Chapter 15 (lecture 8)

Chapter 15(lecture - Nitration of Benzene Benzene reacts only slowly with hot concentrated nitric acid to give nitrobenzene The reaction is much

Info iconThis preview shows pages 1–9. Sign up to view the full content.

View Full Document Right Arrow Icon
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%)
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
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 : :
Background image of page 2
(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 +
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
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 . . . .
Background image of page 4
(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 . . . .
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
(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
Background image of page 6
Friedel-Crafts Alkylation Alkylation: Introduction of an Alkyl Group into an Arene + R -X AlCl 3 R + HX alkyl halide alkylbenzene
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
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
Background image of page 8
Image of page 9
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 01/20/2012 for the course CHEM 325BL at USC.

Page1 / 27

Chapter 15(lecture - Nitration of Benzene Benzene reacts only slowly with hot concentrated nitric acid to give nitrobenzene The reaction is much

This preview shows document pages 1 - 9. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online