CHM138H1 Chapter all: Aromatic Substitution Reactions
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CHM136H1 Full Course Notes
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Electrophilic aromatic substitution is a reaction in which one of the aromatic protons is substituted by an electrophile, keeping the aromatic ring intact. First step: the sigma complex (arenium ion) is resonance stabilized. This step requires energy because aromaticity is lost. Second step: sigma complex is deprotonated, aromaticity is restored, and the catalyst is regenerated. There is no addition product because addition results to permanent lost of aromaticity. Chlorination is similar reaction with analogous mechanism; benzene reacts with cl2 using alcl3 as catalyst. Fluorination and iodination are less common. omatic it. Sulfonation is achieved by reacting with fuming sulfuric acid (mixture of h2so4 and so3 gas). The s=o bond is like a single bond with charge separation (s+ and o-); s uses 3p orbital and o uses 2p leading to inefficient overlap. The reaction is reversible because of the sensitivity of the reaction to the concentrations of the reagents.
