Understanding Benzene and its Electrophilic Aromatic Substitution Reactions

Aromatic compounds like benzene undergo electrophilic aromatic substitution reactions (EAS). This process involves the reaction of an electrophile with the aromatic ring, leading to the formation of various derivatives. Examples include halogenation, nitration, and sulphonation of benzene, each with its own unique mechanism and requirements for reaction conditions.

• Benzene
• Aromatic compounds
• Electrophilic substitution
• Reactions
• Mechanisms

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1. UNIT I : Benzene & its derivatives Topic : Reactions of Benzene PART -I

2. Electrophilic Aromatic Substitution Reactions Aromatic compounds (Benzene) undergo electrophilic aromatic substitution (EAS). Arene (Ar-H) is the generic term for an aromatic hydrocarbon. The aryl group (Ar) is derived by removal of a H atom from an arene.

3. Benzene shows following reactions

4. General Mechanism for ESR : In step 1: the electrophile reacts with two p electrons from the aromatic ring The arenium ion is stabilized by resonance which delocalizes the charge

5. In step 2, a proton is removed and the aromatic system is regenerated.

6. 1. Halogenation of Benzene: Benzene reacts with chlorine or bromine in an electrophilic substitution reaction, but only in the presence of a catalyst and gives halobenzene.

7. Mechanism :

8. We should know; Halogenation of benzene requires the presence of a Lewis acid. Fluorination occurs so rapidly it is hard to stop at mono- fluorination of the ring (A special apparatus is used to perform this reaction). Iodine is so unreactive that an alternative method must be used.

9. 2. Nitration of Benzene : Nitration of benzene occurs with a mixture of concentrated nitric and sulfuric acids.

10. Mechanism :

11. 3. Sulphonation of Benzene: Sulphonation occurs most rapidly using fuming sulfuric acid (concentrated sulfuric acid that contains SO3)

12. Mechanism :