1. Benzene and Aromaticity
Chapter 15
Benzene and Aromaticity
Suggested Problems –
1-12,18-20,23,
32,37,39-41
CHE2202, Chapter 15
Learn, 1

2. Naming Aromatic Compounds
Coal and petroleum are the major sources of simple aromatic compounds
Coal is primarily comprised of large arrays of conjoined benzene-like rings
When heated to 1000°C, coal thermally breaks down to yield coal tar
Petroleum is primarily comprised of alkanes and few aromatic compounds

3. Some Aromatic Hydrocarbons
The term aromatic was initially used to describe fragrant substances.
We now use the word aromatic to refer to the class of compounds that contain six-membered benzene-like rings with three double bonds.

4. Naming Aromatic Compounds
Aromatic compounds possess the largest number of nonsystematic names, of which some are allowed by the IUPAC
One should know the structures of toluene, phenol, aniline, benzaldehyde, and benzoic acid. These are not IUPAC names but they are used commonly.

5. Naming Aromatic Compounds
Monosubstituted benzenes have systematic names with –benzene being the parent name
What are the systematic names for toluene, phenol, and aniline?
Methylbenzene, hydroxybenzene, and aminobenzene – but these systematic names are rarely if ever used when referring to these molecules.

6. The Phenyl Group Arenes are alkyl-substituted benzenes
Based on the size of the alkyl substituents, they are termed alkyl-substituted or phenyl-substituted benzene
The term phenyl (Ph or Φ) is used in cases of a substituent benzene ring as in –C6H5
The term benzyl is used for the C6H5CH2– group
If the alkyl substituent has more carbons than the six in benzene, the molecule is named as a phenyl-substituted alkane. (Analagously where an arene is considered a substituent, the molecule is named as an aryl-substituted alkane.)
If the phenyl group has more carbons, the alkyl group is named as a substituent such as in the case for ethylbenzene.

7. Disubstituted Benzenes
Names based on the placement of substituents
Ortho (o), meta (m) , and para (p)
Provides clarity in the discussion of reactions
Often the first letter (o,m, p) is used in naming a disubstituted compound – eg. o-xylene, m-xylene, p-xylene. (xylene is a dimethyl substituted benzene).
We might refer to a site of reaction as in the example at lower right on the slide in which a bromine is introduced into the para position (relative to the methyl group).

8. Benzenes With More Than Two Substituents
Numbers with the lowest possible values are chosen
List substituents alphabetically with hyphenated numbers
Common names, such as toluene can serve as root name(as in TNT)
Where there are more than two substituents on a benzene ring, the lowest possible numbering scheme is employed with the groups being numbered and listed in alphabetical order.
Note the parent name in the second and third cases are not benzene but instead phenol and toluene.

9. Worked Example Provide the IUPAC name for the following compound
Solution:
The compound is 1-Ethyl-2,4-dinitrobenzene
Substituents on trisubstituted rings receive the lowest possible numbers

10. Structure and Stability of Benzene
The reactivity of benzene is much less than that of alkenes despite having six fewer hydrogens and three double bonds
Benzene - C6H6
Cycloalkane - C6H12
Cyclohexene reacts rapidly with bromine affording 1,2-dibromocyclohexane. Benzene reacts only slowly with bromine to afford a substitutution product, bromobenzene, rather than an addition product.

11. Heats of Hydrogenation as Indicators of Stability
Comparison of the heats of hydrogenation proves the stability of benzene
Cyclohexatriene would be expected to have a heat of hydrogenation equal to three times that for cyclohexene (-356 kJ/mol).
The heat of hydrogenation of benzene is 150 kJ/mol less than expected. As a result benzene must have 150 kJ/mol less energy than would be expected if it contained three isolated double bonds.
In other words, benzene is more stable than expected by 150 kJ/mol.

12. Structure of Benzene
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
Structure is planar, hexagonal
Further evidence for the unusual nature of benzene is that all its carbon-carbon bonds have the same length.
We might have expected the double bonds to be shorter than the single bonds.
All C-C-C bond angles are 120o, all six carbons are sp2-hybridized, and each carbon has a p orbital perpendicular to the plane of the six-membered ring.

13. Structure of Benzene
All six carbon atoms and p orbitals in benzene are equivalent
Impossible to define three localized  bonds in which a given p orbital overlaps only one neighboring p orbital
All  electrons move freely in the entire ring due to equal overlap of all p orbitals
Resonance of benzene is another factor that influences its rate of reactivity

14. Indicating Carbon–Carbon Bond Equivalence in Benzenes
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
The ring does not indicate the number of  electrons in the ring but is a reminder of the delocalized structure
In essence benzene needs to be viewed as a resonance hybrid of two alternate contributing resonance structures.
Because of this resonance, benzene is more stable and less reactive than a typical alkene.
Chemists sometimes represent the two benzene ressonance forms by using a circle to indicate the equivalence of the carbon-carbon bonds.

15. Molecular Orbital Description of Benzene
The 6 p-orbitals combine to give:
Three bonding orbitals with 6  electrons
Three antibonding with no electrons
Orbitals with the same energy are degenerate
We can construct pi molecular obitals for benzene. If six p atomic orbitals combine in a cyclic manner, six benzene molecular orbitals result. The three low-energy molecular orbitals, denoted by psi 1, 2, and 3 are bonding combinations, and the three high-energy orbitals are antibonding.
Note that the two bonding orbitals psi 2 and 3 have the same energy as do the antibonding orbitals psi 4and 5. Orbitals with the same energy are said to be degenerate.
The six p electrons of benzen occupy the three bonding molecular orbitals and are delocalized over the entire conjugated system, leading to the observed 150 kJ/mol stabilization of benzene.

16. Worked Example
Pyridine (C5H5N) - A flat, hexagonal molecule with bond angles of 120°undergoes substitution rather than addition and generally behaves like benzene
Draw a picture of the  orbitals of pyridine to explain its properties

17. Worked Example Solution:
The pyridine ring is formed by the σ overlap of carbon and nitrogen sp2 orbitals
Six p orbitals perpendicular to the plane of the ring hold six electrons
Remember that nitrogen has five valence electrons which are represented by the covalent bonds, lone pair, and single p electron in the depiction.

18. Worked Example
These six p orbitals form six  molecular orbitals that allow electrons to be delocalized over the  system of the pyridine
The lone pair of nitrogen occupies an sp2 orbital that lies in the plane of the ring

19. Aromaticity and the Hückel 4n+2 Rule
Unusually stable - Heat of hydrogenation 150 kJ/mol less negative than a hypothetical cyclic triene
Planar hexagon - Bond angles are 120°, carbon-carbon bond length is 139 pm
Undergoes substitution rather than electrophilic addition
Resonance hybrid with structure between two line-bond structures
A summary of what we know about benzene.

20. The Hückel 4n + 2 Rule Developed by Erich Hückel in 1931
States that a molecule can be aromatic only if:
It has a planar, monocyclic system of conjugation
It contains a total of 4n + 2 molecules
n = 0,1,2,3…
4n  electrons are considered antiaromatic
Cyclobutadiene possesses four electrons and is antiaromatic
Huckel 4n + 2 Rule – A rule used to determine whether a system is aromatic by looking at the number of pi electrons in a ring.
n is an integer value beginning with 0.
Only molecules with 2, 6, 10, 14, 18…. Pi electrons can be aromatic.
Molecules with 4n pi electrons (4, 8, 12, 16…) can’t be aromatic even though they may be cyclic, planar, and apparently conjugated.
Planar, conjugated molecules with 4n pi electrons are said to antiaromatic because delocalization of their pi electrons would lead to their destabilization.

21. The Hückel 4n + 2 Rule
It reacts readily and exhibits none of the properties corresponding to aromaticity
It dimerizes by a Diels-Alder reaction at –78 °C
Benzene possesses six  electrons (4n + 2 = 6 when n = 1) and is aromatic
Cyclobutadiene, not having the stability associated with aromaticity, is highly reactive (unstable).

22. The Hückel 4n + 2 Rule
Cyclooctatetraene possesses eight  molecules and is not aromatic
Comprises four double bonds
The molecule is tub-shaped rather than planar. It has no cyclic conjugation because neighboring p orbitals don’t have the necessary parallel alignment for overlap, and it resembles an open-chain polyene in its reactivity.

23. Aromatic Stability and the Molecular Orbital Theory
Calculation of energy levels of molecular orbitals for cyclic conjugated molecules shows that there is always a single lowest-lying MO above which MOs come in degenerate pairs
Lowest lying molecular orbital is filled by a pair of electrons and higher orbitals are filled by two pairs of electrons

24. Energy Levels of the Six Benzene  Molecular Orbitals
The stability of aromatic systems comes from the filling of the bonding MOs. If psi 1 is the only bonding MO and it contains two electrons, n = 0 in the 4n + 2 rule. Subsequently bonding MOs above psi 1 in energy always occur in pairs (ie. 4n = 2 for higher values of n).

25. Worked Example
To be aromatic, a molecule must have 4n + 2  electrons and must have a planar, monocyclic system of conjugation
Explain why cyclodecapentaene has resisted all attempts at synthesis though it has fulfilled only one of the above criteria

26. Worked Example Solution:
Cyclodecapentaene possesses 4n + 2  (n = 2) but is not flat
If cyclodecapentaene were flat, the starred hydrogen atoms would crowd each other across the ring
To avoid this interaction, the ring system is distorted from planarity

27. Aromatic Ions
The 4n + 2 rule applies to ions as well as neutral substances
Both the cyclopentadienyl anion and the cycloheptatrienyl cation are aromatic
To be aromatic a molecule must by cyclic, conjugated (nearly planar with a p orbital on each atom), and have 4n + 2 pi electrons.
The cyclopentadienyl anion and the cycloheptatrienyl cation are both aromatic even though the number of pi electrons differs from the number of ring atoms.

28. Aromatic Ions
When one hydrogen is removed from the saturated CH2 in an aromatic ion, rehybridization of the carbon from sp3 to sp2 would result in a fully conjugated product with a p orbital on every product
Methods to remove the hydrogen molecule
Removing the hydrogen with both electrons (H:–) from the C–H bond results in a carbocation
Removing the hydrogen with one electron (H·) from the C–H bond results in a carbon radical
Removing the hydrogen without any electrons (H+) from the C–H bond results in a carbanion

29. Cyclopentadienyl Anion and Cycloheptatrienyl Cation
Only the cyclopentadienyl anion and the cycloheptatrienyl cation are aromatic.

30. Aromaticity of Cyclopentadienyl Anion
Disadvantages of the four--electron cyclopentadienyl cation and the five--cyclopentadienyl radical
Highly reactive
Difficult to prepare
Not stable enough for aromatic systems
Advantages of using the six--electron cyclopentadienyl cation
Easily prepared
Extremely stable
pKa =16
The cyclopentadienyl cation, the cyclopentadienyl radical, the cycloheptatrienyl radical, and the cycloheptatrienyl anion, are all unstable, highly reactive, and difficult to prepare.
Cyclopentadiene is one of the most acidic hydrocarbons known owing to the stability of the cyclopentadienyl anion.

31. The Aromatic Cyclopentadienyl Anion and the Aromatic Cycloheptatrienyl Cation

32. Worked Example
Cyclooctatetraene readily reacts with potassium metal to form the stable cyclooctatetraene dianion, C8H82–
Explain why this reaction occurs so easily
Determine the geometry for the cyclooctatetraene dianion

33. Worked Example Solution:
When cyclooctatetrene accepts two electrons, it becomes a (4n + 2)  electron aromatic ion
Cyclooctatetraenyl dianion is planar with a carbon–carbon bond angle of 135°, that of a regular octagon

34. Aromatic Heterocycles: Pyridine and Pyrrole
Heterocycle: Cyclic compound that comprises atoms of two or more elements in its ring
Carbon along with nitrogen, oxygen, or sulfur
Aromatic compounds can have elements other than carbon in the ring

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

36. Pyridine and Pyrimidine
The  structure of pyridine is quite similar to that of benzene
All five sp2-hybridized ions possess a p orbital perpendicular with one to the plane of the ring
Each p orbital comprises one  electron
The nitrogen atom is also sp2-hybridized and possesses one electron in a p orbital
Pyrimidine comprises two nitrogen atoms in a six-membered, unsaturated ring
The sp2-hybridized nitrogen atoms share an electron each to the aromatic  system

37. Pyridine and Pyrimidine
Each nitrogen, whether in pyridine or pyrimidine, contributes one electron to the aromatic pi system.
Lone pairs are in sp2 orbitals in the plane of the ring.

38. Pyrrole and Imidazole
Both pyrrole and imidazole are aromatic having six pi electrons.
In pyrrole, each sp2-hybridized carbon contributes one pi electron and the sp2-hybridized nitrogen atom contributes two from the lone pair, which occupies a p orbital.
In imidazole, both nitrogens are sp2-hybridized, but one is in a double bond and contributes only one electron to the aromatic pi system whereas the other is not in a double bond and contributes two from its lone pair.

39. Rings of Pyrimidine and Imidazole
Significant in biological chemistry
Pyrimidine is the parent ring system present in cytosine, thymine, and uracil
Histidine contains an aromatic imidazole ring
Both pyrimidine and imidazole rings are important in biological chemistry, the former comprising nucleotide bases in nucleic acids and the latter being present in histidine, one of the 20 amino acids found in proteins.

40. Worked Example
Draw an orbital picture of furan to show how the molecule is aromatic

41. Worked Example Solution: Furan is an oxygen analog of pyrrole
It possesses 6  electrons on a cyclic, conjugated system; it is aromatic
Oxygen contributes two lone-pair electron from a p orbital perpendicular to the plane of the ring

42. Polycyclic Aromatic Compounds
While the Hückel rule is relevant only to monocyclic compounds, the concept of aromaticity can also be applied to polycyclic aromatic compounds

43. Naphthalene Orbitals Three resonance forms and delocalized electrons
Naphthalene and other polycyclic aromatic hydrocarbons possess certain chemical properties that correspond to aromaticity
Heat of hydrogenation in naphthalene is approximately 250 kJ/mol
All polycyclic aromatic hydrocarbons can be represented by a number of different resonance forms. Napthalene has three.
Napthalene shows a heat of hydrogenation consistent with aromatic stabilization. It also reacts slowly with electrophiles such as bromine to give substitution rather than double-bond addition products.

44. Aromaticity of Naphthalene
Naphthalene possesses a cyclic, conjugated electron system
p orbital overlap is present along the ten-carbon periphery of the molecule and across the central bond
Aromaticity is due to the  electron delocalization caused by the presence of ten  electrons (Hückel number)

45. Heterocyclic Analogs of Naphthelene
Quinolone, isoquinolone, and purine have pyridine-like nitrogens that share one  electron
Indole and purine have pyrrole-like nitrogens that share two  electrons
Just as there are heterocyclic analogs of benzene, there are also many heterocyclic analogs of napthalene.
Quinoline, isoquinoline, and purine all contain pyridine-like nitrogens that are part of a double bond and contribute one electron to the aromatic pi system.
Indole and purine both contain pyrrole-like nitrogens that contribute two pi electrons.

46. Worked Example
Azulene, a beautiful blue hydrocarbon, is an isomer of naphthalene
Determine whether it is an aromatic
Draw a second resonance form of azulene in addition to the form shown below

47. Worked Example Solution:
Azulene is aromatic because it has a conjugated cyclic  electron system containing ten  electrons (a Hückel number)

48. Spectroscopy of Aromatic Compounds
Infrared Spectroscopy
C–H stretching absorption is seen at 3030 cm–1
Usually of low intensity
A series of peaks are present between 1450 and 1600 cm–1
Caused by the complex molecular motions of the ring

49. Ultraviolet Spectroscopy
Presence of a conjugated  system makes ultraviolet spectroscopy possible
Intense absorption occurs near 205 nm
Less intense absorption occurs between 255 nm and 275 nm
The UV spectrum shown is for benzene.

50. Nuclear Magnetic Resonance Spectroscopy
The aromatic ring deshields hydrogens
Absorption occurs between 6.5 and 8.5 δ
The ring current is responsible for the difference in chemical shift between aromatic and vinylic protons
Ring current is the magnetic field caused by the circulation of delocalized  electrons when the aromatic ring is perpendicular to a strong magnetic field
The effective magnetic field is greater than the applied field

51. The Origin of Aromatic Ring Current
When an aromatic ring is oriented perpendicular to a strong magnetic field, the delocalized pi electrons circulate around the ring, producing a small local magnetic field. This induced field opposes the applied filed 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 a lower applied field.

52. Nuclear Magnetic Resonance Spectroscopy
Aromatic protons appear as two doublets at 7.04 and 7.37 δ
Benzylic methyl protons appear as a sharp singlet at 2.26 δ
Benzylic protons are deshielded by the ring current and appear at higher ppm than they otherwise would.

53. 13C NMR of Aromatic Compounds
Carbons in aromatic ring absorb between 110 and 140 δ
Shift is distinct from alkane carbons but in same range as alkene carbons
One cannot distinguish aromatic carbons from vinylic ones in CMR as readily as protons can be identified in NMR.

54. 13C NMR of Aromatic Compounds
The mode of substitution influences the formation of two, three, or four resonances in the proton-decoupled 13C NMR spectrum
Symmetry of aromatic carbons limits the number of signals in a 13C NMR spectrum. Where there are more than one identical carbon, the peaks tend to be taller. Quaternary carbons tend to be smaller due to the longer time it takes for quaternary carbons to relax following the RF pulses.

55. The Proton-Decoupled 13C NMR Spectra of the Three Isomers of Dichlorobenzene
Subsitution patterns on aromatic rings can be discerned in the CMR spectra because of the symmetry which “duplicates” identical carbons.