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Organic Lecture Series
Benzene &
Aromaticity
Chapter 21
Concept of Aromaticity
1
Organic Lecture Series
The underlying criteria for aromaticity were
recognized in the early 1930s by Erich Hückel,
based on molecular orbital (MO) calculations
To be aromatic, a compound must:
1. be cyclic
2. have one p orbital on each atom of the
ring
3. be planar or nearly planar so that there is
continuous or nearly continuous overlap
of all p orbitals of the ring
4. have a closed loop of (4n + 2) π electrons
in the cyclic arrangement of p orbitals
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Organic Lecture Series
Benzene - Resonance Model
• The concepts of hybridization of atomic orbitals
and the theory of resonance, developed in the
1930s, provided the first adequate description of
benzene’s structure
– the carbon skeleton is a regular hexagon
– all C-C-C and H-C-C bond angles 120°
H 120°
H
sp2 -sp2
120° C
C
σ
σ
120°
H
C
C H
C
C
109 pm
sp2 -1 s
H
H
139 pm
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Organic Lecture Series
Cyclohexane-Chair conformation
Benzene- “flat” unsaturated system
4
Organic Lecture Series
Benzene – Molecular Orbitals
The π system of benzene:
1. the carbon framework with the six 2p orbitals
2. overlap of the parallel 2p orbitals forms one torus
above the plane of the ring and another below it
3. this orbital represents the lowest-lying pi-bonding
molecular orbital
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Organic Lecture Series
Benzene - Resonance Model
• Benzene is represented as a hybrid of two
equivalent Kekulé structures
– each makes an equal contribution to the
hybrid and thus the C-C bonds are neither
double nor single, but something in between
Ben zene as a hybrid of tw o equ ivalen t
contributin g s tru ctures
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Organic Lecture Series
Benzene - Resonance Model
• Resonance energy: the difference in
energy between a resonance hybrid and
the most stable of its hypothetical
contributing structures in which electrons
are localized on particular atoms and in
particular bonds
– one way to estimate the resonance energy of
benzene is to compare the heats of
hydrogenation of benzene and cyclohexene
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Organic Lecture Series
3(-119.7 kJ/mol)
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Organic Lecture Series
Nomenclature-Monosubstituted
• Monosubstituted alkylbenzenes are
named as derivatives of benzene
– many common names are retained
Toluene
OH
Ethylbenzene
CHO
NH 2
Phenol
Aniline
Cumene
Styrene
OCH 3
COOH
Benzaldehyde Benzoic acid
Anisole
9
(commit to memory)
Organic Lecture Series
Benzyl and phenyl groups:
CH 2 -
CH 3
Benzene
Phenyl group, PhO
Toluene
Benzyl group, Bn-
O
H 3 CO
Ph
1-Phenyl-1pentanone
4-(3-Methoxyphenyl)2-butanone
(Z)-2-Phenyl2-butene
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Organic Lecture Series
Disubstituted Benzenes
• Locate two groups by numbers or by the
locators ortho (1,2-), meta (1,3-), and para
(1,4-)
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Organic Lecture Series
Disubstituted Benzenes
– where one group imparts a special name,
name the compound as a derivative of that
molecule:
CH3
NH2
COOH
NO2
CH3
Cl
CH3
Br
4-Bromotolu ene 3-Chloroan iline
2-N itrobenzoic acid m-Xylene
(p-Bromotoluen e) (m-Chloroan iline) (o-N itrob enzoic acid )
Toluene is
the parent
name
Aniline is
the parent
name
Benzoic Acid
is the parent
name
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Organic Lecture Series
Disubstituted Benzenes
where neither group imparts a special name,
locate the groups and list them in
alphabetical order
CH2 CH3
4
2
NO2
3
Br
1
2
1
Cl
1-Chloro-4-ethylben zene
(p-Ch loroethylbenzen e)
1-Bromo-2-nitrob enzene
(o-Bromon itroben zene)
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Organic Lecture Series
Phenols
• The functional group of a phenol is an -OH
group bonded to a benzene ring
OH
OH
OH
OH
OH
CH 3
Phenol
3-Methylphenol
(m-Cresol)
OH
1,2-Benzenediol 1,4-Benzenediol
(Catechol)
(Hydroquinone)
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Organic Lecture Series
Acidity of Phenols
• Phenols are significantly more acidic
than alcohols, compounds that also
contain the OH group
Phenoxide:
O- + H3 O+
OH + H2 O
pK a = 9.95
+
CH3 CH2 O + H3 O
CH3 CH2 OH + H2 O
pK a = 15.9
Ethoxide
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Organic Lecture Series
Acidity of Phenols
– the greater acidity of phenols compared
with alcohols is due to the greater
stability of the phenoxide ion relative to
an alkoxide ion:
O
O
O
O
O
H
H
These 2 Kekulé
s tru ctures are
equivalent
H
These th ree con trib utin g s tru ctures
delocalize th e negative charge
on to carb on atoms of th e rin g
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Organic Lecture Series
Acidity of Phenols
Alkyl and halogen substituents effect
acidities by inductive effects:
– alkyl groups are electron-releasing
– halogens are electron-withdrawing
OH
OH
OH
OH
Cl
CH3
Phen ol
pK a 9.95
m-Cres ol
p Ka 10.01
OH
CH3
p-Cres ol
pK a 10.17
Cl
m-Chlorop henol p-Chororophen ol
pK a 8.85
p Ka 9.18
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Acidity of Phenols
Organic Lecture Series
– nitro groups increase the acidity of phenols by
both an electron-withdrawing inductive
effect and a resonance effect
OH
OH
OH
NO2
Ph e no l
p K a 9.95
NO2
m - N itrop h e n ol p- N itrop h e n ol
p K a 8.28
p K a 7.15
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Organic Lecture Series
Acidity of Phenols
– part of the acid-strengthening effect of -NO2 is
due to its electron-withdrawing inductive effect
– in addition, -NO2 substituents in the ortho and
para positions help to delocalize the negative
charge
O
O
N+
–
O
–
O
delocalization of n egative
charge on to oxygen fu rth er
increases the resonance
stabilization of phen oxid e ion
N+
O
O
–
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Organic Lecture Series
Acidity of Phenols
• Phenols are weak acids and react with
strong bases to form water-soluble salts
– water-insoluble phenols dissolve in NaOH(aq)
OH +
Ph en ol
p K a 9.95
(s tronger aci d)
NaOH
Sod iu m
h ydroxid e
O- Na+ + H2 O
Sod iu m
p hen oxi de
W ater
pK a 15.7
(we ak er acid )
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Organic Lecture Series
Acidity of Phenols
– most phenols do not react with weak
bases such as NaHCO3; they do not
dissolve in aqueous NaHCO3
OH + NaHCO3
Ph en ol
p K a 9.95
(w eak er aci d)
Sod iu m
bi carbon ate
O- Na+ + H2 CO3
Sod iu m
p hen oxi de
Carbon ic acid
p K a 6.36
(stronge r acid )
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Organic Lecture Series
Alkyl-Aryl Ethers
• Alkyl-aryl ethers can be prepared by the
Williamson ether synthesis
– but only using phenoxide salts and
haloalkanes
– haloarenes are unreactive to SN2 reactions
X + RO - N a +
no reaction
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Organic Lecture Series
AlkylAlkyl-Aryl Ethers
The following two examples illustrate
1. the use of a phase-transfer catalyst
2. the use of dimethyl sulfate as a methylating agent
OH + CH2 =CHCH2 Cl
Ph enol
NaOH, H2 O, CH2 Cl2
+
Bu4 N Br
-
3-Chloroprop ene
(A llyl chlorid e)
OCH2 CH=CH2
Phen yl 2-p ropen yl eth er
(Allyl phen yl eth er)
O
OH + CH3 OSOCH3
Phen ol
NaOH, H2 O, CH2 Cl2
O
D imeth yl s ulfate
+
Bu4 N Br
-
OCH3 + Na2 SO4
Methyl ph enyl ether
(An isole)
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Organic Lecture Series
Benzylic Oxidation
• Benzene is unaffected by strong oxidizing
agents such as H2CrO4 and KMnO4
– halogen and nitro substituents are also
unaffected by these reagents
– an alkyl group with at least one hydrogen on
its benzylic carbon is oxidized to a carboxyl
group
O
CHR
C
OH
H2CrO4
or
KMnO4
Arene
Not responsible for mechanism
Benzoic Acids
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Organic Lecture Series
Benzylic Oxidation
CH3
O2 N
COOH
H2 Cr O4
Cl
O2 N
2-Chloro-4-nitrotoluene
Cl
2-Chloro-4-nitrobenzoic acid
– if there is more than one alkyl group on the benzene
ring, each is oxidized to a -COOH group
H3 C
CH 3
K2 Cr 2 O 7
H2 SO4
1,4-Dimethylbenzene
(p-xylene)
O
COH
O
HOC
1,4-Benzenedicarboxylic acid
(terephthalic acid)
Not responsible for mechanism
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Organic Lecture Series
Benzylic Chlorination
• Chlorination (and bromination) occurs by a
radical mechanism
X
CHR2
R2C
X2
heat or h
Arene
Benzyl Halides
CH3
+
Toluen e
Cl2
h eat
or ligh t
CH2 Cl
+ HCl
Benzyl ch loride
26
Organic Lecture Series
Benzylic Bromination
• Bromination can be conducted with NBS:
Br
NBS
( PhCO2 ) 2 , CCl4
Ethylbenzen e
1-Bromo-1-p henylethan e
(racemic)
27
Organic Lecture Series
Benzoyl Peroxide as a Radical initiator
B e n z o y l P e ro x id e
O
O
O
O
hν
or
Δ
O
O
C
2
O
O
P h e n y l R a d ic a l
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Organic Lecture Series
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
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Benzylic Halogenation
Organic Lecture Series
– benzylic bromination is highly regioselective
Br
NBS
(PhCO2 ) 2 , CCl4
Eth ylb enzene
1-Bromo-1-phen yleth ane
(the only product formed )
– benzylic chlorination is less regioselective
Cl
+ Cl2
Eth ylb enzene
heat
or ligh t
Cl
+
1-Chloro-1p henylethan e
(90%)
1-Ch loro-2ph enylethan e
(10%)
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Hydrogenolysis
Organic Lecture Series
• Hydrogenolysis: cleavage of a single bond by H2
– among ethers, benzylic ethers are unique in that they
are cleaved under conditions of catalytic
hydrogenation
H2
C
H2
O
R
Pt or Pd
HO
R
CH3
the
desired
product
the side product
this bond
is cleaved
O
Me
Pd/ C
+ H2
Benzyl butyl ether
OH +
1-Butanol
Toluene
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Organic Lecture Series
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
2 . BH3 • THF
1 . ClCH2 Ph
OH
2-(2-Propen yl)p henol
(2-A llylp henol)
Et 3 N
O
OH
O
Ph
H2
Pd/ C
Ph
3 . H2 O2 / NaOH
OH
OH
2-(3-Hyd roxyprop yl)p henol
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