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1
Created byProfessor William Tam & Dr. Phillis Chang
Chapter 15
Reactions ofAromatic Compounds
Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.
1. Electrophilic AromaticSubstitution Reactions
v Overall reaction
© 2014 by John Wiley & Sons, Inc. All rights reserved.
R
RClAlCl3
R
O R Cl
O
AlCl3
SO3H
H2SO4SO3
© 2014 by John Wiley & Sons, Inc. All rights reserved.
v Different chemistry with alkene
CC
Br2Br C
C Br
Br2
+
+ No Reaction
2. A General Mechanism for Elec-trophilic Aromatic Substitutions
© 2014 by John Wiley & Sons, Inc. All rights reserved.
2
v Benzene does not undergo electrophilicaddition, but it undergoes electrophilicaromatic substitution
+H E A
H AE
(H substituted by E)
© 2014 by John Wiley & Sons, Inc. All rights reserved.
v Mechanism● Step 1
E+ E
slowr.d.s.
E
E
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E
HB
v Mechanism● Step 2
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3
Reactions of Benzene Reactions of Benzene
v Benzene does not react with Br2 or Cl2unless a Lewis acid is present (a catalytic amount is usually enough)
3. Halogenation of Benzene
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v Examples
● Reactivity: F2 > Cl2 > Br2 > I2© 2014 by John Wiley & Sons, Inc. All rights reserved.
4
v Mechanism
Br BrFeBr3
(weakelectrophile)
d-d+
Br Br FeBr3
Br + FeBr4
(very reactiveelectrophile)
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BrBrBr
Brslow r.d.s.
v Mechanism (Cont’d)
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v Mechanism (Cont’d)
Br
HBr FeBr3
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v F2: too reactive, gives a mixture of mono-, di- and polysubstitutedproducts
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5
v I2: very unreactive even in the presence of Lewis acids; usually need to add an oxidizing agent (e.g. HNO3, Cu2+, H2O2)
© 2014 by John Wiley & Sons, Inc. All rights reserved. © 2014 by John Wiley & Sons, Inc. All rights reserved.
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v Electrophile in this case is NO2Å
(nitronium ion)
4. Nitration of Benzene
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6
v MechanismOSO
OHO H NO
OHO+
HSO4- N
O
OO
H
H+ N OO H2O
(NO2)
+
© 2014 by John Wiley & Sons, Inc. All rights reserved.
NO2slow r.d.s.
NO2NO2NO2
v Mechanism (Cont’d)
© 2014 by John Wiley & Sons, Inc. All rights reserved.
v Mechanism (Cont’d)
NO2
HH2O NO2
+ H3O+
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v Mechanism● Step 1
5. Sulfonation of Benzene
SO3 is protonated to form SO3H+
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7
● Step 2
SO3H+ reacts as an electrophile
with the benzene ring to form an
arenium ion© 2014 by John Wiley & Sons, Inc. All rights reserved.
● Step 3
Loss of a proton from the arenium ion restores
aromaticity to the ring and regenerates the acid
catalyst© 2014 by John Wiley & Sons, Inc. All rights reserved.
SO3H
SO3, conc. H2SO4
25oC - 80oC
v Sulfonation & Desulfonation
dil. H2SO4
H2O, 100oC
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8
v Electrophile in this case is RÅ
● R = 2o or 3o
● or (R = 1o)R ClAlCl3d+ d-
6. Friedel–Crafts Alkylation
© 2014 by John Wiley & Sons, Inc. All rights reserved.
v Mechanism
Cl AlCl3 Cl AlCl3
AlCl4+
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v Mechanism (Cont’d)
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v Mechanism (Cont’d)
HCl AlCl3
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9
v Note: Not necessary to start with alkyl halide, other possible functional groups can be used to generate a reactive carbocation
+ H+e.g.
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OH
BF3
60oC+
O BF3
Hvia
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v Acyl group:
v Electrophile in this case is R–C≡OÅ
(acylium ion)
7. Friedel–Crafts Acylation
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10
v Mechanism
O
R ClAlCl3+
R C O R C OAlCl4 +
OCR Cl AlCl3
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v Mechanism (Cont’d)
R C O
R
O
R
O
R
O
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v Mechanism (Cont’d)
H
OR
Cl AlCl3
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v Acid chlorides (or acyl chlorides)
RCO
Cl
RCO
OH RCO
Clor
SOCl2
PCl5
● Can be prepared by
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11
v When the carbocation formed from an alkyl halide, alkene, or alcohol can rearrange to one or more carbocations that are more stable, it usually does so, and the major products obtained from the reaction are usually those from the more stable carbocations.
8. Limitations of Friedel–CraftsReactions
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(How is thisformed?)
(not formed)v For example
AlCl3
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1o cation (not stable)v Reason
Cl AlCl3H
AlCl4+ +
1,2-hydride shift
H 3o cation(more stable)
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v Friedel–Crafts reactions usually give poor yields when powerful electron-withdrawing groups are present on the aromatic ring or when the ring bears an –NH2, –NHR, or –NR2 group. This applies to both alkylations and acylations.
NO2 N(CH3)3 CF3 SO3H NH2O OH O R
These usually give poor yields in Friedel-Crafts reactions
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12
v The amino groups, –NH2, –NHR, and –NR2, are changed into powerful electron-withdrawing groups by the Lewis acids used to catalyze Friedel-Crafts reactions
N NH H
HH AlCl3
>
AlCl3+
Does not undergo a Friedel-Crafts
reaction© 2014 by John Wiley & Sons, Inc. All rights reserved.
v Aryl and vinylic halides cannot be used as the halide component because they do not form carbocations readily
, AlCl3
Cl
Cl , AlCl3
No Friedel-Craftsreaction
No Friedel-Craftsreaction
sp2
sp2
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v Polyalkylations often occur
+OH
+BF3
60oC
(24%) (14%)
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v Rearrangements of the carbon chain do not occur in Friedel–Crafts acylations
v The acylium ion, because it is stabilized by resonance, is more stable than most other carbocations. Thus, there is no driving force for a rearrangement.
9. Synthetic Applications ofFriedel-Crafts Acylations:The Clemmensen Reduction & Wolff–Kishner Reductions
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13
v The carbonyl group of an aryl ketone can be reduced to a CH2 group
[H]R
O
R
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9A. The Clemmensen Reduction
HClreflux
R
O
RZn/Hg
© 2014 by John Wiley & Sons, Inc. All rights reserved.
© 2014 by John Wiley & Sons, Inc. All rights reserved.
v Clemmensen reduction of ketones● A very useful reaction for making
alkyl benzenes that cannot be made via Friedel-Crafts alkylations
?e.g.
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14
v Clemmensen reduction of ketones● Cannot use Friedel-Crafts alkylation
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v Rearrangements of carbon chains do not occur in Friedel-Crafts acylations
(no rearrangement of the R group)
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Zn/Hgconc. HClreflux
© 2014 by John Wiley & Sons, Inc. All rights reserved.
9B. The Wolff–Kishner Reduction
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15
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Quiz 1
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Quiz 2
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Quiz 3
16
v Two questions have to be addressed:● Reactivity● Regiochemistry
10. Substituents Can Affect Boththe Reactivity of the Ring and the Orientation of the Incoming Group
© 2014 by John Wiley & Sons, Inc. All rights reserved.
● Reactivity
faster or slower than
Y = EDG (electron-donating group) or EWG (electron-withdrawing group)
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● Regiochemistry
Statistical mixture of o-, m-, p-products, or any preference?
© 2014 by John Wiley & Sons, Inc. All rights reserved.
G
E A+
GE
Hotherresonancestructure
d+ d-
A substituted benzene
Electrophilic reagent Arenium
ion
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17
Z> Y
>
Y withdraws electrons
Z donates electrons
The ring is electron poor and reacts more slowly with an electrophile
The ring is more electron rich and reacts faster with an electrophile
© 2014 by John Wiley & Sons, Inc. All rights reserved.
–EDG
–H
–EWG
Incr
easin
g ac
tivity
● Reactivity
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Substituent
● Reactivity towards electrophilic aromatic substitution
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18
The energy diagrams below illustrate the effect of electron-withdrawing and electron-donating groups on the transition stateenergy of the rate-determining step.
Figure 18.6 Energy diagrams comparing the rate of electrophilic substitution of substituted benzenes
v Regiochemistry: directing effect
● General aspectst Either o-, p- directing or m-
directingt Rate-determining step is p-
electrons on the benzene ring attacking an electrophile (EÅ)
© 2014 by John Wiley & Sons, Inc. All rights reserved.
attack
YYY
-I -II -III
EEE
Y
E
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attack
YYY
-I -II -IIIE E E
Y
E
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19
Y
E
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v Classification of different substituentsY
Y (EDG)–NH2, –NR2–OH, –O-
Strongly activating
o-, p-directing
–NHCOR–OR
Moderately activating
o-, p-directing
–R (alkyl)–Ph
Weakly activating
o-, p-directing
–H NA NA© 2014 by John Wiley & Sons, Inc. All rights reserved.
v Classification of different substituentsY
Y (EWG)
–Halide(F, Cl, Br, I)
Weakly deactivating
o-, p-directing
–COOR, –COR,–CHO, –COOH,–SO3H, –CN
Moderately deactivating
m-directing
–CF3, –CCl3,–NO2, –⊕NR3
Strongly deactivating
m-directing
© 2014 by John Wiley & Sons, Inc. All rights reserved.
20
11. How Substituents AffectElectrophilic AromaticSubstitution:A Closer Look
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v Two types of EDG(i)
11B.Inductive & Resonance Effects: Theory of Orientation
by resonance effect (donates electron towards the benzene ring through resonance)
OR NR2
or
CH3>(ii) by positive inductive effect (donates electron towards the benzene ring through the s bond)
© 2014 by John Wiley & Sons, Inc. All rights reserved.
v Two types of EDG
● The resonance effect is usually stronger than the positive inductiveeffect if the atoms directly attached to the benzene ring are in the same row as carbon in the periodic table
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21
v Similar to an EDG, an EWG can withdraw electrons from the benzene ring by the resonance effect or by the negative inductive effect
CO
CH3e.g.>
C F
F
F>>
Deactivate the ring by the resonance effect
Deactivate the ring by the negative inductive effect
© 2014 by John Wiley & Sons, Inc. All rights reserved.
v EWG = –COOR, –COR, –CHO, –CF3, –NO2, etc.
11C. Meta-Directing Groups
(EWG ≠ halogen)
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v For example
(highly unstable due to the negative inductive effect of –CF3)
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(highly unstable due to negative inductive effect of –CF3)
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22
(positive charge never attaches to the carbon directly attached to the EWG: –CF3) Þ relatively more favorable
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v EDG = –NR2, –OR, –OH, etc.
11D. Ortho/Para-Directing Groups
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23
v For example
(extra resonance structure due to positive mesomeric effect of –OCH3)
© 2014 by John Wiley & Sons, Inc. All rights reserved.
OCH3 OCH3OCH3
OCH3
OCH3
(para)
NO2 NO2NO2
NO2
- H+
OCH3
NO2
(para)(favorable)
NO2
(extra resonance structure due to resonanceeffect of –OCH3)
© 2014 by John Wiley & Sons, Inc. All rights reserved.
(3 resonance structures only, no extra stabilization by resonance effect of –OCH3) Þ less favorable
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24
v For halogens, two opposing effects
Negative inductive effect:withdraws electron density from the
benzene ring
ClCl
>
Resonance effect:donates electron
density to thebenzene ring
© 2014 by John Wiley & Sons, Inc. All rights reserved.
v Overall● Halogens are weak deactivating
groupst Negative inductive effect >
resonance effect in this case
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With the exception of fluorine, halogens must use 3p, 4p, and 5p orbitals to overlap with the 2p orbital of carbon - this overlap becomes progressively weaker as the size of the halogen increases.
v Regiochemistry
(extra resonance structure due to resonance effect of –Cl)
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25
(extra resonance structure due to resonanceeffect of –Cl)
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(3 resonance structures only, no extra stabilization by the resonance effect of –Cl) Þ less favorable
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99
Ortho/Para-Directing Deactivators: Halogens• Electron-withdrawing inductive effect outweighs weaker
electron-donating resonance effect.Energy Diagram
26
11E. Ortho/Para Direction andReactivity of Alkylbenzenes
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CH3
E
CH3
ECH3
E
CH3
E
>
v Ortho attack
Relatively stable contributor
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v Meta attack
© 2014 by John Wiley & Sons, Inc. All rights reserved.
CH3
E
CH3 CH3 CH3
E E E
>
v Para attack
Relatively stable contributor
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27
Summary
-H (unsubstituted)
-R or -Ar, ortho- and para- -R or -Ar, meta-
-NH2 or -OH, meta- -NH2 or -OH, ortho- and para-
-Cl or -Br, ortho- and para- -Cl or -Br, meta-
-NO2 or -CO2H, meta-
-NO2 or -CO2H, ortho- and para-
Deactivators
Activators
Reactants
[CarbocationIntermediate]
Reaction Progress
Energy
11F. Summary of Substituent Effects on Orientation and Reactivity
Y Y (EDG)
–NH2, –NR2–OH, –O-
Strongly activating
o-, p-directing
–NHCOR–OR
Moderately activating
o-, p-directing
–R (alkyl)–Ph
Weakly activating
o-, p-directing
–H NA NA© 2014 by John Wiley & Sons, Inc. All rights reserved.
Y Y (EWG)
–Halide(F, Cl, Br, I)
Weakly deactivating
o-, p-directing
–COOR, –COR,–CHO, –COOH,–SO3H, –CN
Moderately deactivating
m-directing
–CF3, –CCl3,–NO2, –⊕NR3
Strongly deactivating
m-directing
© 2014 by John Wiley & Sons, Inc. All rights reserved. © 2014 by John Wiley & Sons, Inc. All rights reserved.
28
© 2014 by John Wiley & Sons, Inc. All rights reserved. © 2014 by John Wiley & Sons, Inc. All rights reserved.
CH3
Methylbenzene(toluene)
Ethylbenzene Isopropylbenzene(cumene)
Phenylethene(styrene or
vinylbenzene)
12. Reactions of the Side Chainof Alkylbenzenes
© 2014 by John Wiley & Sons, Inc. All rights reserved.
12A. Benzylic Radicals and Cations
Methylbenzene(toluene)
CH2HR
- RH
CH2
The benzylradical
CC C C
Benzylic radicals are stabilized by resonance© 2014 by John Wiley & Sons, Inc. All rights reserved.
29
CC C C
Benzylic cations are stabilized by resonance© 2014 by John Wiley & Sons, Inc. All rights reserved.
12B.Benzylic Halogenation of the Side Chain
© 2014 by John Wiley & Sons, Inc. All rights reserved.
v Mechanism● Chain initiation
2 XX Xperoxidesheat orlight
● Chain propagation
XHCC6H5 HH
+H
CC6H5H
H X+
© 2014 by John Wiley & Sons, Inc. All rights reserved.
● Chain propagation
● Chain termination
XHCC6H5 XH
+H
CC6H5H
+X X
XHCC6H5 XH
+H
CC6H5H
© 2014 by John Wiley & Sons, Inc. All rights reserved.
30
v e.g.
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C CC
CC C
conjugatedsystem
non-conjugatedsystem
is morestable than
13A. Stability of Conjugated Alkenyl-benzenes
v Alkenylbenzenes that have their side-chain double bond conjugated with the benzene ring are more stable than those that do not
13. Alkenylbenzenes
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v ExampleH+
heatOH
(not observed)
Ha Hb
- Ha
- Hb
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31
13B. Additions to the Double Bond ofAlkenylbenzenes
HBr
RO ORheat
HBr(no
peroxides)
Br
Br
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v Mechanism (top reaction)2 RORO OR
H Br+RO Br RO H+
+ Br Br
Br
(more stablebenzylic radical)
(less stable)
Br+ H Br
Br© 2014 by John Wiley & Sons, Inc. All rights reserved.
v Mechanism (bottom reaction)
H Br
H
H
(more stablebenzylic cation)
(less stable)d+ d-
Br
Br
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13C. Oxidation of the Side Chain
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32
© 2014 by John Wiley & Sons, Inc. All rights reserved.
v Using hot alkaline KMnO4, alkyl, alkenyl, alkynyl and acyl groups all oxidized to –COOH group
v For alkyl benzene, 3o alkyl groups resist oxidation
● Need benzylic hydrogen for alkyl group oxidation
© 2014 by John Wiley & Sons, Inc. All rights reserved.
CH3
NO2
How?
14. Synthetic Applications
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CH3
NO2
CH3
CH3Cl
AlCl3
conc. HNO3
conc. H2SO4heat
CH3
NO2
+
v CH3 group: ortho-, para-directingv NO2 group: meta-directing
© 2014 by John Wiley & Sons, Inc. All rights reserved.
33
NO2
CH3Cl
AlCl3
conc. HNO3
conc. H2SO4heat
CH3
NO2
CH3
NO2
NOT
v If the order is reversed Þ the wrong regioisomer is produced
© 2014 by John Wiley & Sons, Inc. All rights reserved.
v We do not know how to substitute a hydrogen on a benzene ring with a –COOH group. However, side chain oxidation of alkylbenzene could provide the –COOH group
v Both the –COOH group and the NO2group are meta-directing
COOH
NO2
© 2014 by John Wiley & Sons, Inc. All rights reserved.
v Route 1
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CH3Cl
AlCl3
conc. HNO3
conc. H2SO4heat
COOH
COOH
NO2
1. KMnO4, HO-, D
2. H3O+
CH3
v Route 2
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34
v Which synthetic route is better?● Recall “Limitations of Friedel-Crafts
Reactions, Section 15.8”t Friedel–Crafts reactions usually give
poor yields when powerful electron-withdrawing groups are present on the aromatic ring or when the ring bears an –NH2, –NHR, or –NR2group. This applies to both alkylations and acylations
t Route 2 is a better route
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v Both Br and Et groups are ortho-, para-directing
v How to make them meta to each other?
v Recall: an acyl group is meta-directing and can be reduced to an alkyl group by Clemmensen reduction
Br
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Br
O
Cl
AlCl3
O
O
Br
Br2FeBr3
Zn/Hg
HCl, heat
© 2014 by John Wiley & Sons, Inc. All rights reserved.
14A. Use of Protecting and BlockingGroups
v Protected amino groups● Example
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35
Problemv Not a selective synthesis, o- and p-
products + dibrominated and tribrominated products will form
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v The amino groups, –NH2, –NHR, and –NR2, are changed into powerful electron-withdrawing groups by the Lewis acids used to catalyze Friedel-Crafts reactions
N NH H
HH AlCl3
>
AlCl3+
Does not undergo a Friedel-Crafts
reaction© 2014 by John Wiley & Sons, Inc. All rights reserved.
Solutionv Introduce a deactivating group on
–NH2
© 2014 by John Wiley & Sons, Inc. All rights reserved.
v The amide group is less activating than –NH2 group ● No problem for over bromination
v The steric bulkiness of this group also decreases the formation of the o-brominated product
© 2014 by John Wiley & Sons, Inc. All rights reserved.
36
© 2014 by John Wiley & Sons, Inc. All rights reserved.
Problemv Difficult to get o-product without
getting p-product v Over nitration
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Solutionv Use of a –SO3H blocking group at the
p-position which can be removed later
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14B. Orientation in DisubstitutedBenzenes
v Directing effect of EDG usually outweighs that of EWG
v With two EDGs, the directing effect is usually controlled by the stronger EDG
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37
Examples [only major product(s) shown]
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39
C CCH2X
C CC
R
XH
C CC
R'
XR
1o Allylic 2o Allylic 3o Allylic
1o Benzylic 2o Benzylic 3o Benzylic
CArR
HX CAr
R'
RXCAr
H
HX
15. Allylic and Benzylic Halides inNucleophilic Substitution Reactions
© 2014 by John Wiley & Sons, Inc. All rights reserved.
H3C X R CH2 X R CH XR'
v A Summary of Alkyl, Allylic, & Benzylic Halides in SN Reactions● These halides give mainly SN2
reactions:
● These halides may give either SN1 or SN2 reactions:
Ar CH2 X Ar CH XR
C CCH2 X
C CC
R
XH
© 2014 by John Wiley & Sons, Inc. All rights reserved.
v A Summary of Alkyl, Allylic, & Benzylic Halides in SN Reactions● These halides afford mainly SN1
reactions:
C CC
R'
XR
C XR'R
R"C XArR
R'
© 2014 by John Wiley & Sons, Inc. All rights reserved.
H2/Ni
slowH2/Ni
fast
H2/Nifast
+
benzene cyclohexadienes cyclohexene
cyclohexane
16. Reduction of AromaticCompounds
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40
16A. The Birch Reduction
benzene
NaNH3, EtOH
1,4-cyclohexadiene
© 2014 by John Wiley & Sons, Inc. All rights reserved.
electride salt [Na(NH3)x]+ e−
v Mechanism
benzene
Na
benzene radical anion
- -etc.
EtOH
cyclohexadienyl radical
etc.
H
H
H
HNa
cyclohexadienyl anion
etc.
H
H
H
H-
- H
H
H
H
1,4-cyclohexadiene
EtOH
© 2014 by John Wiley & Sons, Inc. All rights reserved.
The solvated electrons add to the aromatic ring to give a radical anion.
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kinetic product is produced
because the largest orbital coefficient of the HOMO of the conjugated pentadienyl anion intermediate is on the central carbon atom.
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41
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v Synthesis of 2-cyclohexenones
OCH3 Liliq. NH3EtOH
OCH3
O2-cyclohexenone
H3O+
H2O
(84%)
© 2014 by John Wiley & Sons, Inc. All rights reserved. © 2014 by John Wiley & Sons, Inc. All rights reserved.
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