Transcript Substituent Effects in Electrophilic Aromatic Substitution

Chapter 12: Reactions of Arenes: Electrophilic Aromatic
Substitution
12.1: Representative Electrophilic Aromatic Substitution
Reactions of Benzene
E= -Cl, -Br, -I (halogenation)
-NO2 (nitration)
-SO3H (sulfonation)
-R (alkylation)
(Table 12.1)
(acylation)
275
12.2: Mechanistic Principles of Electrophilic Aromatic
Substitution
Recall the electophilic addition of HBr (or Br2) to alkenes (Ch. 6)
H
+
n u cleo p h ile
H
Br
H
+
Br
Br
electro p h ile
Most aromatic rings (benzene) are not sufficiently nucleophilic
to react with electrophiles. Catalysts are often needed to
increase the reactivity of the electrophiles.
Mechanism: a -bond of benzene acts as a nucleophile and
“attacks” the electrophile leading to a resonance stabilized
cyclohexadienyl carbocation. Loss of a proton gives the
substitution product and restores aromaticity.
276
H
H
H
+
H
E -X
H
H
H
H
H
E
H
Electrophilic substitution:
product regains
aromatic stabilization
H
H
H
H
E
E
H
HX +
H
H
H
E
H
H
H
+
X
H
H
Resonance stabilized
cyclohexadienyl cation
intermediate
H
H
H
X
H
X
H
Electrophilic addition:
products lose
aromatic stabilization
Aromaticity is worth ~ 130-150 KJ/mol
277
12.3: Nitration of Benzene
O
H N O 3, H 2S O 4
N
O
H 2O
O
HO
O
+
N
H 2S O 4
O
H 2O
N
N
O
N itric acid
Su lfu ric a cid
p K a ~ -1.3
p K a ~ -2 .0
O
+
H 2O
O
N itro n iu m
Io n
278
12.4: Sulfonation of Benzene
O
O
sulfur analogue of
a carboxylic acids
S
S O 3, H 2S O 4
OH
H 2O
B e n ze n e
su lfo nic a cid
O
O
O
+
S
O
H 2 SO 4
HO
S
+
H SO 4 -
O
279
12.5: Halogenation of Benzene
C l2 , F e C l3
Cl
B r2, F e B r3
Br
I2 , C u C l2
I
For X= C l or Br
F eX 3
X X
I2
+
C u 2+
+
X
X
FeX
2 I+
+
2 Cu+
280
12.6: Friedel-Crafts Alkylation of Benzene
Cl
A lC l 3
+
+ HCl
a lk yl h a lide
(ele ctro ip h ile )
Cl
+ A lC l 4
A lC l 3
stro n g
L ew is a cid
c a rb o c a tio n
281
Since the Friedel-Crafts alkylation goes through a carbocation
intermediate, skeletal rearrangements of the alkyl halide are
common
+
Cl
A lC l 3
+
(n o t o b serv ed )
+
A lC l 3
+
Cl
(6 5 % )
(3 5 % )
282
alkyl halide:
halide = F, Cl, Br, I
must be an alkyl halide; vinyl and aryl halides do not react
the aromatic substrate:
can not have strong electron withdrawing substituents,
nor an amino group
Y ≠ NO2, CN, -SO3H
O
(R= ketone, aldehyde, carboxylic acids,
R
ester)
-NH2, NHR, NR2, -N+R3,
Y
F-C alkylation is often difficult to stop after one alkylation reaction
(H 3 C ) 3 C -C l
A lC l 3
+
(H 3 C ) 3 C -C l
A lC l 3
in itial p ro d u ct is
m o re rea ctiv e
th a n b en zen e
m ajo r p ro d u ct
283
12.7: Friedel-Crafts Acylation of Benzene
O
C
O
O
+
O
+
R
Cl
R
C
Cl
C
A lC l 3
A lC l 3
R
R
O
O
C
C
R
acyl group
A lC l 4
R
The acylated product is less reactive than benzene toward
electrophilic aromatic substitution. F-C acylation can be
stopped after one acyl group is added
284
12.8: Synthesis of Alkylbenzenes by Acylation-Reduction
Ketones and aldehydes can be reduced to the alkanes with:
Zn(Hg), HCl (Clemmensen Reduction)
H2NNH2, KOH (Wolff-Kishner Reduction)
O
Z n (H g ), H C l
-o rH 2N N H 2, K O H
A ryl K e to n e
Cl
+
+
+
A lC l 3
O
O
Cl
A lC l 3
Z n (H g ), H C l
-o rH 2N N H 2, K O H
Rearrangements and multiple alkylations
are not observed for the F-C acylation
285
12.9: Rate and Regioselectivity in Electrophilic Aromatic
Substitution - The nature of a substituent already present on
the benzene ring affects the rate and regioselectivity (relative
position) of electrophilic aromatic substitution.
A substituent (-X) is said to be activating if the rate of electrophilic
aromatic substitution of the substituted benzene (C6H5X) is
faster than benzene.
A substituent (-X) is said to be deactivating if the rate of
electrophilic aromatic substitution of the substituted benzene
(C6H5X) is slower than benzene.
Relative rate of nitration:
CF3
(triflu o ro m e th y l)b e n z e n e
2.5 x 10-5
deactivating
CH3
benzene
1
to lu e n e
20-25
activating
286
CH3
CH3
CH3
CH3
NO2
H 2S O 4, H N O 3
+
+
NO2
NO2
to lu e n e
o -n itro to lu e n e
(6 3 % )
m -n itro to lu e n e
(3 % )
CF3
CF3
H 2S O 4, H N O 3
p -n itro to lu e n e
(3 4 % )
CF3
CF3
NO2
+
+
NO2
NO2
(triflu o ro m e th yl)b e n ze n e
o -n itro -(triflu o ro m e th yl) m -n itro -(triflu o ro m e th yl)
b e n ze n e
b e n ze n e
(6 % )
(9 1 % )
p -n itro -(triflu o ro m e th yl)
b e n ze n e
(3 % )
A substituent (-X) is said to be an ortho-para director if it directs
an incoming electrophile to positions ortho and/or para to itself.
A substituent (-X) is said to be an meta director if it directs
an incoming electrophile to position meta to itself.
287
Substituents are characterized as either electron-donating or
electron-withdrawing and alter the electron density of the
aromatic ring through:
1. Inductive effects: ability of a substituent to donate or withdraw
electron density through -bonds due to electronegativity
differences and bond polarities of a functional group
+ N
C
+ X
O
C
-
O 
+
-
N
R
CH3
O
X = F , C l,
B r, I
E lectro n -w ith d ra w in g g ro u p s
E lectro n -d o n atin g g ro u p
2. Resonance effects: ability of a substituent to donate or
withdraw electrons through non-bonding pairs of electrons or
overlap -bonds (conjugation).
O
O
N
C
C
R
E lectro n -w ith d ra w in g g ro u p s
X
N
OCH3
O
E lectro n -d o n a tin g g ro u p s
288
The rate (activating or deactivating) and regiochemistry
(ortho-para vs meta directing) can be understood by examining
the influence of the substituent on the stability of the cyclohexadienyl cation intermediate.
12.10: Rate and Regioselectivity in the Nitration of Toluene:
Regioselectivity: The carbocation intermediate from o- or
p-addition can be stabilized by the substituent through inductive
effects and hyperconjugation.
o rth o
6 3%
CH3
m eta
3%
p a ra
34%
289
Activating groups increase the rate of electrophilic aromatic
substitution at all positions of the ring.
Partial rate factors - relative rate of electrophilic aromatic
substitution compared to benzene
H 3C
CH3
H 3C
C
CH3
42
42
4.5
4 .5
2 .5
2 .5
3
3
58
75
Electron rich aromatic rings are more nucleophlic.
All activating group donate electrons through inductive effects
and/or resonance. Electron-donating groups stabilize the
carbocation intermediate of electrophilic aromatic substitution.
290
12.11: Rate and Regioselectivity in the Nitration of
(Trifluoromethyl)benzene - Regioselectivity: The carbocation
intermediate from o- or p-addition is destabilized by the
electron-withdrawing substituent. This directs addition to the
m-position.
o rth o
6%
CF3
m eta
91%
p a ra
3%
291
Dactivating groups decrease the rate of electrophilic aromatic
substitution at all positions of the ring.
Partial rate factors - relative rate of electrophilic aromatic
substitution compared to benzene
CF3
4 .5 x 1 0 -6
4 .5 x 1 0 -6
6 .7 x 1 0 -5
6 .7 x 1 0 -5
4 .5 x 1 0 -6
Electron deficient aromatic rings are less nucleophlic.
All deactivating group withdraw electrons through inductive
effects and/or resonance. Electron-withdrawing groups
destabilize the carbocation intermediate of electrophilic aromatic
substitution.
292
12.12: Substituent Effects in Electrophilic Aromatic
Substitution: Activating Substituents
All activating substituents increase the rate of electrophilic
aromatic substitution and are ortho-para directors.
Nitration of phenol: the -OH is a very strong activating group
o rth o
5 0%
OH
m eta
0%
p a ra
50%
293
Substituents that have an O or N atom directly attached to the
aromatic ring are strong activators. Phenol, anisole, and anilines
are very strong activators and do not require strong Lewis Acid
catalysts to undergo electrophilic aromatic substutution.
O
O
-alkyl, -vinyl, -aryl
O
activators
C
H
N
R
C
-OH, -OCH3, -NH2
R
strong activators
very strong activators
12.13: Substituent Effects in Electrophilic Aromatic
Substitution: Strongly Deactivating Substituents
Strong deactivators are meta directors
O
C
O
O
H
C
R
C
O
OH
C
O
OR
C
N
S
O
strong deactivators
O
O
N
CF3
O
very strong deactivators
294
12.14: Substituent Effects in Electrophilic Aromatic
Substitution: Halogens - Halogens are deactivating because
they are strong electron withdrawing groups (inductive effect);
however, they have non-bonding pairs of electrons and can also
donate electrons (resonance effect), and are ortho-para directors.
o rth o
3 0%
Cl
m eta
1%
p a ra
69%
295
Table 12.2, p. 491
O
O
-N O 2
-N R 3
-S O 3 H -C O 2 H
C
N
O
-B r
C H
-C O 2 R
-I
alk y l
-F
-C l
-H
C R
stro n g d eactiv ato rs
(m eta d irecto rs)
d eactiv ato rs
(o rth o /p ara d irecto rs)
O C R
O
H
N C R
-O R
-N H 2
-O H
stro n g activ ato rs
(o rth o /p ara d irecto rs)
12.15: Multiple Substituent Effects - The individual directing
effect of each substituent must be considered in order to
determine the overall directing effect of a disubstituted
benzene toward further electrophilic substitution.
296
1. When the individual directing effects of the two groups
reinforce, further electrophilic substitution is directed to the
common position.
-C H 3 d irects h ere
CH3
-C H 3 d irects h ere
-N O 2 d irects h ere
-N O 2 d irects h ere
CH3
Br
B r 2 , F eB r 3
NO2
NO2
2. When the individual directing effects of two groups oppose, the
stronger activating group has the dominant influence; however,
mixtures of products are often produced.
-O H d irects h ere
OH
OH
-O H d irects h ere
Br
B r 2 , FeB r 3
-C H 3 d irects h ere
-C H 3 d irects h ere
CH3
CH3
297
3. Further substitution between two existing substituents
rarely occurs. Start with an ortho-disubstituted benzene to
synthesize 1,2,3-trisubstituted benzenes
-C H 3 d irects h ere
CH3
-C H 3 d irects h ere
-C l d irects h ere
-C l d irects h ere
CH3
CH3
B r 2 , F eB r 3
CH3
Br
Br
+
+
Cl
Cl
Cl
Cl
Br
-C H 3 d irects h ere
n o t o b serv ed
-C l d irects h ere
-B r d irects h ere
CH3
-C H 3 d irects h ere
-C l d irects h ere
Br
Br
-B r d irects h ere
-B r d irects h ere
-C l d irects h ere
-C H 3 d irects h ere
CHO
C l 2 , F eC l 3
Br
Cl
-B r d irects h ere
CHO
CHO
Br
Br
+
-B r d irects h ere
-C H O d irects h ere
-B r d irects h ere
Cl
Cl
-C H O d irects h ere
298
12.16: Regioselective Synthesis of Disubstituted Aromatic
Compounds
Consider the directing effects of the substituents to determine
the order of their introduction to ensure the correct orientation
Friedel-Crafts reactions (alkylation, acylation) cannot be carried
out on strongly deactivated aromatics
Sometimes electrophilic aromatic substitution must be combined
with a functional group transformation
C O 2H
Br
NO2
299
NO2
o ,p -d ire cto r
Cl
d e a ctiva tin g
m -d ire c to r
d e a ctiva tin g
o ,p -d ire cto r
a ctiva tin g
300
Summary of electrophilic aromatic substitution of benzene
Zanger, M.; Gennaro, A. R.; McKee, J. R. J. Chem. Ed. 1993, 70 (12) , 985-987
S O 3,
H 2S O 4
S O 3H
X 2,
ca ta lyst
X
H N O 3,
H 2S O 4
R C O C l,
A lC l 3
R C H 2X ,
A lC l 3
NO2
O
C H 2R
R
[H ]
(m -)
(o -, p -)
(m -)
(o -, p -)
(m -)
NBS,
h
Br
[O ]
[O ]
R
C O 2H
(o -, p -)
(m -)
12.17: Substitution in Naphthalene (please read)
12.18: Substitution in Heterocyclic Aromatic Compounds
301
(please read)