Ch_322b_15.03

Ch_322b_15.03 - Chapter 15.3 lecture note

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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 + Remember, the nitro group is zwitterionic: NO 2 - + N :O: O
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Sulfonation of Benzene Benzene reacts with
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Ch_322b_15.03 - Chapter 15.3 lecture note

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