Question no. 6 The heat of hydrogenation (delta H degree) of cyclohexane to cycl

Question

Question no. 6 The heat of hydrogenation (delta H degree) of cyclohexane to cyclohexane is -28.4 kcal/mole. The corresponding value for benzene to cyclohexene is -49.8 kcal/mole. Using these values, calculate an empirical resonance energy for benzene. delta H degree for cyclooctene to cyclohexane is -23.3 kcal/mole, while that for 1, 3, 5, 7-cyclooctatetraene to cyclooctane is -100.9 kcal/mole. Calculate an empirical resonance energy for 1, 3, 5, 7-cyclooctatetraene. Theoretical heats of combustion can be calculated by adding up the contributions of the different types of bonds that are present in te molecule. Using the values below, calculate a heat of combustion for both benzene and 1, 3, 5, 7-cyclooctatetrene. The experimental heats of combustion for benzene and cyclooctatetrene are - 789.1 kcal/mole and - 1099 kcal/mole, respectively. Compare these values with those calculated above. Would you consider cyclooctatetrene to show aromatic behavior? Which of the following compounds are aromatic? (Consider possible resonance forms that fit the 4 pi + 2 rule.)

Explanation / Answer

6 a) the heat of hydrogenation of cyclohexene to cyclohexane = -28.4 Kcal / mole

so the heat of hydrogenation of cyclohextriene to cyclohexane = 3 X (-28.4) = -85.2

The heat of hydrogenation of benzene = -49.8 Kcal / mole

so the empirical resonance energy of benzene = 3 X Heat of hydrogenation of cyclohexene- heat of hydrogenation of benzene

Empirical resonance energy = -85.2 - (-49.8) = -35.4 Kcal / mole

b) Again the empirical resoance energy of given cyclooctatetraene = 4 X heat of hydrogenation of cyclooctene - heat of hydrogenation of given cyclooctatetraene

Empirical resonance energy = 4 X (-23.3) - (-100.9) = 7.7 Kcal / mole

So this is unstable due to its antiaromatic character

c) The combustion equation for benzene will be

C6H6 + 15/2 O2 ---> 6CO2 + 3H2O

Heat of comubstion = sum of bond energy of reactants- sum of bond energy of reactants

Heat of combustion = [ 6 X C-H bond energy + 3 C=C bond energy + 3X C-C bond energy + 15/2 O=O) - (12 X C=O bond energy + 6 X OH bond energy)

Heat of combustion = [ 6 X 54 + 3 X 117.4 + 3X 49.3 + 7.5x 119] -[ 12 X 178 + 6 X 110.8]

Heat of combustion =[1716] - [2800.8] = -1084.8 Kcal / mole

(ii) heat of combustion of cyclooctatetraene

C8H8 + 10O2 ---> 8CO2 + 4H2O

Heat of combustion = [ 8 X C-H bond energy + 4 C=C bond energy + 4X C-C bond energy + 10 O=O) - (16X C=O bond energy + 8 X OH bond energy)

Heat of combustion = [ 8 X 54 + 4 X 117.4 + 4X 49.3 + 10 x 119] -[ 16 X 178 + 8 X 110.8]

Heat of combustion =[2288.8] - [3734.4] = -1445.6 Kcal / mole

d) The values are different from the obtained values.

e) No cyclooctatetraene is not an aromatic compound. It is unstable as the compound is antiaromatic.

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