21-21-11
Chapter 21, BenzeneChapter 21, Benzene and and the Concept ofand and the Concept ofAromaticityAromaticity
21-21-22
Benzene - Kekulé Benzene - Kekulé
In 1872, August Kekulé proposed the following structure for benzene.
This structure, however, did not account for the unusual chemical reactivity of benzene.
CH
CH
CH
CHC
H
CH
CCC
CCC
H
HHH
HH
21-21-33
Benzene - Resonance Benzene - Resonance
We often represent benzene 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.
Benzene as a hybrid of two equivalentcontributing structures
21-21-44
Benzene - Resonance ModelBenzene - 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 planar regular hexagon.• All C-C-C and H-C-C bond angles 120°.
sp2-sp2
sp2-1s109 pm
120°
120°120°
139 pm
C
C
C
C
C CH
H H
H
H H
21-21-55
The Pi System of BenzeneThe Pi System of Benzene
• (a) The carbon framework with the six 2p orbitals.• (b) Overlap of the parallel 2p orbitals forms one torus
above the plane of the ring and another below it• this orbital represents the lowest-lying pi-bonding
molecular orbital.
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Benzene-Molecular Orbital ModelBenzene-Molecular Orbital Model
The molecular orbital representation of the pi bonding in benzene.
21-21-77
Orbitals of the pi System of BenzeneOrbitals of the pi System of BenzeneNumber of nodal surfaces
0
1
2
3
21-21-88
Benzene - ResonanceBenzene - Resonance
Resonance energy:Resonance energy: The difference in energy between a resonance hybrid in which the electrons are delocalized
and the most stable one 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.
21-21-99
Benzene- Resonance EnergyBenzene- Resonance Energy
Experimental data
Model
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Concept of AromaticityConcept of Aromaticity
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• Be cyclic.• Have one p orbital on each atom of the ring.• Be planar or nearly planar so that there is continuous
or nearly continuous overlap of all p orbitals of the ring.
• Have a closed loop of (4n + 2) pi electrons in the cyclic arrangement of p orbitals.
21-21-1111
Frost CirclesFrost Circles
Frost circle:Frost circle: A graphic method for determining the relative order of pi MOs in planar, fully conjugated monocyclic compounds.• Inscribe in a circle a polygon of the same number of
sides as the ring to be examined such that one of the vertices of the polygon is at the bottom of the circle.
• The relative energies of the MOs in the ring are given by where the vertices of the polygon touch the circle.
Those MOs• Below the horizontal line through the center of the ring
are bonding MOs.• on the horizontal line are nonbonding MOs.• above the horizontal line are antibonding MOs.
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Frost CirclesFrost Circles
• Frost circles describing the MOs for monocyclic, planar, fully conjugated four-, five-, and six-membered rings.
21-21-1313
Relationship of hexa-1,3,5-triene to benzene
How does the linear triene relate to benzene?
?
Relationship of hexa-1,3,5-triene to benzene
?
Relationship of hexa-1,3,5-triene to benzene
?
Look at orbitals 2 and 3.
2
3
Curve around
Antibonding, destabilizing
Bonding, stabilizing
21-21-1616
Aromatic HydrocarbonsAromatic Hydrocarbons
Annulene:Annulene: A cyclic hydrocarbon with a continuous alternation of single and double bonds.• [14]Annulene is aromatic according to Hückel’s
criteria.
[14]Annulene (aromatic)
HH
HH
H
H
H
H
HH
H
H
H H
n = 3
21-21-1717
Aromatic HydrocarbonsAromatic Hydrocarbons
• [18]Annulene is also aromatic.
[18]Annulene (aromatic)
H
H
HH
HH
H
H
HH
H
HH
H
HH
H
H
n = 4
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Aromatic HydrocarbonsAromatic Hydrocarbons
• According to Hückel’s criteria, [10]annulene should be aromatic; it has been found, however, that it is not.
• Nonbonded interactions between the two hydrogens that point inward toward the center of the ring force the ring into a nonplanar conformation in which overlap of the ten 2p orbitals is no longer continuous.
[10]Annulene
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Aromatic HydrocarbonsAromatic Hydrocarbons
• What is remarkable relative to [10]annulene is that if the two hydrogens facing inward toward the center of the ring are replaced by a methylene (CH2) group, the ring is able to assume a conformation close enough to planar that it becomes aromatic.
CH2
Bridged [10]annulene
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Antiaromatic HydrocarbonsAntiaromatic Hydrocarbons
Antiaromatic hydrocarbon:Antiaromatic hydrocarbon: A monocyclic, planar, fully conjugated hydrocarbon with 4n pi electrons (4, 8, 12, 16, 20...).• An antiaromatic hydrocarbon is especially unstable
relative to an open-chain fully conjugated hydrocarbon of the same number of carbon atoms.
Cyclobutadiene is antiaromatic.• In the ground-state electron configuration of this
molecule, two electrons fill the 1 bonding MO.
• The remaining two electrons lie in the 2 and 3 nonbonding MOs.
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CyclobutadieneCyclobutadiene
• The ground state of planar cyclobutadiene has two unpaired electrons, which make it highly unstable and reactive.
21-21-2222
CyclooctatetraeneCyclooctatetraene
• Cyclooctatetraene, with 8 pi electrons is not aromatic; it shows reactions typical of alkenes.
• X-ray studies show that the most stable conformation is a nonplanar “tub” conformation.
• Although overlap of 2p orbitals occurs to form pi bonds, there is only minimal overlap between sets of 2p orbitals because they are not parallel.
viewed from above viewed through an edge
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CyclooctatetraeneCyclooctatetraene
MO energy diagram for a planar conformation of cyclooctatetraene.
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Heterocyclic AromaticsHeterocyclic Aromatics
Heterocyclic compound:Heterocyclic compound: A compound that contains more than one kind of atom in a ring. • In organic chemistry, the term refers to a ring with one
or more atoms that differ from carbon.
Pyridine and pyrimidine are heterocyclic analogs of benzene; each is aromatic.
Pyridine
N
N
N••••
Pyrimidine
12
34
5
61
2
3 4
5
6
21-21-2525
PyridinePyridine
• The nitrogen atom of pyridine is sp2
hybridized.• The unshared pair of
electrons lies in an sp2 hybrid orbital and is not a part of the six pi electrons of the aromatic system (the aromatic sextet).
• Resonance energy of pyridine is134 kJ (32 kcal)/mol.
21-21-2626
Furan and PyrroleFuran and Pyrrole
• The oxygen atom of furan is sp2 hybridized.• one unshared pairs of electrons on oxygen lies in an
unhybridized 2p orbital and is a part of the aromatic sextet.
• The other unshared pair lies in an sp2 hybrid orbital and is not a part of the aromatic system.
• The resonance energy of furan is 67 kJ (16 kcal)/mol.
O NH
21-21-2727
Other HeterocyclicsOther Heterocyclics
Purine
Indole
N
N
NN
N
H
H
N
H
CH2CH2NH2
Serotonin(a neurotransmitter)
HO
Caffeine
N
NN
N
O
O
H3CCH3
CH3
21-21-2828
Aromatic Hydrocarbon IonsAromatic Hydrocarbon Ions
Any neutral, monocyclic, unsaturated hydrocarbon with an odd number of carbons must have at least one CH2 group and, therefore, cannot be aromatic.
• Cyclopropene, for example, has the correct number of pi electrons to be aromatic, 4(0) + 2 = 2, but does not have a closed loop of 2p orbitals.
Cyclopropene Cyclopentadiene Cycloheptatriene
CH2 CH2CH2
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Cyclopropenyl CationCyclopropenyl Cation
• If, however, the CH2 group of cyclopropene is transformed into a CH+ group in which carbon is sp2 hybridized and has a vacant 2p orbital, the overlap of orbitals is continuous and the cation is aromatic.
Cyclopropenyl cation represented as a hybrid of three equivalent contributing structures
+
H
H
H
H
H
H
H
H
H+
+
21-21-3030
Cyclopropenyl CationCyclopropenyl Cation
• When 3-chlorocyclopropene is treated with SbCl5, it forms a stable salt.
• This chemical behavior is to be contrasted with that of 5-chloro-1,3-cyclopentadiene, which cannot be made to form a stable salt.
+
Cyclopropenyl hexachloroantimonate
+
3-Chloro-cyclopropene
HH
ClSbCl5 SbCl6
-
Antimony(V) chloride(a Lewis acid)
21-21-3131
Cyclopentadienyl CationCyclopentadienyl Cation
• If planar cyclopentadienyl cation were to exist, it would have 4 pi electrons and be antiaromatic.
• Note that we can draw five equivalent contributing structures for the cyclopentadienyl cation. Yet this cation is not aromatic because it has only 4 pi electrons.
Cyclopentadienyltetrafluoroborate
++
5-Chloro-1,3-cyclopentadiene
H
ClHAgBF4 BF4
- + AgCl
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Cyclopentadienyl Anion, CCyclopentadienyl Anion, C55HH55--
To convert cyclopentadiene to an aromatic ion, it is necessary to convert the CH2 group to a CH group in which carbon becomes sp2 hybridized and has 2 electrons in its unhybridized 2p orbital.
••
• • H
HH
H
H
the origin of the 6 pi electronsin the cyclopentadienyl anion
Cyclopentadienyl anion (aromatic)
HH
HH
H:
H
HH
HH
n = 1
21-21-3333
Cyclopentadienyl Anion, CCyclopentadienyl Anion, C55HH55--
• As seen in the Frost circle, the six pi electrons of cyclopentadienyl anion occupy the 1, 2, and 3 molecular orbitals, all of which are bonding.
21-21-3434
Cyclopentadienyl Anion, CCyclopentadienyl Anion, C55HH55--
The pKa of cyclopentadiene is 16.• In aqueous NaOH, it is in equilibrium with its sodium
salt.
• It is converted completely to its anion by very strong bases such as NaNH2 , NaH, and LDA.
pKa 15.7pKa 16.0
Na+ + H2OH
H
H
H
H
CH2 + NaOH :
21-21-3535
Cycloheptatrienyl Cation, CCycloheptatrienyl Cation, C77HH77++
Cycloheptatriene forms an aromatic cation by conversion of its CH2 group to a CH+ group with its sp2 carbon having a vacant 2p orbital.
+
Cycloheptatrienyl cation (Tropylium ion) (aromatic)
H
HH
H
H
HH
H
HH
H
H
HH
+
21-21-3636
NomenclatureNomenclature
Monosubstituted alkylbenzenes are named as derivatives of benzene.• Many common names are retained.
Toluene CumeneEthylbenzene Styrene
Phenol Aniline Benzoic acid Anisole
COOHNH2 OCH3OH
Benzaldehyde
CHO
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NomenclatureNomenclature
Benzyl and phenyl groups
(Z)-2-Phenyl-2-butene
4-(3-Methoxyphenyl)-2-butanone
1-Phenyl-1-pentanone
O OH3CO
Ph
BenzenePhenyl group, Ph- Toluene Benzyl group, Bn-
CH3 CH2
21-21-3838
Disubstituted BenzenesDisubstituted Benzenes
Locate two groups by numbers or by the locators orthoortho (1,2-), metameta (1,3-), and parapara (1,4-).• Where one group imparts a special name, name the
compound as a derivative of that molecule.
CH3
Br
COOHNO2
Cl
NH2
CH3
CH3
2-Nitrobenzoic acid
(o-Nitrobenzoic acid)
3-Chloroaniline(m-Chloroaniline)
4-Bromotoluene(p-Bromotoluene)
m-Xylene
21-21-3939
Disubstituted BenzenesDisubstituted Benzenes
• Where neither group imparts a special name, locate the groups and list them in alphabetical order.
CH2CH3
Cl
NO2Br
1-Bromo-2-nitrobenzene (o-Bromonitrobenzene)
1-Chloro-4-ethylbenzene (p-Chloroethylbenzene)
12
3
4 2
1
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Polysubstituted DerivativesPolysubstituted Derivatives
• If one group imparts a special name, name the molecule as a derivative of that compound.
• If no group imparts a special name, list them in alphabetical order, giving them the lowest set of numbers.
CH3
NO2
OH
Br
Br
NO2
CH2CH3
Br
4
2
1
6
4
21
4
1 2
4-Chloro-2-nitro-toluene
2,4,6-Tribromo-phenol
2-Bromo-1-ethyl-4-nitrobenzene
Br
Cl
21-21-4141
PhenolsPhenols
The functional group of a phenol is an -OH group bonded to a benzene ring.
1,2-Benzenediol(Catechol)
1,4-Benzenediol(Hydroquinone)
3-Methylphenol(m-Cresol)
Phenol
OH OHOHOH
OHCH3
OH
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Acidity of PhenolsAcidity of Phenols
Phenols are significantly more acidic than alcohols.
OH H2O
CH3CH2OH H2O
O-
CH3CH2O-
H3O+
H3O+
pKa = 9.95+
+
+
+ pKa = 15.9
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Acidity of PhenolsAcidity of Phenols
Separation of water-insoluble phenols from water-insoluble alcohols.
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Acidity of Phenols (Resonance)Acidity of Phenols (Resonance)
• The greater acidity of phenols compared with alcohols is due to the greater stability of the phenoxide ion relative to an alkoxide ion.
These 2 Kekuléstructures areequivalent
HH
OO O O
H
O
These three contributing structuresdelocalize the negative chargeonto carbon atoms of the ring
H
OO O O
H
O
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Phenol Subsitituents (Inductive Effect)Phenol Subsitituents (Inductive Effect)
Alkyl and halogen substituents effect acidities by inductive effects:• Alkyl groups are electron-releasing.• Halogens are electron-withdrawing.
p-ChororophenolpKa 9.18
m-ChlorophenolpKa 8.85
PhenolpKa 9.95
m-CresolpKa 10.01
p-CresolpKa 10.17
OH OH OH OH OH
CH3
CH3
ClCl
21-21-4747
Phenol Subsitituents(Resonance, Inductiion)Phenol Subsitituents(Resonance, Inductiion)
• Nitro groups increase the acidity of phenols by both an electron-withdrawing inductive effect and a resonance effect.
OH
NO2
OH OH
NO2PhenolpKa 9.95
p-NitrophenolpKa 7.15
m-NitrophenolpKa 8.28
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Acidity of PhenolsAcidity 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.
+ +
delocalization of negativecharge onto oxygen furtherincreases the resonancestabilization of phenoxide ion
O
O O
NO
O
NO
21-21-5151
SynthesisSynthesis: Alkyl-Aryl Ethers: Alkyl-Aryl Ethers
Alkyl-aryl ethers can be prepared by the Williamson ether synthesis:• but only using phenoxide salts and haloalkanes.• haloarenes cannot be used because they are
unreactive to SN2 reactions.
no reaction+X RO-Na
+
21-21-5252
SynthesisSynthesis: Alkyl-Aryl Ethers: Alkyl-Aryl Ethers
OH CH2=CHCH2ClNaOH, H2O, CH2Cl2
OCH2CH=CH2
Phenyl 2-propenyl ether(Allyl phenyl ether)
+
Phenol 3-Chloropropene(Allyl chloride)
OH
O
O
CH3OSOCH3NaOH, H2O, CH2Cl2
OCH3 Na2SO4+
Methyl phenyl ether(Anisole)
+
Phenol Dimethyl sulfate
21-21-5353
SynthesisSynthesis: Kolbe Carboxylation: Kolbe Carboxylation
Phenoxide ions react with carbon dioxide to give a carboxylate salt.
OH
NaOHH2O
O-Na
+
CO2
H2O
OHCO
-Na
+O
HClH2O
OH O
COH
Phenol Sodiumphenoxide
Sodium salicylate Salicylic acid
21-21-5454
Mechanism: Mechanism: Kolbe CarboxylationKolbe Carboxylation
• The mechanism begins by nucleophilic addition of the phenoxide ion to a carbonyl group of CO2.
O
C
O
O
OC
H
OO OH
CO
O
A cyclohexadienoneintermediate
+
Sodium phenoxide
Salicylate anion
keto-enoltautomerism
(1) (2)
Go back to aromatic structure
21-21-5555
Synthesis: Synthesis: QuinonesQuinones
Because of the presence of the electron-donating -OH group, phenols are susceptible to oxidation by a variety of strong oxidizing agents.
H2 CrO4
Phenol 1,4-Benzoquinone(p-Quinone)
O
O
OH
21-21-5656
QuinonesQuinones
OHOH K2Cr2O7
OH
OH
H2SO4
K2Cr2O7
H2SO4
O
O
OO
1,4-Benzoquinone (p-Quinone)
1,2-Benzenediol (Catechol)
1,2-Benzoquinone (o-Quinone)
1,4-Benzenediol(Hydroquinone)
21-21-5757
QuinonesQuinones
Readily reduced to hydroquinones.
1,4-Benzoquinone(p-Quinone)
(reduction)
1,4-Benzenediol(Hydroquinone)
O
O
OH
OH
Na2S2O4, H2O
21-21-5858
Coenzyme QCoenzyme Q
Coenzyme Q is a carrier of electrons in the respiratory chain.
O
O
MeO
HMeO MeO
MeO
OH
OH
Hn n
Coenzyme Q(oxidized form)
Coenzyme Q(reduced form)
reduction
oxidation
21-21-6060
Benzylic OxidationBenzylic 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.
2-Chloro-4-nitrotoluene 2-Chloro-4-nitrobenzoic acid
H2CrO4
O2N Cl
CH3
O2N Cl
COOH
21-21-6161
Benzylic OxidationBenzylic Oxidation
• If there is more than one alkyl group on the benzene ring, each is oxidized to a -COOH group.
1,4-Dimethylbenzene (p-xylene)
1,4-Benzenedicarboxylic acid (terephthalic acid)
CH3 H2 SO4
K2Cr2O7H3C COH
O
HOCO
21-21-6262
Benzylic ChlorinationBenzylic Chlorination
Chlorination and bromination occur by a radical chain mechanism.
CH3
Cl2+
CH2Cl
HCl+
Toluene
heat or light
Benzyl chloride
(PhCO2)2, CCl4
NBS
Br
Ethylbenzene 1-Bromo-1-phenylethane(racemic)
21-21-6363
Mechanism: Mechanism: Benzylic ReactionsBenzylic 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
21-21-6464
Benzylic HalogenationBenzylic Halogenation
• Benzylic bromination is highly regioselective.
• Benzylic chlorination is less regioselective.
(PhCO2)2, CCl4
NBS
Br
Ethylbenzene 1-Bromo-1-phenylethane(the only product formed)
Cl
Cl2 +
heat or light
1-Chloro-2-phenylethane
(10%)
Ethylbenzene
+
1-Chloro-1-phenylethane
(90%)
Cl
21-21-6565
HydrogenolysisHydrogenolysis
Hydrogenolysis:Hydrogenolysis: Cleavage of a single bond by H2
• Benzylic ethers are unique in that they are cleaved under conditions of catalytic hydrogenation.
O H2Pd/C
OHMe
+
Benzyl butyl ether Toluene1-Butanol
+
this bondis cleaved
21-21-6666
Synthesis, Protecting Group: Synthesis, Protecting Group: Benzyl EthersBenzyl 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.
OH
1. ClCH2Ph
O Ph
2. BH3•THF
Et3N 3. H2O2/NaOH
H2
Pd/CO Ph
OH
OH
OH
2-(3-Hydroxypropyl)phenol
2-(2-Propenyl)phenol(2-Allylphenol)
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