September 15

September 15 - Conformat ional analysis of cycloalkanes:...

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Conformational analysis of cycloalkanes: Lecture #9, September 15, 2008 Small rings suffer from angle strain . Angle strain arises when bond angles deviate from aceepted values . Heats of combustion and ring strain, or strain energy (steric strain) : Strain energy is the difference between calculated values for heats of combustion and experimental values obtained from bomb calorimetry experiments. Heats of combustion data for alkanes (Table 2.3, page 83) Hexane: - 4163 kJ/mol Heptane: - 4817 kJ/mol Octane: - 5471 kJ/mol About 654 kJ/ mol for CH 2 ; accepted value is 653 kJ/mol of CH 2 Calculated Δ H comb ( n x 653 kJ/mol) Measured Δ H comb (kJ/mol) Δ H comb /CH 2 (kJ/mol) 1959 2612 3265 3918 4571 2091 2721 3291 3920 4599 697 681 658 653 657 Strain energy/ ring strain (Measured - Calculated) (kJ/mol) 132 109 26 2 28 potential energy (kJ/mol) 2 C 3 H 6 + 9 O 2 6 CO 2 + 6 H 2 O Measured Δ H comb 2091 1959 Strain energy Calculated Δ H comb We calculate the heat of combustion of cyclopropane based on a hypothetical strain-free system (three CH 2 groups).
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September 15 - Conformat ional analysis of cycloalkanes:...

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