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Chapter 3: Problem 121

Give, in the form of simple equations, the method of generation of electrophiles in the following aromatic electrophilic substitutions. (i) Sulphonation (ii) Friedel-Crafts alkylation (iii) Friedel-Crafts acylation

Short Answer

Expert verified
Question: Explain the generation of electrophiles in sulphonation, Friedel-Crafts alkylation, and Friedel-Crafts acylation of aromatic rings. Answer: In sulphonation, the electrophile SO3 is generated by the reaction of concentrated sulfuric acid (H2SO4) with the aromatic compound. In Friedel-Crafts alkylation, the carbocation electrophile (R+) is created by reacting an alkyl halide (RX) with a Lewis acid catalyst, such as aluminum chloride (AlCl3). In Friedel-Crafts acylation, the acylium ion electrophile (RCO+) is formed by reacting an acid chloride (RCOCl) with a Lewis acid catalyst, typically aluminum chloride (AlCl3).

Step by step solution

01

(i) Sulphonation of Aromatic Rings

Sulphonation is the process of introducing a sulphonic acid group (-SO3H) to an aromatic ring. The most common electrophile used in sulphonation is the sulphur trioxide (SO3) molecule. It is generated by the reaction of concentrated sulfuric acid (H2SO4) with the aromatic compound. The equation for the generation of the electrophile is: H2SO4 -> H+ + HSO4- HSO4- + SO3 -> H2SO4SO3 (Electrophile: SO3) The aromatic compound then reacts with the generated SO3 electrophile, leading to the sulphonation.
02

(ii) Friedel-Crafts Alkylation of Aromatic Rings

Friedel-Crafts alkylation is a reaction in which an alkyl group is added to an aromatic ring. This involves an electrophile in the form of a carbocation (R+). The carbocation is usually generated by reacting an alkyl halide (RX) with a Lewis acid catalyst, mostly aluminum chloride (AlCl3). The equation for the generation of the electrophile is: AlCl3 + R-Cl -> R+ + AlCl4- (Electrophile: R+) The aromatic compound reacts with the generated R+ electrophile, leading to alkylation.
03

(iii) Friedel-Crafts Acylation of Aromatic Rings

Friedel-Crafts acylation is a reaction in which an acyl group (-COR) is added to an aromatic ring. The electrophile in this case is an acylium ion (RCO+). The acylium ion is generated by reacting an acid chloride (RCOCl) with a Lewis acid catalyst, mostly aluminum chloride (AlCl3). The equation for the generation of the electrophile is: AlCl3 + RCO-Cl -> RCO+ + AlCl4- (Electrophile: RCO+) The aromatic compound then reacts with the generated RCO+ electrophile, leading to acylation.

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