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Chapter 15 (lecture 8)

Chapter 15 (lecture 8) - Nitration of Benzene Benzene...

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Nitration of Benzene Benzene reacts only slowly with hot concentrated nitric acid to give nitrobenzene. The reaction is much faster in a mixture of concentrated nitric acid (pKa = -1.3) and concentrated sulfuric acid (pKa = -9), a much stronger acid. + HNO 3 + H 2 SO 4 concentrated acids ~50 o C NO 2 + H 3 O + + HSO 4 - nitrobenzene (~85%)
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Mechanism of Nitration (1) Generation of the Electrophile Nitric acid undergoes reversible dehydration in the presence of concentrated sulfuric acid producing the nitronium ion , a strong electrophile. H-O : : N O = : : O : : : - + pKa = -1 + H-OSO 3 H pKa = -9 H-O : N O = : : O : : : - + H + + OSO 3 H - H-O : N O = : : O : : : - + H + H 2 O + N O = : : + nitronium ion O = : :
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(2) Electrophilic Attack N O = : : + O = : : + slow step H N = : : : : : - + etc. arenium ion + O O (3) Deprotonation and Re-aromatization H N = : : : : : - + + + O-H H : : O O NO 2 + H 3 O +
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Sulfonation of Benzene Benzene reacts with fuming sulfuric acid (concentrated sulfuric acid with SO 3 ) to give benzenesulfonic acid. In concentrated sulfuric acid alone, sulfonation proceeds slowly. fuming H 2 SO 4 25 o C SO 3 H benzenesulfonic acid (1) Generation of the Electrophile In concentrated sulfuric acid, dehydration of the acid produces sulfur trioxide, SO 3 , a strong electrophile . H-O-S-O-H :O: == :O: : : : : + H-O-S-O-H :O: == :O: : : : : H-O-S-O: :O: == :O: : : : : - + H-O-S-O-H :O: == :O: : : : H + H-O-S-O-H :O: == :O: : : : H + H 3 O + + S=O : : = = : : sulfur trioxide O O . . . .
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(2) Electrophilic Attack + slow S=O : : : : O O . . . . H S = O: : : - :O: = + etc. arenium ion O : : (3) Deprotonation and Re-aromatization H S = O: : : - :O: = + + :O-S-O-H :O: = = :O: : : : : - hydrogen sulfate anion fast S :O: = = O: : : - + H 2 SO 4 O : : O . . . .
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(4) Acid-Base Equilibrium Synthetic Applications Although the introduction of a sulfonic acid group is generally of more limited interest than other electrophilic substitution reactions, the reversibility of sulfonation leads to its use as a synthetic strategy. Heating arylsulfonic acids in dilute sulfuric acid removes the sulfonic acid function. SO 3 H + H 2 O heat dilute H 2 SO 4 + H 2 SO 4 S :O: = = O: : : - + H 3 O + S :O: = = O : : + H 2 O benzenesulfonic acid K a = 2 x 10 -1 fast O . . . . O . . . . H
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Friedel-Crafts Alkylation Alkylation: Introduction of an Alkyl Group into an Arene + R -X AlCl 3 R + HX alkyl halide alkylbenzene
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A Mechanism for the Alkylation Reaction The Lewis acid catalysts that are often required in the Friedel-Crafts reactions promote formation of strong electrophiles . (1) Generation of the Electrophile R-Cl: : : + AlCl 3 Lewis acid R-Cl-AlCl 3 : : - + complex Lewis base With 2 o and 3 o R-Cl compounds, dissociation to carbocation intermediates seems to occur, with the R + species reacting with the arene. In some reactions, it is believed that the complex serves as the transfer agent for R + in the electrophilic addition step.
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