1. Benzene and Aromaticity
Chapter 15
Benzene and Aromaticity

2. Introduction
In early 19th century, the term aromatic was used to describe some fragrant compounds
Not correct: later they are grouped by chemical behavior (unsaturated compounds that undergo substitution rather than addition).
coal distillate cherries, peaches and almonds Tolu balsam

3. Currently, the term aromatic is used to refer to the class of compounds related structurally to benzene
They are distinguished from aliphatic compounds by electronic configuration
steroidal hormone analgesic tranquilizer

4. 1. Sources of Aromatic Hydrocarbons
There are two main sources of simple aromatic hydrocarbons:
coal
petroleum

5. High temperature distillation of coal tar
Coal is a mixture of benzene-like rings joined together. Under high temperature, it produces coal tar which, upon fractional distillation, yields:

6. Heating petroleum at high temperature under high pressure over a catalyst
Petroleum consists mainly of alkanes which, at high temperature under pressure over a catalyst, convert into aromatic compounds.

7. 2. Naming Aromatic Compounds
Aromatic compounds are named according to the system devised by the International Union of Pure and Applied Chemistry (IUPAC).

8. Aromatic compounds have many common names that have been accepted by IUPAC:
Toluene = methylbenzene
Phenol = hydroxybenzene
Aniline = aminobenzene

10. Monosubstituted benzenes
Monosubstituted benzenes, like hydrocarbons, are systematically named with –benzene as the parent name
C6H5Br C6H5NO2 C6H5CH2CH2CH3

11. Arenes Arenes are alkyl-substituted benzenes
If # Csubstituent < or = 6, then the arene is named as an alkyl-substituted benzene
If # Csubstituent > 6, then the arene is named as a phenyl-substituted alkane

12. Aryl groups “Phenyl” refers to C6H5 “Benzyl” refers to “C6H5CH2”
It is used when a benzene ring is a substituent
“Ph” or “f” can also be in place of “C6H5”
“Benzyl” refers to “C6H5CH2”

13. Disubstituted benzenes
Relative positions on a disubstituted benzene ring:
ortho- (o) on adjacent carbons (1,2 disubstituted)
meta- (m) separated by one carbon (1,3 disubstituted)
para- (p) separated by two carbons (1,4 disubstituted)

14. The ortho- (o), meta- (m), and para- (p) nomenclature is useful to describe reaction patterns
Example: “Reaction of toluene with Br2 occurs at the para position”

15. Multisubstituted benzenes
Multisubstituted benzenes (more than two substituents) are named as follows:
Choose the sequence when the substituents have the lowest possible number
List substituents alphabetically with hyphenated numbers
Use common names, such as “toluene”, as parent name (as in TNT)

16. Use common names, such as “toluene”, as parent name
The principal substituent is assumed to be on C1
See Table 15.1

17. Practice Problem: Tell whether the following compounds are
Practice Problem: Tell whether the following compounds are ortho-, meta-, or para-disubstituted
(a) Meta (b) Para (c) Ortho

18. Practice Problem: Give IUPAC names for the following compounds
(a) m-Bromochlorobenzene (b) (3-Methylbutyl)benzene
(c) p-Bromoaniline (d) 2,5-Dichlorotoluene
(e) 1-Ethyl-2,4-dinitrobenzene (f) 1,2,3,5-Tetramethylbenzene

19. Practice Problem: Draw structures corresponding to the
Practice Problem: Draw structures corresponding to the following IUPAC names:
p-Bromochlorobenzene
p-Bromotoluene
m-Chloroaniline
1-Chloro-3,5-dimethylbenzene

20. 3. Structure and Stability of Benzene
Benzene is very stable
It undergoes substitution rather than the rapid addition reaction common to compounds with C=C, suggesting that in benzene there is a higher barrier
Example: Benzene reacts slowly with Br2 to give bromobenzene (where Br replaces H)

21. Heats of Hydrogenation as Indicators of Stability
The addition of H2 to C=C normally gives off about 118 kJ/mol – 3 double bonds would give off 356 kJ/mol
Two conjugated double bonds in cyclohexadiene add 2 H2

22. Benzene has 3 unsaturations but gives off only 206 kJ/mol on reacting with 3 H2 molecules
Therefore it has about 150 kJ/mol more “stability” than an isolated set of three double bonds

23. Benzene has 150 kJ/mol more “stability” than expected for “cyclohexatriene”

24. Benzene’s Unusual Structure
All its C-C bonds are the same length: 139 pm — between single (154 pm) and double (134 pm) bonds
Electron density in all six C-C bonds is identical

25. Structure is planar, hexagonal
All C–C–C bond angles are 120°
Each C is sp2-hybridized and has a p orbital perpendicular to the plane of the six-membered ring

26. Drawing Benzene and Its Derivatives
The two benzene resonance forms can be represented by a single structure with a circle in the center to indicate the equivalence of the carbon–carbon bonds
This does not indicate the number of  electrons in the ring but shows the delocalized structure
One of the resonance structures will be used to represent benzene for ease in keeping track of bonding changes in reactions

27. 4. Molecular Orbital Description of Benzene
In benzene, 6 p orbitals combine to form 6 p molecular orbitals (MO):
3 bonding orbitals with 6  electrons
3 antibonding orbitals

28. 3 bonding, low-energy MO: y1, y2, and y3
3 antibonding high-energy MO: y4*, y5*, and y6*

29. Orbitals with the same energy are degenerate
2 bonding orbitals, y2 and y3
2 antibonding orbitals, y4* and y5*

30. y3 and y4* have no p electron density on 2 carbons because of a node passing through these atoms

31. Practice Problem: Pyridine is flat, hexagonal with bond angles of 120°. It undergoes electrophilic substitution rather than addition and generally behaves like benzene. Draw the p orbitals of pyridine.

32. Recall: Key Ideas on Benzene
Benzene is a cyclic conjugated molecule
Benzene is unusually stable - DHhydrogenation = 150 kJ/mol less negative than a cyclic triene
Benzene is planar hexagon: bond angles are 120°; carbon–carbon bond lengths, 139 pm
Benzene undergoes substitution rather than electrophilic addition
Benzene is a resonance hybrid with structure between two line-bond structures
Benzene has 6 p electrons, delocalized over the ring

33. 5. Aromaticity and the Hückel 4n + 2 Rule
was devised by Eric Hückel in 1931
states that planar, monocyclic conjugated systems with a total of 4n + 2 p electrons where n is an integer (n = 0, 1, 2, 3,…) are aromatic

34. Aromatic compounds with 4n + 2 p electrons
Benzene
It has 6 p electrons: 4n + 2 = 6, thus n = 1
It is aromatic: it is stable and the electrons are delocalized

35. Compounds with 4n  electrons are NOT aromatic (May be Antiaromatic)
Planar, cyclic conjugated molecules with 4n  electrons are antiaromatic
They are much less stable than expected
They will distort out of plane and behave like ordinary alkenes
Which of the above is antiaromatic?

36. Cyclobutadiene
It has 4 p electrons: 4n + 2 = 4, thus n = ½ (not an integer)
It is antiaromatic: The p electrons are localized into two double bonds
localized p
electrons

37. Cyclobutadiene
It has 4 p electrons: 4n + 2 = 4, thus n = ½ (not an integer)
It is antiaromatic: The p electrons are localized into two double bonds
It is so unstable that it dimerizes by a self-Diels-Alder reaction at low temperature
dienophile diene

38. Cyclooctatetraene
It has 8 p electrons: 4n + 2 = 8, thus n = 3/2 (not an integer)
It is nonaromatic:
the p electrons are localized into four double bonds
it is tub-shaped not planar
it has four double bonds, reacting with Br2, KMnO4, and HCl as if it were four alkenes
p orbitals
not parallel
for overlap

39. Practice Problem: To be aromatic, a molecule must have 4n + 2
Practice Problem: To be aromatic, a molecule must have 4n p electrons and must have cyclic conjugation Is cyclodecapentaene aromatic?
It has 10 p electrons: 4n + 2 = 10, thus n = 2 (an integer)
It is not planar due to steric strain, thus the neighboring p
orbitals are not properly aligned for overlap. It is not
conjugated. Thus it is not aromatic.

40. 6. Aromatic Ions
The Hückel 4n + 2 rule applies to ions as well as to neutral species:
To be aromatic, a molecule must be planar, cyclic conjugated system with 4n + 2 p electrons
Example: Both the cyclopentadienyl anion and the cycloheptatrienyl cation are aromatic.

41. Example:. Both the cyclopentadienyl anion and the
Example: Both the cyclopentadienyl anion and the cycloheptatrienyl cation are aromatic.
The key feature of both is that they contain 6  electrons in a ring of continuous p orbitals

42. Aromaticity of the Cyclopentadienyl Anion
Not fully conjugated and not aromatic
Unstable and nonaromatic
Stable and aromatic

43. Cyclopentadiene is relatively acidic (pKa = 16) because its conjugate base, the aromatic cyclopentadienyl anion, is so stable.
Other hydrocarbons have pKa > 45

44. Aromaticity of the Cyclopentadienyl Anion
1,3-Cyclopentadiene contains conjugated double bonds joined by a CH2 that blocks delocalization
Removal of H+ at the CH2 produces a cyclic 6-electron system, which is stable
Removal of H- or H• generate nonaromatic 4 and 5 electron systems
Relatively acidic (pKa = 16) because the anion is stable

45. Aromaticity of the Cycloheptatrienyl Cation
Not fully conjugated and not aromatic
Stable and aromatic
Unstable and nonaromatic

46. The cycloheptatrienyl cation (six p electrons) is aromatic and very stable
Reaction of cycloheptatriene with Br2 yields cycloheptatrienylium bromide, an ionic substance containing the cycloheptatrienyl cation

47. Aromaticity of the Cycloheptatrienyl Cation
Cycloheptatriene has 3 conjugated double bonds joined by a CH2
Removal of H- at the CH2 produces the cycloheptatrienyl cation
The cation is a cyclic 6-electron system, which is stable and is aromatic
Removal of H+ or H• generate nonaromatic 7 and 8 electron systems

48. Practice Problem: Draw the five resonance structures of the
Practice Problem: Draw the five resonance structures of the cyclopentadienyl anion. Are all carbon-carbon bonds equivalent? How many absorption lines would be in the 1H and 13C NMR spectra of the anion?

49. Practice Problem: Cyclooctatetraene readily reacts with potassium
Practice Problem: Cyclooctatetraene readily reacts with potassium metal to form the stable the cyclooctatetraene dianion, C2H82-. Why does the reaction occur so easily? What is the geometry of the dianion?

50. 7. Aromatic Heterocycles: Pyridine and Pyrrole
A heterocycle is a cyclic compound that contains an atom or atoms other than carbon in its ring, such as N, O, S, P
There are many heterocyclic aromatic compounds and many are very common
Cyclic compounds that contain only carbon are called carbocycles (not homocycles)
Nomenclature is specialized
Example: Pyridine and Pyrrole

51. Pyridine
Pyridine is a six-membered heterocycle with a nitrogen atom in its ring
 electron structure resembles benzene (6 electrons)
The nitrogen lone pair electrons are in sp2 orbital, not part of the p aromatic system (perpendicular orbital)
Pyridine is a relatively weak base compared to normal amines but protonation does not affect aromaticity

52. Pyrrole
Pyrrole is a five-membered heterocycle with a nitrogen atom in its ring
 electron system is similar to that of cyclopentadienyl anion
Four sp2-hybridized carbons with 4 p orbitals perpendicular to the ring and 4 p electrons

53. Pyrrole
Nitrogen atom is sp2-hybridized, and lone pair of electrons occupies a p orbital (6  electrons)
Since lone pair electrons are in the aromatic ring, protonation destroys aromaticity, making pyrrole a very weak base

54. Practice Problem: Thiophene, a sulfur-containing heterocycle,
Practice Problem: Thiophene, a sulfur-containing heterocycle, undergoes typical aromatic substitution reactions rather than addition reactions Explain why thiophene is aromatic.
It has 6 p electrons: 4n + 2 = 6, thus n = 1 (an integer)
It has a lone pair of electrons in a p orbital perpendicular to the plane

55. Practice Problem: Draw an orbital picture of furan to show how
Practice Problem: Draw an orbital picture of furan to show how the molecule is aromatic
It has 6 p electrons: 4n + 2 = 6, thus n = 1 (an integer)
It has a lone pair of electrons in a p orbital perpendicular to the plane

56. Practice Problem: Draw an orbital picture of imidazole, and account
Practice Problem: Draw an orbital picture of imidazole, and account for its aromaticity. Which nitrogen atom is pyridine-like, and which is pyrrole-like? Which nitrogen atom is more electron-rich, and why?

57. 8. Why 4n + 2? According to the molecular orbital (MO) theory:
Cyclic conjugated molecules always have a single lowest-lying MO, above which the MOs come in degenerate pairs
When electrons fill the various molecular orbitals, it takes two electrons (one pair) to fill the lowest-lying orbital and four electrons (two pairs) to fill each of n succeeding energy level
Only a total of 4n + 2 electrons fill bonding MOs

58. Benzene Benzene has its bonding orbitals filled (6 p electrons)
y1, the lowest-energy MO, is single and has two electrons
y2 and y3, the next two lowest-energy MOs, are degenerate and have four electrons

59. Cyclopentadienyl anion
Cyclopentadienyl anion has its bonding orbitals filled (6 p electrons)
y1 is single and has two electrons; y2 and y3 are degenerate and have four electrons
Cyclopentadienyl cation and radical do not have their bonding orbitals filled

60. Practice Problem: Show the relative energy levels of the seven p
Practice Problem: Show the relative energy levels of the seven p MOs of the cycloheptatrienyl system. Tell which of the seven orbitals are filled in the cation, radical, and anion, and account for the aromaticity of the cycloheptatrienyl cation

61. 9. Polycyclic Aromatic Compounds: Naphthalene
Polycyclic aromatic compounds are
aromatic compounds with rings that share a set of carbon atoms (fused rings)
compounds from fused benzene or aromatic heterocycle rings
carcinogenic
(tobacco)

62. Characteristics of Polycyclic Aromatic Compounds
They are cyclic, planar and conjugated molecules
They are unusually stable
They react with electrophiles to give substitution products, in which cyclic conjugation is retained, rather than electrophilic addition products
They can be represented by different resonance forms
They have 4n + 2 p electrons, delocalized over the ring

63. Naphthalene Naphthalene has three resonance forms
Naphthalene reacts slowly with electrophiles to give substitution products

64. Naphthalene
Naphthalene is a cyclic, conjugated p electron system, with p orbital overlap both around the ten-carbon periphery of the molecule and across the central bond
It has ten delocalized p electrons (Hückel number)

65. Practice Problem: Azulene is an isomer of naphthalene. Is
Practice Problem: Azulene is an isomer of naphthalene. Is azulene aromatic? Draw a second resonance form of azulene.

66. Practice Problem: Naphthalene is sometimes represented with
Practice Problem: Naphthalene is sometimes represented with circles in each ring to represent aromaticity How many p electrons are in each circle?

67. 10. Spectroscopy of Aromatic Compounds
Aromatic compounds can be identified by:
Infrared (IR) Spectroscopy
Ultraviolet (UV) Spectroscopy
Nuclear Magnetic Resonance (NMR) Spectroscopy

68. Infrared Spectroscopy
Aromatic rings have C–H stretching at 3030 cm1 and peaks in the range of 1450 to 1600 cm1
Substitution pattern of the aromatic ring:
Monosubstituted: cm-1
cm-1
o-Disubstituted: cm-1
m-Disubstituted: cm-1
cm-1
p-Disubstituted: cm-1

69. Example: Toluene (IR) 3030 cm1 Monosubstituted: 690-710 cm-1

70. Ultraviolet Spectroscopy
Aromatic rings have peaks near 205 nm and a less intense peak in nm range
Aromatic compounds are detectable by UV spectroscopy since they have a conjugated p electron system

71. Nuclear Magnetic Resonance Spectroscopy
1H NMR:
Aromatic H’s are strongly deshielded by ring and absorb between 6.5 and 8.0 
Peak pattern is characteristic positions of substituents

72. Ring Current is a property unique to aromatic rings
When aromatic ring is oriented perpendicular to a strong magnetic field, delocalized  electrons circulate producing a small local magnetic field
This opposes applied field in middle of ring but reinforces applied field outside of ring

73. Ring Current accounts for the downfield shift of aromatic ring protons in the 1H NMR spectrum
Aromatic 1H’s experience an effective magnetic field greater than applied
It results in outside H’s resonance at lower field

74. Ring Current produces different effects inside and outside the ring
Outside 1H are deshielded and absorb at a lower field
Inside 1H are shielded and absorb at a higher field

75. Ring Current is characteristic of all Hückel aromatic compounds
Aryl 1H absorb between d
Benzylic 1H absorb between d downfield from other alkane 1H

76. Example: p-bromotoluene (1H NMR)
The 4 aryl protons: Two doublets at 7.02 and 7.45 d
The benzylic CH3 protons: a singlet at 2.29 d
Integration 2:2:3

77. 13C NMR Carbons in aromatic ring absorb between 110 to 140 
Shift is distinct from alkane carbons but is in same range as alkene carbons

79. Chapter 15
The End