The document discusses the structure of benzene. It describes Kekulé's proposal in 1865 that benzene has a ring structure with alternating single and double bonds between the six carbon atoms. This resonance structure accounts for benzene's stability and its tendency to undergo substitution rather than addition reactions. The orbital model of benzene involves sp2 hybridization of the carbon atoms and delocalization of the pi electrons above and below the plane of the ring, contributing to its additional stability over cyclohexatriene.

1. Structure of benzene

2. Syllabus Unit I
• Analytical, synthetic and other evidences in the
derivation of structure of benzene, Orbital picture,
resonance in benzene, aromatic characters, Huckel’s
rule
• Reactions of benzene - nitration, sulphonation,
halogenation-reactivity, Friedel crafts alkylation-
reactivity, limitations, Friedel crafts acylation.
• Substituents, effect of substituents on reactivity and
orientation of monosubstituted benzene compounds
towards electrophilic substitution reaction.

3. Benzene
Organic compound
• Aliphatic compound [ Fatty ]
• Aromatic compound [ Fragrant ]

4. Aliphatic compound
• It is open chain compound and those cyclic
compound that resemble open chain compound.
• Aromatic are benzene and the compound that
resemble benzene in chemical behaviour.
• Benzene molecule is a ring: A ring of special kind.
• Aliphatic undergo additional, free radical
substitution
• Aromatic hydrocarbon undergoes substitution
reaction.

5. KEKKULE’S STRUCTURE

6. • Benzene has molecular formula C6H6 from its
elemental composition and molecular weight.
• Benzene was known to contain six carbon
atom and six hydrogen atom.
• The question was how they [ the atoms ] are
arranged.

8. • Other structures are, of course, consistent
with the formula C6H6 : for example,
• II- V. Of all these, KekuU's structure was
accepted as the most nearly satisfactory;

9. Kekule’s dream 1890
"I was sitting writing at my textbook, but the work did not
progress; my thoughts were elsewhere. I turned my chair to
the fire, and dozed. Again the atoms were gamboling before
my eyes. This time the smaller groups kept modestly in the
background. My mental eye, rendered more acute by
repeated visions of this kind, could now distinguish larger
structures of manifold conformations; long rows, sometimes
more closely fitted together; all twisting and turning in snake-
like motion. But look ! What was that ? One of the snakes had
seized hold of its own tail, and the form whirled mockingly
before my eyes. As if by a flash of lightning I woke ; . . . I spent
the rest of the night working out the consequences of the
hypothesis. Let us learn to dream, gentlemen, and then
perhaps we shall learn the truth.“ August Kekule, 1865.

10. In 1858 August Kekkule proposed that
carbon atoms can join to one another
to form chains.

11. Honey comb

12. In 1865, he offered an answer to the question of
benzene: these carbon chains can. sometimes
be closed, to form rings.

16. All structures consistent with formula C6H6
Benzene yield only mono substituted product.
This fact places a severe limitation on the
structure of benzene: each hydrogen must be
exactly equivalent to every other hydrogen,
since the replacement of any one of them yields
the same product.

17. • Structure V must now be rejected, since it
would yield two isomeric mono bromo
derivatives, the 1-bromo and the 2-bromo
compounds; all hydrogens are not equivalent
in V.
• Similar reasoning shows us that II and III are
likewise unsatisfactory.
• Structure I and IV, among others, are still
possible as benzene structure.

18. Benzene yields three isomeric di
substitution products, This fact further
limits our choice of a structure; for
example, IV must now be rejected.

19. Closure examination of structure I shows
that two 1,2 dibomo isomers VI, VII differ
in positions of bromine relative to the
double bond should be possible
VI VII

20. But Kekkule visualized the benzene
molecule as a dynamic thing.
He visualized the structure as
Thus two 1,2 dibromo benzene VI and VII would be in rapid
equilibrium and hence could not be separated

21. • Later when the idea of tautomerism defined.
It was assumed that kekkule’s alternation
essentially amounted to tautomerism.
• Kekkule had intuitively anticipated by present
concept of delocalised electrons and drew two
pictures.
• Rightly or wrongly the term kekkule structure
has come to mean a molecule with alternating
single or double bond.

22. Stability of benzene
• The most striking evidence of this stability is
found in the chemical reactions of benzene.
• Benzene undergoes substitution rather than
addition
• Cyclohexadiene and cyclohexene, readily
undergoes readily the addition reactions
characteristic of the alkene structure.
• Benzene reacts either not at all or very slowly.

23. Benzene readily undergoes a new set
of reactions involving substitution

24. It would appear that benzene resists addition, in
which the benzene ring system would be
destroyed, whereas it readily undergoes
substitution, in which the ring system is
preserved.

25. Stability of the benzene ring. Heats of
hydrogenation and combustion
• Heats of hydrogenation and combustion of
benzene are lower than expected.
• Heat of hydrogenation is the quantity of heat
evolved when one mole of unsaturated
compound is hydrogenated.
• The value for each double bond is about 28-30
K cal

26. • Heat of hydrogenation of cyclohexene= 28.6
Kcal
• Heat of hydrogenation of cyclohexadiene ie
twice = 55.4 Kcal
• Expect benzene is cyclohexatriene it should
have 85.8 Kcal
• Actually the value for benzene [ 49.8 Kcal] is
36KCal less than expected

28. C-C bond length in Benzene
• All carbon-carbon bond in benzene are equal and
are intermediate in length between single and
double bond.
• C=C in a wide variety of compounds are found to
be about 1.34 A long
• C-C single bonds in which the nuclei are held
together by only one pair of electrons, are
considerably longer.
• Ethane 1.53 A propylene 1.50 A 1,3 butadiene
1.48 A

29. • If benzene actually possessed three single and
three double bond as in kekule structure.
• We expect 3 short bond 1.34A and 3 long
bond 1.48A
• X ray diffraction studies show that 6 C-C bond
in benzene are equal and have a length of
1.39A.
• Thus intermediate between single and double
bond.

30. Resonance Structure of Benzene

31. • The Kekule structures I and II meet the conditions
for resonance: structures that differ only in the
arrangement of electrons.
• Benzene is hybrid of I and II
• Six bond lengths are identical because the six
bonds are identical.
• When it is realised that all C-C bonds in benzene
are equivalent there is no longer any difficulty in
accounting for the number of isomeric di
substitution product.

32. The 36 kcal of energy that
Benzene does not contain compared with
cyclohexatriene is resonance energy.
It is the 36 kcal of resonance energy that is
responsible for the new set of properties
we call aromatic properties.

33. • Addition reactions convert an alkene into a
more stable saturated compound.
• Hydrogenation of cyclohexene, for example, is
accompanied by the evolution of 28.6 kcal;
the product lies 28.6 kcal lower than the
reactants on the energy scale.

34. • But addition would convert benzene into a
less stable product by destroying the
resonance-stabilized benzene ring system.
• for example
• The first stage of hydrogenation of benzene
requires 5.6 kcal to convert benzene into the
less stable cyclohexadiene.

35. ORBITAL PICTURE OF BENZENE

36. • A more detailed picture of the benzene
molecule is obtained from a consideration of
the bond orbitals in this molecule.
• Carbon is bonded to three other atoms, it uses
sp2 orbitals as in ethylene.
• These lie in the same plane, that of the
carbon nucleus, and are directed toward the
corners of an equilateral triangle.

37. The six carbons and six hydrogens
of benzene to permit maximum
overlap of these orbitals,

39. • Benzene is a flat molecule, with every carbon
and every hydrogen lying in the same plane.
• It is a very symmetrical molecule, too, with
each carbon atom lying at the angle of a
regular hexagon; every bond angle is 120.
• Each bond orbital is cylindrically symmetrical
about the line joining the atomic nuclei and
hence, as before, these bonds are designated
as a bonds.

40. • The molecule is not yet complete, however. There
are still six electrons to be accounted for.
• In addition to the three orbitals already used,
each carbon atom has a fourth orbital, a p orbital.
• P orbital consists of two equal lobes, one lying
above and the other lying below the plane of the
other three orbitals, that is, above and below the
plane of the ring; it is occupied by a single
electron.

41. • In the case of ethylene, the P orbital of one carbon can
overlap the p orbital of an adjacent carbon atom,
permitting the electrons to pair and an additional bond
to be formed .
• But the overlap here is not limited to a pair of p
orbitals as it was in ethylene; the P orbital of any one
carbon atom overlaps equally well the p orbitals of
both carbon atoms to which it is bonded.
• The result is two continuous doughnut-shaped
electron clouds, one lying above and the other below
the plane of the atoms.

42. • As with the allyl radical, it is the overlap of the
p orbitals in both directions, and the resulting
participation of each electron in several bonds
that corresponds to our description of the
molecule as a resonance hybrid of two
structures.
• Again it is the delocalization of the pie
electrons their participation in several bonds
that makes the molecule more stable.

43. • To accommodate six pie electrons, there must be
three orbitals.
• The orbital approach reveals the importance of
the planarity of the benzene ring.
• The ring is flat because the trigonal (sp2) bond
angles of carbon just fit the 120 angles of a
regular hexagon; it is this flatness that permits
the overlap of the p orbitals in both directions,
with the resulting delocalization and stabilization.

44. • The facts are consistent with the orbital
picture of the benzene molecule.
• X-ray and electron diffraction show benzene
to be a completely flat, symmetrical molecule
with all carbon-carbon bonds equal, and all
bond angles 120A
• Despite delocalization, the pie electrons are
nevertheless more loosely held than the a
sigma electrons.

45. • The pie electrons are thus particularly
available to a reagent that is seeking
electrons: the typical reactions of the benzene
ring are those in which it serves as a source of
electrons for electrophiiic (acidic)reagents.
• Because of the resonance stabilization of the
benzene ring, these reactions lead to
substitution, in which the aromatic character
of the benzene ring is preserved.

46. Representation of the benzene ring
For convenience we shall represent the benzene
ring by a regular hexagon containing a circle (I);
it is understood that a hydrogen atom is
attached to each angle of the hexagon unless
another atom or group is indicated.

47. • The straight lines stand for the sigma bonds
joining carbon atoms. The circle stands for the
cloud of six delocalized pie electrons.
• I is a particularly useful representation of the
benzene ring, since it emphasizes the equivalence
of the various carbon-carbon bonds. The
presence of the circle distinguishes the benzene
ring from the cyclohexane ring, which is often
represented today by a plain hexagon.

48. Aromatic character. The Huckel 4n + 2
rule
• We have defined aromatic compounds as those
that resemble benzene.
• But just which properties of benzene must a
compound possess before we speak of it as being
aromatic?
• Besides the compounds that contain benzene
rings, there are many other substances that are
called aromatic; yet some of these superficially
bear little resemblance to benzene.
• What properties do all aromatic compounds have
in common?

49. • From the experimental standpoint, aromatic
compounds are compounds whose molecular
formulas would lead us to expect a high degree of
unsaturation, and yet which are resistant to the
addition reactions generally characteristic of
unsaturated compounds.
• Instead of addition reactions, we often find that
these aromatic compounds undergo electrophilic
substitution reactions like those of benzene.

50. • Along with this resistance toward addition and
presumably the cause of it we find evidence of
unusual stability: low heats of hydrogenation and
low heats of combustion.
• Aromatic compounds are cyclic generally
containing five-, six-, or seven-membered rings
and when examined by physical methods, they
are found to have flat (or nearly fiat) molecules.
• Their protons show the same sort of chemical
shift in nmr spectra as the protons of benzene
and its derivatives.

51. • From a theoretical standpoint, to be aromatic
a compound must have a molecule that
contains cyclic clouds of delocalized pie
electrons above and below the plane of the
molecule.
• The pie cloud must contain a total of [4n + 2 ]
pie electron.

52. Erich Huckel

53. • An aromatic compound, delocalization alone is
not enough. There must be a particular number
of pie electrons: 2, or 6, or 10, etc. This
requirement, called the 4n + 2 rule or HUckel rule
(after Erich Hiickel, of the Institut fiir theoretische
Physik, Stuttgart), is based on quantum
mechanics, and has to do with the filling up of
the various orbitals that make up the pie cloud.
• The HUckel rule is strongly supported by the
facts.

54. The evidence supporting the Huckel
rule
• Benzene has six pie electrons, the aromatic
sextet six is, of course, a Huckel number,
corresponding to n = 1.
• Besides benzene and its relatives
(naphthalene, anthracene, phenanthrene), we
shall encounter a number of heterocyclic
compounds that are clearly aromatic; these
aromatic heterocycles, we shall see, are just
the ones that can provide an aromatic sextet.

55. • Each molecule is a hybrid of either five or
seven equivalent structures, with the charge
or odd electron on each carbon.
• Among the six compounds, only two give
evidence of unusually high stability: the
cyclopentadienyl anion and the
cycloheptatrienyl cation (tropylium ion).

56. For a hydrocarbon, cyclopentadiene is an
unusually strong acid indicating that loss of a
hydrogen ion gives a particularly stable anion. (It
is,for example, a much stronger acid than
cycloheptatriene, despite the fact that the latter
gives an anion that is stabilized by seven
contributing structures.)

57. • Dicyclopentadienyliron (ferrocene), [(C5H5)-]
2Fe + + , is a stable molecule that has been
shown to be a "sandwich" of an iron atom
between two flat five-membered rings.
• All carbon-carbon bonds are 1.4 A long. The
rings of ferrocene undergo two typically
aromatic substitution reactions: sulfonation
and the Friedel-Crafts reaction.

58. Ferrocene Structure

59. Of the cycloheptatrienyl derivatives, on the other
hand, it is the cation that is unusual.
Tropylium bromide, C7H7 Br, melts above 200, is
soluble in water but insoluble in non-polar solvents,
and gives an immediate precipitate of AgBr.
When treated with- silver nitrate. This is strange
behavior for an organic bromide, and strongly
suggests that, even in the solid, we are dealing with
an ionic compound, R+Br-, the cation of which is
actually a stable carbonium ion.

61. • Six is the Huckel number most often
encountered, and for good reason. To provide
p orbitals, the atoms of the aromatic ring must
be trigonally (sp2) hybridized, which means,
ideally, bond angles of 120.
• To permit the overlap of the p orbitals that
gives rise to the pie cloud, the aromatic
compound must be flat, or nearly so.

62. • The number of trigonally hybridized atoms that
will fit a flat ring without undue angle strain (i.e.,
with reasonably good overlap for n bond
formation) is five, six, or seven.
• Six is the Huckel number of pie electrons that can
be provided.
• Benzene, our model for aromatic character, is the
"perfect“ specimen: six carbons to provide six pie
electrons and to make a hexagon whose angles
exactly match the trigonal angle.)

63. • Huckel numbers 2, 10, 14, etc.
• The rings will be too small or too large to
accommodate trigonally hybridized atoms
very well, so that any stabilization due to
aromaticity may be largely offset by angle
strain or poor overlap of p orbitals, or both.

64. • Thank you