Benzene and

1

• Benzene and
• and the Concept of
• Aromaticity

Chapter 21
2
Benzene - Kekulé

• 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 - 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 regular hexagon
• all C-C-C and H-C-C bond angles 120

4
Benzene - The Resonance 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

5
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

6
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

7
Benzene
8
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

9
Frost Circles

• 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

10
Frost Circles

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

11
Aromatic Hydrocarbons

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

12
Aromatic Hydrocarbons

• 18annulene is also aromatic

13
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

14
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

15
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

16
Cyclobutadiene

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

17
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

18
Cyclooctatetraene

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

19
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.

20
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

21
Furan

• 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

22
Other Heterocyclics
Furan
23
Other Heterocyclics
Pyrimidine
Purine
24
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

25
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

26
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

27
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.

28
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

29
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

30
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

31
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

H
H
H
H

H
H
H