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StudentNotesSection11 - Section 11: Chemistry of Benzene...

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Section 11: Chemistry of Benzene 136 Section 11: Chemistry of Benzene 11.1 Electrophilic Aromatic Substitution We recall from our earlier discussion of the aromaticity of benzene that benzene does not undergo addition reactions with electrophiles. However, benzene does undergo substitution reactions with a wide range of electrophiles – electrophilic aromatic substitution is the most common reaction a benzene ring can undergo. 11.1.1 Generalized Mechanism The mechanism of substitution is a two-step process, where E + is the electrophilic species:
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Section 11: Chemistry of Benzene 137 11.1.2 Deuteration The substitution of deuterium ( 2 H or D) for a proton on an aromatic ring is a relatively straightforward reaction that may be achieved using a strong deuterated acid such as D 2 SO 4 . The mechanism is identical to that described above where E + is D + . 11.1.3 Halogenation X 2 FeX 3 (cat.) X + HX The halogenation of an alkene may be achieved using the halogen (such as Br 2 or Cl 2 ) dissolved in ether. In the case on benzene, the halogen itself is not sufficiently electrophilic for the first addition step to proceed. We can activate the halogen using Lewis acid, (FeBr 3 or FeCl 3 ), to catalyze the reaction.
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Section 11: Chemistry of Benzene 138 Iodine does not readily react with benzene. However, I + may be readily generated by the oxidation of I 2 with a species such as hydrogen peroxide or a copper (II) salt: I 2 + 2 CuCl 2 2 I + 2CuCl + 2 Cl The reaction of I+ with benzene proceeds in the same manner as indicated in the generalized mechanism. Note that in this case, the copper (II) salt is not acting as a catalyst. 11.1.4 Nitration HNO 3 NO 2 H 2 SO 4 Aromatic nitration requires quite forcing conditions –a combination of nitric and sulfuric acids, often in very high concentrations. The electrophile in this case is a nitronium ion, generated by the protonation of nitric acid.
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Section 11: Chemistry of Benzene 139 11.1.5 Sulfonation SO 3 H 2 SO 4 S O O OH also written S O O SO 3 H Sulfonation also requires very forcing conditions. In this case, we use “fuming” sulfuric acid – a saturated solution of sulfur trioxide in sulfuric acid. Depending on the reaction conditions, the electrophile may be sulfur trioxide, or its protonated conjugate acid.
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This note was uploaded on 10/28/2010 for the course CHEM 235 taught by Professor Dr.poole during the Spring '10 term at Ball State.

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StudentNotesSection11 - Section 11: Chemistry of Benzene...

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