This document discusses the Kekulé structure of benzene proposed in 1865, highlighting its limitations such as inability to explain benzene's stability and its reaction patterns. It outlines the concept of resonance, explaining that benzene is better represented as a resonance hybrid of two contributing structures rather than a fixed structure with alternating single and double bonds. The resonance hybrid model accounts for benzene's uniform bond lengths and enhanced stability, clarifying why benzene resists typical reactions seen in unsaturated compounds.

1. PRESENTATION ON
OF
BACHELOR OF PHARMACY
PREPARED BY:
PALAK KHETAN
K.P.SINGH MEMORIAL INSTITUTE OF
PHARMACY
KEKULE’s STRUCTURE OF BENZENE AND IT’s
OBJECTIONS
RESONANCE IN BENZENE

2. KEKULE's
STRUCTURE OF
BENZENE
⮚In 1865, August Kekule suggested
that benzene consists of a cyclic
planar structure of six carbons
with alternate single and double
bond.
⮚Each carbon was attached to one
hydrogen.
⮚Benzene according to this
proposal was simple
1,3,5-cyclohexatriene.
Atoms are gambling like
snake(twisting) and twinning
But look at
One of the snake has seized its own tail

3. OBJECTION’s FOR KEKULE’S
STRUCTURE FOR BENZENE
If Kekule’s structure was correct
EX- Two isomers of dibromobenzene should be formed

4. BUT,
Only one ortho – dibromobenzene could be prepared.
⮚To overcome this objection , Kekule further suggested that
benzene was a mixture of two forms (I and II ) in rapid
equilibrium and the position of single and double bonds are not
fixed but they oscillate back and forth.
.

5. ⮚Kekule’s structure could not explain the stability of benzene
towards oxidising agents (all unsaturated compounds undergo
oxidation but benzene resists oxidation like all saturated
compounds).
⮚This structure could not explain the ease of benzene to
undergo substitution reactions rather then additon reactions
like other alkenes after having three double bonds ( being
unsaturated in nature benzene does not reacts with HBr or Br2
and does not decolourize bromine in CCl4).
⮚Kekule’s formula for benzene contains two kinds of bonds i.e.
single and double bonds but X- ray diffraction shows that
benzene is a regular hexagon with an angle of 120˚and all the
C–C bonds lengths are equal.
⮚So, Kekule’s structure failed to explain the UNUSAUL
STABILITY AND EQUALITY OF C–C BOND LENGTHS in
benzene.

6. RESONANCE IN
BENZENE
⮚ The phenomenon in which two or more structures can be
written for a substance which involves the identical position
of atoms is referred to as resonance.
⮚ The actual structure of the molecule is referred to as
resonance hybrid.
⮚ The alternative structures are known as resonating
structures .
⮚To represent the resonance a double headed arrow ( )will
be used.
⮚ In case of benzene, the Kekule’s structure I and II represent
the
resonating structures.

7. ⮚The structure A represent the resonance hybrid or a hybrid of
these two resonating structures.
To explain all the limitations of Kekule’s structure, it has been
proposed that benzene is a resonance hybrid of the Kekule’s
structure I and II.
⮚ These two structures are the canonical forms of benzene.
⮚ In actual, benzene is a resonance hybrid of these two structures
(A)
⮚The hybrid structure is represented by inserting a circle or a
dotted circle in the hexagon.
⮚ This circle represents the π – electrons which are delocalised
between the six carbon atoms of benzene ring.

8. ⮚It means that the any two adjacent carbons atoms of the
benzene molecule are neither joined by a pure single(–) nor by a
pure double(=) bond.
⮚ Spectroscopic measurements show that benzene is planar
and all the carbon – carbon (C—C) bonds are of equal length,
1.40Å.
⮚carbon – carbon single bond( C–C) length is 1.54Å.
⮚carbon –carbon double bond(C C) length is 1.34Å.

9. ⮚This value lies between the carbon–carbon single bond (C–C)
and carbon–carbon double bond (C=C) length.
⮚The resonance hybrid is more stable than any
of it’s contributing structures.
⮚This explains that why benzene is more stable
then either of it’s kekule’s structures.
⮚It also explains the unusual stability and equality
of C–C bond length in benzene.
⮚For benzene, the stability due to resonance is so great that π
– bonds of the molecule will normally resists breaking.
⮚THIS EXPLAINS LACK OF REACTIVITY OF
BENZENE TOWARDS ADDITION