Organic Chemistry

1
Chapter 21, Benzene and and the Concept
of Aromaticity
2
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.

3
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.

4
Benzene - Resonance Model

• 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 planar regular hexagon.
• All C-C-C and H-C-C bond angles 120.

5
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.

6
Benzene-Molecular Orbital Model

• The molecular orbital representation of the pi
  bonding in benzene.

7
Orbitals of the pi System of Benzene
Number of nodal surfaces
3
2
1
0
8
Benzene - Resonance

• 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.

9
Benzene- Resonance Energy
Model
Experimental data
10
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
• 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.

11
Frost Circles

• 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.

12
Frost Circles

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

13
Relationship of hexa-1,3,5-triene to benzene
How does the linear triene relate to benzene?
14
Relationship of hexa-1,3,5-triene to benzene
?
15
Relationship of hexa-1,3,5-triene to benzene
Look at orbitals 2 and 3.
p3
?
p2
Bonding, stabilizing
Curve around
Antibonding, destabilizing
16
Aromatic Hydrocarbons

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

n 3
17
Aromatic Hydrocarbons

• 18Annulene is also aromatic.

n 4
18
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.

19
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.

20
Antiaromatic Hydrocarbons

• 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.

21
Cyclobutadiene

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

22
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.
• 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.

23
Cyclooctatetraene

• MO energy diagram for a planar conformation of
  cyclooctatetraene.

24
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 that differ from carbon.
• Pyridine and pyrimidine are heterocyclic analogs
  of benzene each is aromatic.

25
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 (the aromatic
  sextet).
• Resonance energy of pyridine is134 kJ (32
  kcal)/mol.

26
Furan 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.

27
Other Heterocyclics
28
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 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.

32
Cyclopentadienyl Anion, C5H5-

• 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.

n 1
33
Cyclopentadienyl Anion, C5H5-

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

34
Cyclopentadienyl Anion, C5H5-

• 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
Cycloheptatrienyl Cation, C7H7

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

36
Nomenclature

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

37
Nomenclature

• Benzyl and phenyl groups

38
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.

39
Disubstituted Benzenes

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

40
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.

41
Phenols

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

42
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.

43
Acidity of Phenols

• Phenols are significantly more acidic than
  alcohols.

44
Acidity of Phenols

• Separation of water-insoluble phenols from
  water-insoluble alcohols.

45
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.

46
Phenol Subsitituents (Inductive Effect)

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

47
Phenol Subsitituents(Resonance, Inductiion)

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

48
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.

49
Acidity of Phenols

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

50
Acidity of Phenols

• Most phenols do not react with weak bases such as
  NaHCO3 they do not dissolve in aqueous NaHCO3.
• Carbonic acid is a stronger acid than phenol.
  Therefore, the position of this equilibrium lies
  far to the left.

51
Synthesis 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.

52
Synthesis Alkyl-Aryl Ethers
53
Synthesis Kolbe Carboxylation

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

54
Mechanism Kolbe Carboxylation

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

Go back to aromatic structure
55
Synthesis Quinones

• Because of the presence of the electron-donating
  -OH group, phenols are susceptible to oxidation
  by a variety of strong oxidizing agents.

56
Quinones
57
Quinones

• Readily reduced to hydroquinones.

58
Coenzyme Q

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

59
Vitamin K

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

60
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.

61
Benzylic Oxidation

• If there is more than one alkyl group on the
  benzene ring, each is oxidized to a -COOH group.

62
Benzylic Chlorination

• Chlorination and bromination occur by a radical
  chain mechanism.

63
Mechanism 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.

64
Benzylic Halogenation

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

65
Hydrogenolysis

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

66
Synthesis, Protecting Group Benzyl 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.