Kekule structure

AN INTRODUCTION

1. Kekule Structure
•Submitted to
•Inam-ul-haq
•Submitted by
•Mehwish Farooq
• Roll no 3006
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30 January 2015

2. Contents
•Introduction
•History
•Summary
•Explanation
•Example
•Results
•Review
•References
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3. Introduction
Kekulé was the first to suggest a sensible structure for benzene. The carbons are
arranged in a hexagon, and he suggested alternating double and single bonds between
them. Each carbon atom has a hydrogen attached to it.
•Problems with the chemistry
•Problems with the shape
•Problems with the stability of benzene

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4. History
Benzene (C6H6) is the simplest aromatic hydrocarbon (or
arene).
Benzene has four degrees of unsaturation, making it a highly
unsaturated
hydrocarbon.
Whereas unsaturated hydrocarbons such as alkenes, alkynes
and
dienes readily undergo addition reactions, benzene does not.
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5. History
•Benzene does react with bromine, but only in the presence of FeBr3
(a Lewis acid), and the reaction is a substitution, not an addition.
•August Kekulé proposed that benzene was a rapidly equilibrating
mixture of two compounds, each containing a six-membered ring with
three alternating p bonds.

6. Lewis/Kekule Structures
•dots = electrons
•line = two shared electrons
•are all unshared electrons are
shown, and referred to as
nonbonding electrons. Each pair
of non-bonding electrons is also
known as a “lone pair”
H
H
H
ClC
H
H
H
ClC
1. Bonding (continued)
H
H
H
ClC
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Summary

7. 1. Bonding (continued):
H
H
CC
H
CC
H
H
H
H
H
CCH
CC
H
HH
H
H
CO
CO
H
H
Lewis
Kekule
ethylene acetylene formaldehyde
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8. Line-Angle Structural Formulas
•You must be able to use these by next
week
•Also known as a skeletal structure or a
stick figure
•Bonds are represented by lines
•Carbon atoms are assumed to be
present wherever two lines meet and at
the beginning or the end of a line.
1. Bonding (continued):
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9. The structure with three double bonds was proposed by Kekule as an attempt
to explain how a molecule whose molecular formula was C6H6 could be built
out of carbons which make four bonds. The ring and the three double bonds fit
the molecular formula, but the structure doesn't explain the chemical behavior
of benzene at all well. Each of the double bonds would be expected to show the
characteristic behavior of an alkene and undergo addition reactions, but this is
not how benzene reacts.
Explanation
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10. •In benzene, the actual bond length (1.39 Å) is
intermediate between the carbon—carbon single bond
(1.53 Å) and the carbon—carbon double bond (1.34 Å).
Explanation
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11. Examples
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12. Examples
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13. Results
•Kekulé was the first to suggest a sensible structure for benzene.
•Benzene does react with bromine, but only in the presence of FeBr3 (a Lewis
acid), and the reaction is a substitution, not an addition.
•Benzene is a planar molecule (all the atoms lie in one plane), and that would also be
true of the Kekulé structure. The problem is that C-C single and double bonds are
different lengths.
C-C 0.154 nm
C=C 0.134 nm
•If the ring had two double bonds in it initially (cyclohexa-1,3-diene), exactly twice as
many bonds would have to be broken and exactly twice as many made. In other words,
you would expect the enthalpy change of hydrogenation of cyclohexa-1,3-diene to be
exactly twice that of cyclohexene - that is, -240 kJ mol-1
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14. •The structure with three double bonds was proposed by Kekule as an attempt
to explain how a molecule whose molecular formula was C6H6 could be built
out of carbons which make four bonds.
•August Kekulé proposed that benzene was a rapidly equilibrating mixture of
two compounds, each containing a six-membered ring with three alternating p
bonds.
Review
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15. •http://chemistry2.csudh.edu/rpendarvis/BenzStr.html
•http://en.wikipedia.org/wiki/August_Kekul%C3%A9
•http://chemwiki.ucdavis.edu/Organic_Chemistry/
Fundamentals/Bonding_in_Organic_Compounds/
Bonding_in_Benzene%3A_the_Kekul
%C3%A9_Structure
•http://www.citycollegiate.com/benzene3.htm
•http://web.chemdoodle.com/kekules-dream
•https://www2.chemistry.msu.edu/faculty/
reusch/virttxtjml/Questions/General/honc
str1.htm
References
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