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4
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- 1825 Michael Faraday isolates a new
hydrocarbon from illuminating gas.
- 1834 Eilhardt Mitscherlich isolates same
substance and determines its empirical formula to be CnHn. Compound comes to be called benzene.
- 1845 August W. von Hofmann isolates benzene from coal tar.
- 1866 August Kekulé proposes structure of
benzene.
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6
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- Kekulé proposed a cyclic structure for C6H6
with alternating single and double bonds.
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- Later, Kekulé revised his proposal by suggesting
a rapid equilibrium between two equivalent
structures.
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- However, this proposal suggested isomers of the
kind shown were possible.
Yet, none were ever
found.
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9
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- Structural studies of benzene do not support the
Kekulé formulation.
Instead of alternating single
and double bonds, all of the C—C bonds are the
same length.
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10
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11
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- 140 pm is the average between the C—C single bond distance and
the double bond distance in 1,3-butadiene.
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13
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- Instead of Kekulé's suggestion of a rapid
equilibrium between two structures:
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14
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- express the structure of benzene as a resonance
hybrid of the two Lewis structures. Electrons are
not localized in alternating single and double bonds,
but are delocalized over all six ring carbons.
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15
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- Circle-in-a-ring notation stands for resonance
description of benzene (hybrid of two Kekulé
structures)
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- benzene is the best and most familiar example
of a substance that possesses "special stability"
or "aromaticity"
- aromaticity is a level of stability that is substantially
greater for a molecule than would be expected on
the basis of any of the Lewis structures written for it
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17
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- heat of hydrogenation: compare
experimental
value with "expected" value for hypothetical
"cyclohexatriene"
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18
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19
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- "expected" heat of hydrogenation of benzene is 3 x heat of
hydrogenation of cyclohexene
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- observed heat of hydrogenation is 152 kJ/mol less than
"expected"
- benzene is 152 kJ/mol more stable than
expected
- 152 kJ/mol is the resonance energy of benzene
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- hydrogenation of 1,3-cyclohexadiene (2H2) gives off more
heat than hydrogenation of benzene (3H2)!
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- compared to localized 1,3,5-cyclohexatriene
- compared to 1,3,5-hexatriene
- exact value of resonance energy of benzene
depends on what it is compared to, but
regardless of model, benzene is more stable
than expected by a substantial amount
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25
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- Planar ring of 6 sp2 hybridized carbons
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26
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- Each carbon contributes a p orbital
- Six p orbitals overlap to give cyclic p system;
six p electrons delocalized throughout p system
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- High electron density above and below plane of ring
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