Chapter 15 Benzene and Aromaticity Aromatic Nomenclature
Chapter 15
Benzene and Aromaticity
Aromatic - aromatic was term used to describe fragrant chemicals - e.g. benzaldehyde, toluene - source of aromatics: coal and petroleum
Benzene (C6H6)
resonance structures
alternative representation
Nomenclature - number and position of functional groups define structure - benzene is usually suffix; common names are also used - monosubstituted aromatics:
Cl
chlorobenzene
NO2
nitrobenzene
CH 3
O
CHCH 3
C
cumene
CH3
methylbenzene
vinylbenzene
toluene
styrene
COOH
NH2
CH3
acetophenone
benzoic acid
aniline
1
- alkyl-substituted benzenes referred to as arenes - if alkyl substituent has six or fewer carbons, then the arene is named as an alkyl-substituted benzene - if alkyl substituent has more than six carbons, then the compound is named as a phenyl-substituted alkane CH3
CH2
CHCH2CH2CH2CH2CH3
phenyl group
benzyl group
2-phenylheptane
- disubstituted:
X
ortho - 1,2-relationship
ortho
ortho meta
meta - 1,3-relationship
meta
para - 1,4-relationship
para CH3
Cl
Cl Cl
CH3
meta-xylene
ortho-dichlorobenzene - when discussing reactions:
CHO
para-chlorobenzaldehyde
CH3
CH3 Br2 FeBr3
Br
- benzenes with more than two substituents: 1) number such that lowest possible numbers used 2) list substituents alphabetically in name Br
Br
Br
Cl Cl
Br
1,2,4-tribromobenzene
4-bromo-1,2-dimethylbenzene
(not 1,3,4) NO2
OH Br
O2N
Br
NO2
2,4,6-trinitrotoluene (TNT)
2,6-dibromophenol
2
Structure and Stability of Benzene - benzene is much more stable than other alkenes - cyclohexene reacts rapidly with two equivalents of Br2 Br
Br2
Br
- benzene reacts slowly with Br2 to give substitution product C6H5Br Br
H
Br2 FeBr3
H
Br Br
not formed - reaction permits cyclic conjugation of benzene ring to be retained
Compare Heats of Hydrogenation H2 Pd/C
cyclohexene, ∆Hohydrog = -118 kJ/mol H2 Pd/C
1,3-cyclohexadiene, ∆Hohydrog = -230 kJ/mol H2 Pd/C
benzene, ∆Hohydrog = -206 kJ/mol - for benzene, expected ~ -356 kJ/mol; benzene ~ -150 kJ/mol ‘extra’ stability
3
Stability of Benzene 1) Resonance theory - benzene is a resonance hybrid of two equivalent forms. Neither form is correct by itself; the true structure of benzene is somewhere in between the two resonance forms.
2) Molecular Orbital Theory - all C-C-C bond angles are 120o, all six carbon atoms are sp2 hybridized, and each carbon atom has a p orbital perpendicular to the plane of the six-membered ring.
Molecular Orbital Description of Benzene
Hückel 4n + 2 Rule - a molecule is aromatic only if it has a planar, monocyclic system of conjugation with a total of 4n+2 π electrons, where n is an integer (n = 0, 1,2,3…) - aromatic: 2,6,10,14,18… π electrons - not aromatic: 4,8,12,16… 4n π electrons
Examples: 1) cyclobutadiene
2) benzene
two double bonds four π electrons
three double bonds six π electrons
not aromatic
aromatic
4
3) cyclooctatetraene
four double bonds eight π electrons
not aromatic
- reacts readily with Br2, KMnO4, HCl - C-C bonds 1.47 Å, C=C bonds 1.34 Å - tube-shaped molecule - 1H NMR shows sharp line at 5.7 δ
Cyclooctatetraene
Problem: 3-Chlorocyclopropene on treatment with AgBF4 gives a precipitate of AgCl and a stable solution of a product that shows a single 1H NMR absorption at 11.04 δ. What is a likely structure for the product, and what is its relation to Hückel’s rule?
5
Benzene
number of p orbitals = 6 number of electrons = 6
Hückel’s 4n + 2 Rule 1) planar, 2) monocyclic system of conjugation, and 3) total of 4n+2 π electrons
Consequences of Hückel Rule: Aromatic Ions Hückel 4n + 2 rule does not say that: 1) number of p orbitals = number of π electrons i.e. an aromatic species must be neutral - therefore, in addition to molecules, ions may also be aromatic
Examples: H H
H
-
H
+
H
H H
H
H
H H
cyclopentadienyl anion
H
cycloheptatrienyl cation
Cyclopentadienyl Anion H
H
- cyclopentadienyl anion is remarkably stable
H
H H
H
- cyclopentadiene is one of the most acidic hydrocarbons known (pKa = 16)
H
H +
H
H
H
H
H
H
-
H
H
H
H H
H
H
cyclopentadienyl cyclopentadienyl cation: four π electrons cyclopentadienyl anion: six π electrons radical: five π electrons
- pKa of the diene is comparable to water (!)
6
Consequences of Hückel Rule: Heterocycles Hückel 4n + 2 rule does not say that: 2) atoms in ring must be carbon - heterocycle: a cyclic compound that contains an atom or atoms other than carbon in its ring e.g. oxygen, sulphur, phosphorus Examples: N
pyridine
N H
pyrrole
- both compounds are crucial to many biochemical processes
7
Molecular Orbitals: Pyridine and Pyrrole - nitrogen atoms play different roles in heterocycles: pyridine: N atom is in double bond, contributes only 1 π electron to the aromatic sextet pyrrole: N atom is not in double bond, contributes 2 π electrons to the aromatic sextet
Why 4n + 2? - molecular orbital theory: Energy levels of molecular orbitals for cyclic conjugated molecules always possess a single lowest-lying MO, above which the MOs come in degenerate pairs - one pair (2 electrons) to fill the bottom and two pairs (4 electrons) to fill each n successive level
Energy Levels of Molecular Orbitals in Benzene
8
Energy Levels of Molecular Orbitals in Cyclopentadienyl Cation, Radical, and Anion
Polycyclic Aromatic Compounds - Hückel rule is strictly applied to monocyclics - however, concept of aromaticity can be extended to include polycyclic aromatic compounds
naphthalene
anthracene
benzo[a]pyrene
- all can be represented by various resonance forms
coronene
Naphthalene - resonance forms of naphthalene:
- heat of hydrogenation: 250 kJ/mole of stabilization - naphthalene reacts slowly with Br2, similar to benzene Br Br2
+ HBr
FeBr3
9
Spectroscopy of Aromatic Compounds Infrared Spectroscopy - characteristic C-H stretching absorption at 3030 cm-1 - characteristic series of peaks from 1450 to 1600 cm-1 - strong absorptions from 690 to 900 cm-1 - diagnostic of substitution pattern of aromatic ring monosubstituted: 690-710 cm-1 730-770 cm-1
m-disubstituted:
690-710 cm-1 810-850 cm-1
735-770 cm-1
p-disubstituted:
810-840 cm-1
o-disubstituted:
Infrared Spectrum of Toluene
10
Ultraviolet Spectroscopy - intense absorption near 205 nm and weaker absorption from 255 to 275 nm is indicative of an aromatic ring Nuclear Magnetic Resonance Spectroscopy - hydrogen atoms directly bonded to an aromatic ring are easily identifiable - hydrogen are strongly deshielded by the ring and absorb between 6.5 and 8.0 δ - deshielding of hydrogens is due to a property of aromatic rings called a ring current
Ring Current in Aromatics
When an aromatic ring is oriented perpendicular to a strong magnetic field, the delocalized π electrons circulate around the ring, producing a small local magnetic field. This induced field opposes the applied field in the middle of the ring but reinforces the applied field outside the ring. Aromatic protons therefore experience an effective magnetic field greater than the applied field and come into resonance at lower field.
Example: 1H NMR Spectrum of toluene
11
Example: 1H NMR Spectrum of p-bromotoluene
- protons on ‘outside’ are deshielded and protons on ‘inside’ are shielded Example:
1H
NMR spectrum of [18]Annulene H
H
H
H
H
H H
H
H H
H H
H
Inside H: -3.0 δ H
Outside H: 9.3 δ
H H
H
H
[18]Annulene
- carbon atoms of aromatics absorb from 110 to 140 δ in 13C spectrum 21.3
CH3
128.4
137.7 129.3 128.5
125.6 Cl
133.8 127.6
133.7
128.1 126.0
128.4 125.4
12
Notes
Notes
13
Benzene and Aromaticity
Aromatic - aromatic was term used to describe fragrant chemicals - e.g. benzaldehyde, toluene - source of aromatics: coal and petroleum
Benzene (C6H6)
resonance structures
alternative representation
Nomenclature - number and position of functional groups define structure - benzene is usually suffix; common names are also used - monosubstituted aromatics:
Cl
chlorobenzene
NO2
nitrobenzene
CH 3
O
CHCH 3
C
cumene
CH3
methylbenzene
vinylbenzene
toluene
styrene
COOH
NH2
CH3
acetophenone
benzoic acid
aniline
1
- alkyl-substituted benzenes referred to as arenes - if alkyl substituent has six or fewer carbons, then the arene is named as an alkyl-substituted benzene - if alkyl substituent has more than six carbons, then the compound is named as a phenyl-substituted alkane CH3
CH2
CHCH2CH2CH2CH2CH3
phenyl group
benzyl group
2-phenylheptane
- disubstituted:
X
ortho - 1,2-relationship
ortho
ortho meta
meta - 1,3-relationship
meta
para - 1,4-relationship
para CH3
Cl
Cl Cl
CH3
meta-xylene
ortho-dichlorobenzene - when discussing reactions:
CHO
para-chlorobenzaldehyde
CH3
CH3 Br2 FeBr3
Br
- benzenes with more than two substituents: 1) number such that lowest possible numbers used 2) list substituents alphabetically in name Br
Br
Br
Cl Cl
Br
1,2,4-tribromobenzene
4-bromo-1,2-dimethylbenzene
(not 1,3,4) NO2
OH Br
O2N
Br
NO2
2,4,6-trinitrotoluene (TNT)
2,6-dibromophenol
2
Structure and Stability of Benzene - benzene is much more stable than other alkenes - cyclohexene reacts rapidly with two equivalents of Br2 Br
Br2
Br
- benzene reacts slowly with Br2 to give substitution product C6H5Br Br
H
Br2 FeBr3
H
Br Br
not formed - reaction permits cyclic conjugation of benzene ring to be retained
Compare Heats of Hydrogenation H2 Pd/C
cyclohexene, ∆Hohydrog = -118 kJ/mol H2 Pd/C
1,3-cyclohexadiene, ∆Hohydrog = -230 kJ/mol H2 Pd/C
benzene, ∆Hohydrog = -206 kJ/mol - for benzene, expected ~ -356 kJ/mol; benzene ~ -150 kJ/mol ‘extra’ stability
3
Stability of Benzene 1) Resonance theory - benzene is a resonance hybrid of two equivalent forms. Neither form is correct by itself; the true structure of benzene is somewhere in between the two resonance forms.
2) Molecular Orbital Theory - all C-C-C bond angles are 120o, all six carbon atoms are sp2 hybridized, and each carbon atom has a p orbital perpendicular to the plane of the six-membered ring.
Molecular Orbital Description of Benzene
Hückel 4n + 2 Rule - a molecule is aromatic only if it has a planar, monocyclic system of conjugation with a total of 4n+2 π electrons, where n is an integer (n = 0, 1,2,3…) - aromatic: 2,6,10,14,18… π electrons - not aromatic: 4,8,12,16… 4n π electrons
Examples: 1) cyclobutadiene
2) benzene
two double bonds four π electrons
three double bonds six π electrons
not aromatic
aromatic
4
3) cyclooctatetraene
four double bonds eight π electrons
not aromatic
- reacts readily with Br2, KMnO4, HCl - C-C bonds 1.47 Å, C=C bonds 1.34 Å - tube-shaped molecule - 1H NMR shows sharp line at 5.7 δ
Cyclooctatetraene
Problem: 3-Chlorocyclopropene on treatment with AgBF4 gives a precipitate of AgCl and a stable solution of a product that shows a single 1H NMR absorption at 11.04 δ. What is a likely structure for the product, and what is its relation to Hückel’s rule?
5
Benzene
number of p orbitals = 6 number of electrons = 6
Hückel’s 4n + 2 Rule 1) planar, 2) monocyclic system of conjugation, and 3) total of 4n+2 π electrons
Consequences of Hückel Rule: Aromatic Ions Hückel 4n + 2 rule does not say that: 1) number of p orbitals = number of π electrons i.e. an aromatic species must be neutral - therefore, in addition to molecules, ions may also be aromatic
Examples: H H
H
-
H
+
H
H H
H
H
H H
cyclopentadienyl anion
H
cycloheptatrienyl cation
Cyclopentadienyl Anion H
H
- cyclopentadienyl anion is remarkably stable
H
H H
H
- cyclopentadiene is one of the most acidic hydrocarbons known (pKa = 16)
H
H +
H
H
H
H
H
H
-
H
H
H
H H
H
H
cyclopentadienyl cyclopentadienyl cation: four π electrons cyclopentadienyl anion: six π electrons radical: five π electrons
- pKa of the diene is comparable to water (!)
6
Consequences of Hückel Rule: Heterocycles Hückel 4n + 2 rule does not say that: 2) atoms in ring must be carbon - heterocycle: a cyclic compound that contains an atom or atoms other than carbon in its ring e.g. oxygen, sulphur, phosphorus Examples: N
pyridine
N H
pyrrole
- both compounds are crucial to many biochemical processes
7
Molecular Orbitals: Pyridine and Pyrrole - nitrogen atoms play different roles in heterocycles: pyridine: N atom is in double bond, contributes only 1 π electron to the aromatic sextet pyrrole: N atom is not in double bond, contributes 2 π electrons to the aromatic sextet
Why 4n + 2? - molecular orbital theory: Energy levels of molecular orbitals for cyclic conjugated molecules always possess a single lowest-lying MO, above which the MOs come in degenerate pairs - one pair (2 electrons) to fill the bottom and two pairs (4 electrons) to fill each n successive level
Energy Levels of Molecular Orbitals in Benzene
8
Energy Levels of Molecular Orbitals in Cyclopentadienyl Cation, Radical, and Anion
Polycyclic Aromatic Compounds - Hückel rule is strictly applied to monocyclics - however, concept of aromaticity can be extended to include polycyclic aromatic compounds
naphthalene
anthracene
benzo[a]pyrene
- all can be represented by various resonance forms
coronene
Naphthalene - resonance forms of naphthalene:
- heat of hydrogenation: 250 kJ/mole of stabilization - naphthalene reacts slowly with Br2, similar to benzene Br Br2
+ HBr
FeBr3
9
Spectroscopy of Aromatic Compounds Infrared Spectroscopy - characteristic C-H stretching absorption at 3030 cm-1 - characteristic series of peaks from 1450 to 1600 cm-1 - strong absorptions from 690 to 900 cm-1 - diagnostic of substitution pattern of aromatic ring monosubstituted: 690-710 cm-1 730-770 cm-1
m-disubstituted:
690-710 cm-1 810-850 cm-1
735-770 cm-1
p-disubstituted:
810-840 cm-1
o-disubstituted:
Infrared Spectrum of Toluene
10
Ultraviolet Spectroscopy - intense absorption near 205 nm and weaker absorption from 255 to 275 nm is indicative of an aromatic ring Nuclear Magnetic Resonance Spectroscopy - hydrogen atoms directly bonded to an aromatic ring are easily identifiable - hydrogen are strongly deshielded by the ring and absorb between 6.5 and 8.0 δ - deshielding of hydrogens is due to a property of aromatic rings called a ring current
Ring Current in Aromatics
When an aromatic ring is oriented perpendicular to a strong magnetic field, the delocalized π electrons circulate around the ring, producing a small local magnetic field. This induced field opposes the applied field in the middle of the ring but reinforces the applied field outside the ring. Aromatic protons therefore experience an effective magnetic field greater than the applied field and come into resonance at lower field.
Example: 1H NMR Spectrum of toluene
11
Example: 1H NMR Spectrum of p-bromotoluene
- protons on ‘outside’ are deshielded and protons on ‘inside’ are shielded Example:
1H
NMR spectrum of [18]Annulene H
H
H
H
H
H H
H
H H
H H
H
Inside H: -3.0 δ H
Outside H: 9.3 δ
H H
H
H
[18]Annulene
- carbon atoms of aromatics absorb from 110 to 140 δ in 13C spectrum 21.3
CH3
128.4
137.7 129.3 128.5
125.6 Cl
133.8 127.6
133.7
128.1 126.0
128.4 125.4
12
Notes
Notes
13
