Benzene and

1

• Benzene and
• and the Concept of
• Aromaticity

Chapter 21
2
21.1 A. Benzene - Kekulé

• Discovered by Michael Faraday in 1825. (C H)
• The first structure for benzene was proposed by
  August Kekulé in 1872.
• This structure, however, did not account for the
  unusual chemical reactivity of benzene.

3
Benzene

• The concepts of hybridization of atomic orbitals
  and the theory of resonance, developed in the
  1930s, provided the first adequate description of
  benzenes structure.
• the carbon skeleton is a regular hexagon
• all C-C-C and H-C-C bond angles 120.

4
B. Benzene - Molecular Orbital Model

• The linear combination of six overlapping p
  orbitals must form six molecular orbitals.
• Three will be bonding, three antibonding.
• Lowest energy MO will have all bonding
  interactions, no nodes.
• As energy of MO increases, the number of nodes
  increases.

5
Benzene - Molecular Orbital Model
Orbitals of equal energy are degenerate. Antibondi
ng orbitals are starred (). Electrons in the
orbitals of lowest energy are in the ground
state.
6
Benzene - Molecular Orbital Model

• The 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.

Figure 21.2
7
Benzene - Molecular Orbitals
Viewed from the top of each carbon atom
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C. Benzene - Resonance Model

• 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.

9
Benzene - Resonance

• 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.

10
Benzene, Fig 21.3
11
21.2 A. Concept 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
• 1. be cyclic.
• 2. have one p orbital on each atom of the ring
  (sp2).
• 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) pi electrons
  in the cyclic arrangement of p orbitals.

12
Frost Circles (Polygon Rule)

• Frost circle a graphic method for determining
  the relative order of pi MOs in planar, fully
  conjugated monocyclic compounds.
• inscribe a polygon of the same number of sides as
  the ring to be examined such that one of the
  vertices is at the bottom of the ring.
• the relative energies of the MOs in the ring are
  given by where the vertices 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.

13
Frost Circles, Fig 21.4

• following are Frost circles describing the MOs
  for monocyclic, planar, fully conjugated four-,
  five-, and six-membered rings.

14
B. Aromatic Hydrocarbons

• Annulene a cyclic hydrocarbon with a continuous
  alternation of single and double bonds.
• 14annulene is aromatic according to Hückels
  criteria.

15
Aromatic Hydrocarbons

• 18annulene is also aromatic.

16
Aromatic Hydrocarbons

• according to Hückels criteria, 10annulene
  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.

17
Aromatic Hydrocarbons

• what is remarkable relative to 10annulene 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.

18
C. Antiaromatic Hydrocarbons

• Antiaromatic hydrocarbon a monocyclic, planar,
  fully conjugated hydrocarbon with 4n pi electrons
  (4, 8, 12, 16, 20...), it does not obey Huckels
  rule.
• 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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Cyclobutadiene, Fig. 21.5

• planar cyclobutadiene has two unpaired electrons,
  which make it highly unstable and reactive.

20
Cyclooctatetraene

• 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.

21
Cyclooctatetraene

• 2p orbital overlap forms each pi bond, there is
  essentially no overlap between adjacent alkenes.

22
Cyclooctatetraene, Fig. 21.6

• planar cyclooctatetraene, if it existed, would be
  antiaromatic.
• it would have unpaired electrons in the ?4 and ?5
  nonbonding MOs.

23
D. Heterocyclic Aromatics

• 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 are other than carbon.
• Pyridine and pyrimidine are heterocyclic analogs
  of benzene each is aromatic.

24
Pyridine

• 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.
• pyridine has a resonance energy of 134 kJ (32
  kcal)/mol, slightly less than that of benzene.

25
Furan, Fig. 21.7

• 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.

26
Other Heterocyclics
27
Other Heterocyclics
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E. Aromatic 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.

29
Cyclopropenyl 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.

30
Cyclopropenyl 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.

31
Cyclopentadienyl Cation

• if planar cyclopentadienyl cation existed, 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.

32
Cyclopentadienyl Anion

• 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.

pKa 16
33
Cyclopentadienyl Anion

• as seen in the Frost circle, the six pi electrons
  occupy the p1, p2, and p3 molecular orbitals, all
  of which are bonding.

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Cyclopentadienyl Anion

• 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.

35
MOs of Aromatic Ions

• Cyclopropenyl cation and cyclopentadienyl anion.

36
pKa's of some hydrogens
37
Cycloheptatrienyl Cation

• Cycloheptatriene forms an aromatic cation by
  conversion of its CH2 group to a CH group with
  its sp2 carbon having a vacant 2p orbital.

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21.3 A. Nomenclature

• Monosubstituted alkylbenzenes are named as
  derivatives of benzene.
• many common names are retained.

Phenol
Aniline
Benzoic acid
Anisole
Benzaldehyde
39
Nomenclature

• Benzyl and phenyl groups.

Benzene
Toluene
O
O
1-Phenyl-1pentanone
40
B. Disubstituted Benzenes

• Locate two groups by numbers or by the locators
  ortho (1,2-), meta (1,3-), and para (1,4-).
• where one group imparts a special name, name the
  compound as a derivative of that molecule.

41
Disubstituted Benzenes

• where neither group imparts a special name,
  locate the groups and list them in alphabetical
  order.

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C. Polysubstituted 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.

1
4
1
2
6
2
2
4
4
1
43
21.4 A. Phenols

• The functional group of a phenol is an -OH group
  bonded to a benzene ring.

Phenol
1,3-Benzenediol is resorcinol
44
Phenols

• hexylresorcinol is a mild antiseptic and
  disinfectant.
• eugenol is used as a dental antiseptic and
  analgesic.
• urushiol is the main component of the oil of
  poison ivy.

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B. Acidity of Phenols

• Phenols are significantly more acidic than
  alcohols, compounds that also contain the OH
  group.

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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.

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Acidity of Phenols

• Alkyl and halogen substituents effect acidities
  by inductive effects.
• alkyl groups are electron-releasing.
• halogens are electron-withdrawing.

48
Acidity of Phenols

• nitro groups increase the acidity of phenols by
  both an electron-withdrawing inductive effect and
  a resonance effect.

49
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.

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C. Acid-Base Reactions of Phenols

• Phenols are weak acids and react with strong
  bases to form water-soluble salts.
• water-insoluble phenols dissolve in NaOH(aq).

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Acid-Base Reactions of Phenols

• most phenols do not react with weak bases such as
  NaHCO3 they do not dissolve in aqueous NaHCO3

No reaction
52
D. 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.
• The following two examples illustrate
• the use of a phase-transfer catalyst.
• the use of dimethyl sulfate as a methylating
  agent.

X
no reaction
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Alkyl-Aryl Ethers
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E. Kolbe Carboxylation

• Phenoxide ions react with carbon dioxide to give
  a carboxylic salt.

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Kolbe Carboxylation

• the mechanism begins by nucleophilic addition of
  the phenoxide ion to a carbonyl group of CO2.

56
F. Quinones

• Because of the presence of the electron-donating
  -OH group, phenols are susceptible to oxidation
  by a variety of strong oxidizing agents.
• Quinones are six-membered rings with two CO.

O
O
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Quinones
58
Quinones

• Perhaps the most important chemical property of
  quinones is that they are readily reduced to
  hydroquinones by sodium hydrosulfite.

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Coenzyme Q

• Coenzyme Q is a carrier of electrons in the
  respiratory chain.

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Vitamin K

• both natural and synthetic vitamin K (menadione)
  are 1,4-naphthoquinones.

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21.5 A. 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.

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Benzylic Oxidation

• if there is more than one alkyl group on the
  benzene ring, each is oxidized to a -COOH group.
• an aryl-COOH is the oxidation product regardless
  of the alkyl group that was attached to the
  aromatic ring (may be Me, Et, Pr, Bu, vinyl,
  etc.).

O
O
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B. Benzylic Chlorination

• Chlorination (and bromination) occurs by a
  radical mechanism.

64
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.

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Benzylic Halogenation

• benzylic bromination is highly regioselective.
• benzylic chlorination is less regioselective.

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C. Hydrogenolysis

• Hydrogenolysis
• Cleavage of a single bond by H2
• among ethers, benzylic ethers are unique in that
  they are cleaved under conditions of catalytic
  hydrogenation.

67
Benzyl Ethers

• Benzyl ethers are used 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.

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Benzene and and the Concept of Aromaticity
End of Chapter 21