Module 7 Chap 9. Benzene and Derivatives (Arenes)
Read Chapter 9: 268B-288 and 296-300
Learning Outcomes. By the end of this module you should be able to:
• Understand the bonding of benzene (and aryl/phenyl rings)
• Recognize, more generally, the stabilizing effect of aromaticity in organic
molecules, and how this effects the stability and reactivity.
• Know the special reactivity of the benzylic position and predict products from
the oxidation of benzylic positions
• Draw the mechanism of electrophilic aromatic substitution (EAS)
• Predict the products of EAS
• Understand the acidity of phenols and how substituents on the aryl ring can
further affect the acidity. Use the same concepts to expain the basicity of
phenolate ions.
Suggested Problems: Chapter 9: 11, 12, 18,19,21-23, 29, 37, 38
Ÿ2
Structure
• Due to resonance, all carbon-carbon
bonds are equivalent. All carbon and
hydrogen atoms are in the same
plane due to sp2 hybridization.
÷
Ÿ3
How much stability does resonance impart to benzene?
-
O
• Benzene is therefore 360 (3 x 120) – 209 = 151 kJ/mol more stable than a
cyclohexane ring containing 3 C=C is expected to be.
Ÿ4
Resonance imparts stability such that the carbon-carbon
“double bonds” of the Lewis structure of benzene and
compounds containing benzene rings do NOT undergo the
typical alkene reactions.
H2O
H
Br 2
no
reaction
¥4
Born
A
*
*
"
IHt
E- ⑥
nd
.
,
no
.
Ÿ5
Aromaticity
• Aromatic (fragrant) is the term used to molecules that
exhibit this type of stability.
• For a compound to be aromatic, it must meet three criteria:
o Have a p orbital on each ring atom
f- O
, 1,2 , 3
o Be planar or nearly planar
o Have 2, 6, 10, 14, 18, etc. p electrons (4n + 2 number
of p electrons)
.
447+2
=
bite
tote
-
-
-
mothballs
(
noo
,
not
those
kind
)
Hee
Hite
-
.
.
-
Ÿ6
Rings with heteroatoms (atoms other than C or H) can be
aromatic. The heteroatoms are sp2-hybridized so that a lone
pair can be placed in the remaining p orbital, which forms a
molecular orbital with the p orbitals of the other atoms. (See
“How To 9.1” in the text.)
H
O
N
furan
pyrrole
'
Ee
E:
ate
no
-
Ÿ7
⇐¥⇒
bite
-
¥
Ito
to
?
6 Fe
I
there
e-
localized
do
are
1-
to
A-
system
.
-
Ÿ8
Heterocyclic aromatic compounds are common in bioorganic
molecules.
bite
au
•
-
atoms
in
mg
SPN
O
i
@ a
a O
a a
101T
Of
ad
a
-
e-
"
µ
MY7YvaMTFTong7
group
Some nomenclature:
!
ED
Ghb
Ÿ9
Benzene
when a benzene ring is named as a substituent, it is called a
phenyl group. If the benzene ring is attached to a CH2, the
entire group is a benzyl group.
CH2 CH3
phenyl group
aryl group
y€Kf
I
phenylethane
CH2
CH2 Cl
I ✓
benzyl group
benzyl chloride
Benzylic
position
Ÿ 10
A special naming convention is used for di-substituted
benzene compounds.
-
aryl
1,2-substituted à ortho (o); 3-substituted à meta (m);1,4substituted à para (p)
I
3
2
4
3
2
I
1,2
-
=
ortho
2
I
-
1,3
meta
1,4
=
para
Ÿ 11
Reactions of Aromatic Compounds
Ability to stabilize cations and anions
I
Benzylic
.
• Although the aromatic ring is not very reactive, it is able to
stabilize adjoining cations and anions by resonance, as
previously seen.
→
CR2
5
CR2
CR2
&
a
p
q
f
f
CR2
CR2
ACR
CR2
G
2
s
-
u
CR2
T
q
Ÿ 12
Es
:
Acidity of phenols
,
Which of A-E lists the order of decreasing pKa?
CO2H
CH2OH
OH
-
I
II
A. II > I > III
B.
III >- I > II
C. - I > III > II
D. II > III > I
I
¥
III
Coa
21
resonance
Ht
→
on
,
uh
:
a
Oag
resonance
-
o_O
-
o_0
last
:D
.
Ÿ 13
n⇐¥%nE¥*
z
-
-
a
inductively
I
withdrawing
.
stabilizes
Cl
6
Z
-
Nou
Gfl§
"
B
t
.IN?o::o:o?d?o
Il
to
:
inductively withdrawing
④
resonance
stabilizing
:-O
Ÿ 14
(Harder)
Which of A-E lists the order of increasing pKa?
OH
O2N
OH
OH
H 3CO
I
A.
III < II < I
B.
II < I < III
C.
I < III < II
D.
III < I < II
II
III
T
ANALYSIS
RESONANCE
Do
THIS
or
Does
the
.
-
stabilise
relative
Eats
or
to
H
grp
destabilize
?
Benzylic
with
C
a
It
Ÿ 15
-
Oxidation of benzylic carbon atoms
• The p bonds of aromatic rings are not easily oxidized, but
benzylic carbons, are oxidized to carboxylic acids if they
are bonded to one or more H.
c
•
y
.
Chronic
Cstrong
acid
oxidizing
acid )
£
-
OH
Et
[
to
Bothwell
It
H
Ÿ 16
C
C
Chromic
acid
R
EYE
EITI
"
k
no
OH
-
K ¥509
'
-
¥9
benzylic
N R
.
C
-
H
.
OH
nw9f
'
I
-
I
-
OH
Coat
"
no
c
-
'
I I I
it
Ÿ 17
OH
1)
22
identify benzylic
with
these
oxidize
EH
He
Which of A-E is the
product of the
following reaction?
A
O
A.
C
"
H
-
to
oxygen
H2CrO4
H
O
OH
B.
HO
O
OH
O
O
OH
O
C.
Ÿ 18
.
HO
D.
O
OH
HO
OH
OH
O
E.
HO
O
O
Ÿ 19
Electrophilic aromatic substitution (EAS)
[
Super
-
electrophile
deeper
Ÿ 20
¥E÷÷
Aa
Mechanism
a
E
deficihteteetrophi 6
verye#m
E+
H+
B
:
E
÷¥÷÷÷÷÷.¥
7
⇐
⇒
'
⇐
'
i:
.
't
Ÿ 21
• All the reactions we study follow this mechanism and only
differ in the way the electrophile, E+, is formed
Comparison:
Substitute
for
C
an
-
E
aromatic
C
-
H
Ÿ 22
1. Chlorination and bromination
¥3
Bra
-
rea
-
Br
:c ?
:p
,
-
St
t
to
g
Etf
E
t
Ÿ 23
Et
④
a
-
Ÿ 24
o
2. Nitration
HNO3
H2SO4
f-
NOE
\
NO2
-
1+0
N
• H2SO4 reacts with HNO3 to form the electrophile NO2+ and the base HSO4–.
Base
It
I
H
to
IN
And
④
→
conj
.
o=N④=o
Et
base
-
-
-0
Ÿ 25
Synthesis: Making anilines
This is the best way to add an amino group to an aromatic
ring, since the nitro group is easily reduced to an amine.
NO
µ 2
3H
HL
2
NHL
Ni, Pd, or Pt
NH2
+ 2 H2O
Ÿ 26
3. Sulfonation
fuming
SO3H
.
H2SO4
conj
.
of
(
Et
acid
sulfuric
aid
Ÿ 27
1
\
4. Alkylation reactions: Friedel-Crafts alkylation
R–Cl
R
AlCl3
St
"
"
Et
R
-
a
generic
based
abbreviates
groxpp
for
ay
carbon
R
R
Ÿ 28
via
⑦
(
Et
←
)
-
-
U
a
I
-
a
a'
I
a
a
Ÿ 29
This reaction works well for most alkyl halides. But, there are
two major limitations:
1) Reaction does not work if ring contains EWG
2) Note that because a carbocation is involved,
carbocation rearrangements may occur.
Ÿ 30
Carbocation rearrangements:
What is the major product of the reaction of the alkylhalide
below with benzene and AlCl3?
.
I
JEE
-
nice,
It¥9
Mayo
At Cb
"
→
→
¥¥T⇒*
major
Ÿ 31
• Acid-catalyzed alkylation
• You can also alkylate using an alkene and a strong, nonnucleophilic acid.
p
µ&
Ho
-
-
OH
R
OH
H3PO4
• Phosphoric acid protonates the alkene to generate a
carbocation, which acts as the electrophile.
A
H
•
)
Carbocation rearrangements will occur.
R
to
-
it
-
t
t
t
°
"
-
P
ta
o_O
-
÷
R
5. Friedel-Crafts acylation reactions
c
-
-
÷
Ÿ 32
:
• Similar to the F-C alkylation, except here, the electrophile
contains a carbonyl group as the electrophile.
O
O
RCCl
AlCl3
R
Et
7-
Anglian
Ion
p
as
r
-
I
⇐
U
:
Ÿ 33
Cl2
A.
Which of the
following will not
proceed as
B.
written?
Cl
✓
FeCl3
AlCl3
+
✓
Cl
HNO
O3
C.
O
B
⑦
IT
D.
+ Cl
E.
+
O
✓
⑤
a
,
✓
AlCl3
H3PO4
via
?
no
H2SO 4
O
via
NH
002
R