Homework

1
Homework

• Read 22.1
• Answer questions 21.42, 21.43, 21.48, 21.52.

2
Kolbe Carboxylation

• Phenoxide ions react with carbon dioxide to give
  a carboxylic salt

O
O
3
Kolbe Carboxylation

• the mechanism begins by nucleophilic addition of
  the phenoxide ion to a carbonyl group of CO2

4
Quinones

• Because of the presence of the electron-donating
  -OH group, phenols are susceptible to oxidation
  by a variety of strong oxidizing agents

O
O
5
Quinones
6
Quinones

• Perhaps the most important chemical property of
  quinones is that they are readily reduced to
  hydroquinones

7
Coenzyme Q

• Coenzyme Q is a carrier of electrons in the
  respiratory chain

O
H
H
n
n
O
n6-10
8
Vitamin K

• both natural and synthetic vitamin K (menadione)
  are 1,4-naphthoquinones

9
Benzylic Oxidation

• Benzene is unaffected by strong oxidizing agents
  such as H2CrO4 and KMnO4
• an alkyl group with at least one hydrogen on its
  benzylic carbon is oxidized to a carboxyl group
• halogen and nitro substituents are unaffected by
  these reagents but groups sensitive to oxidation,
  e.g., -OH, -NH2, or -NHR must be protected.

2-Chloro-4-nitrotoluene
2-Chloro-4-nitrobenzoic acid
10
Benzylic Oxidation
methylbenzene (toluene)
ethylbenzene
iso-propylbenzene (cumene)
11
Benzylic Oxidation

• if there is more than one alkyl group on the
  benzene ring, each is oxidized to a -COOH group

O
O
1,4-Dimethylbenzene
(
p-
xylene)
12
Benzylic Chlorination

• Chlorination (and bromination) occurs by a
  radical mechanism

13
Benzylic Reactions

• Benzylic radicals (and cations also) are easily
  formed because of the resonance stabilization of
  these intermediates
• the benzyl radical is a hybrid of five
  contributing structures

C
C
C
C
C
14
Benzylic Halogenation

• benzylic bromination is highly regioselective
• benzylic chlorination is less regioselective

Review section 8.5!
15
Hydrogenolysis

• Hydrogenolysis cleavage of a single bond by H2
• among ethers, benzylic ethers are unique in that
  they are cleaved under conditions of catalytic
  hydrogenation

16
Benzyl Ethers

• The value of benzyl ethers is as protecting
  groups for the OH groups of alcohols and phenols
• to carry out hydroboration/oxidation of this
  alkene, the phenolic -OH must first be protected
  it is acidic enough to react with BH3 and destroy
  the reagent

O
O