • Due to the presence of delocalized electrons, benzene is stable.
  • Addition reactions are assisted by benzene. In this reaction, it breaks the delocalization and loses that stability.
  • The symbol of benzene is Benzene where delocalized electrons are represented by circle. Each corner of this hexagon contains a carbon atom by which a hydrogen atom is attached.

Electrophilic Substitution Reaction

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As delocalized electrons are exposed above and below the plane, benzene is highly attractive to electrophiles which seek after electron rich areas in any other molecules. The electrophiles can be either positive ion, or the less positive end of polar molecule.

De localized electrons which are at above and below the benzene plane are open for attacking in same way as those below and above the plane of ethene molecule. Since, the end result will be different.

Like ethene, benzene requires to use some delocalized electrons forming bonds with new groups and atoms. This results in breaking the delocalization. This costs energy.

Instead, in substitution reaction, if a hydrogen atom is replaced, it can maintain delocalization. With delocalized electrons, hydrogen atoms are not involved in any way. In most of the reactions with benzene, the electrophile is a positive ion.

Substitution Reaction Example

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Basic Example of Electrophilic Substitution Reaction

1. Aromatic nitration

In this reaction, nitronium ion is generated from sulfuric acid and nitric acid and nitro compounds are formed.

Aromatic Nitration

2. Aromatic sulfonation

Sulfonation of benzene takes place with fuming sulfuric acid and forms benzene sulfonic acid.

Aromatic Sulfonation

3. Aromatic halogenation of benzene

With chlorine, bromine or iodine, corresponding aryl halogen compounds are formed which are catalyzed by the corresponding iron trihalide.

Aromatic Halogenation of Benzene

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