Briefly explain how the experiment about the comparison of the heats of hydrogenation between cyclohexane, cyclohexene, 1,3-cyclohexadiene and benzene is able to indicate the unusual stability of benzene? (Hint: please refer to Slide 9 in "Benzene" chapter of the lecture notes.)
Briefly explain how the experiment about the comparison of the heats of hydrogenation between cyclohexane, cyclohexene, 1,3-cyclohexadiene and benzene is able to indicate the unusual stability of benzene? (Hint: please refer to Slide 9 in "Benzene" chapter of the lecture notes.)
Chemistry
10th Edition
ISBN:9781305957404
Author:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste
Publisher:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste
Chapter1: Chemical Foundations
Section: Chapter Questions
Problem 1RQ: Define and explain the differences between the following terms. a. law and theory b. theory and...
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Transcribed Image Text:2. Briefly explain how the experiment about the comparison of the heats of
hydrogenation between cyclohexane, cyclohexene, 1,3-cyclohexadiene and benzene
is able to indicate the unusual stability of benzene? (Hint: please refer to Slide 9 in
"Benzene" chapter of the lecture notes.)
Heats of Hydrogenation as Indicators
of Benzene's Unusual Stability
Benzene's Unusual Structure
General Chemistry 2
General Chemistry 2
• The addition of H, to C=C normally gives off about 118 klimol - 3
double bonds would give off 354kJimol
• Two conjugated double bonds in cyclohexadiene add 2 H2 to
give off 230 kJ/mol
• Benzene has 3 unsaturation sites but gives off only 206 kJimol on
reacting with 3 H, molecules
Therefore it has about 150 kJ more "stability" than an isolated set of
three double bonds
Based on resonance theory, benzene's actual structure is a hybrid of
2 equivalent resonance forms
All its C-C bonds are the same length: 139 pm - between single
(154 pm) and double (134 pm) bonds
Electron density in all six C-C bonds is identical
Structure is planar, hexagonal
C-C-C bond angles 120
Each C is sp hybridized and has a p orbital perpendicular to the
plane of the six-membered ring
1.5 bonds on average
Cyhe

Transcribed Image Text:The addition of H, to C=C normally gives off about 118 kJ/mol – 3
double bonds would give off 354kJ/mol
• Two conjugated double bonds in cyclohexadiene add 2 H2 to
give off 230 kJ/mol
Benzene has 3 unsaturation sites but gives off only 206 kJ/mol on
reacting with 3 H, molecules
Therefore it has about 150 kJ more "stability" than an isolated set of
three double bonds
Benzene
150 kJ/mol
(difference)
1,3-Cyclohexadiene
-356 kJ/mol
(expected)
Cyclohexene
-230 kJ/mol
-206 kJ/mol
(actual)
-118 kJ/mol
Cyclohexane
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