Transcript Benzyne Mechanism

Benzyne Mechanism
Reactant is halobenzene with no electronwithdrawing groups on the ring.
Use a very strong base like NaNH2.
1
=>
Benzyne Intermediate
NH2
H
NH2
NH2
_
NH2
H
H
H
_
or
H
H
CH3
H
H
CH3
meta-toluidine
2
H
H
CH3
H
H
CH3
para-toluidine
=>
Chlorination of Benzene
Addition to the benzene ring may occur
with high heat and pressure (or light).
The first Cl2 addition is difficult, but the
next 2 moles add rapidly.
The product, benzene hexachloride, is an
insecticide.
=>
3
Catalytic Hydrogenation
Elevated heat and pressure is required.
Possible catalysts: Pt, Pd, Ni, Ru, Rh.
Reduction cannot be stopped at an
intermediate stage.
CH3
CH3
3 H2, 1000 psi
Ru, 100°C
CH3
4
CH3 =>
Birch Reduction:
Regiospecific
• A carbon with an e--withdrawing group
O
O
is reduced.
C
C
OH
Na, NH3
_
O
H
CH3CH2OH
• A carbon with an e--releasing group
is not reduced.
OCH3
Li, NH3
(CH3)3COH, THF
5
OCH3
=>
Birch Mechanism
=>
6
Side-Chain Oxidation
Alkylbenzenes are oxidized to benzoic acid
by hot KMnO4 or Na2Cr2O7/H2SO4.
CH(CH3)2
-
KMnO4, OH
CH CH2
H2O, heat
_
COO
_
COO
=>
7
Side-Chain Halogenation
Benzylic position is the most reactive.
Chlorination is not as selective as
bromination, results in mixtures.
Br2 reacts only at the benzylic position.
Br
CH2CH2CH3
Br2, h
CHCH2CH3
=>
8
SN1 Reactions
Benzylic carbocations are resonancestabilized, easily formed.
Benzyl halides undergo SN1 reactions.
CH2Br
9
CH3CH2OH, heat
CH2OCH2CH3
=>
SN2 Reactions
Benzylic halides are 100 times more
reactive than primary halides via SN2.
Transition state is stabilized by ring.
=>
10
Reactions of Phenols
Some reactions like aliphatic alcohols:
phenol + carboxylic acid  ester
phenol + aq. NaOH  phenoxide ion
Oxidation to quinones: 1,4-diketones.
O
OH
Na2Cr2O7, H2SO4
CH3
CH3
11
O
=>
Quinones
Hydroquinone is used as a developer for
film. It reacts with light-sensitized AgBr
grains, converting it to black Ag.
Coenzyme Q is an oxidizing agent found
in the mitochondria of cells.
=>
12
Electrophilic
Substitution of Phenols
Phenols and phenoxides are highly reactive.
Only a weak catalyst (HF) required for FriedelCrafts reaction.
Tribromination occurs without catalyst.
Even reacts with CO2.
O
_
_
O
-
CO2, OH
O
C
OH
O
_
+
O
C
H
OH
=>
13
salicylic acid