A Direct Calorimetric Demonstration of Resonance Energy in the Benzene Nucleus This calorimetric experiment is intended as a f i n t contact in chemical education with the aromaticity concept. The need for an experiment for this purpose was brought to our attention by Professor F. Bickelhaupt, Department of Organic Chemistry of the Free University. The doubly unsaturated compound 3,5-cyclohexadiene-l,2-dicarhoxylic acid, available from B.A.S.F., Organic Intermediates Catalogue 1971, item number 157, dissolved in dilute aqueous alkali solution rearranges upon addition of a Pd catalyst a t 65' C completely to phthalic acid and a mono-unsaturated acid, which is either 1-cyclohexene-1.2-dicarboxylicacid or 2-cycloheaene-1,2-dicarboxylicacid. The rearrangement takes twenty minutes. The reaction is (R stands for two COO--groups)
The choice of the reactant was induced by a study by Linstead et al.,' which authors, however, postulated a different course of the reaction. The heat developed per mole of phthalic acid is equal to the resonance energy of the phenyl nucleus (minus a small amount because of loss of the 1.3-diene resonance in the reactant). There is a side reaction. Part of the phthalic acid decarhoxylates to benzoic acid, but in the alkaline ( p H = 8.5) solution the C02 is converted to HC03-and the thermal effect of the deearboxylation with HCOB- (aq) as a final product is negligibly sma1l.z In a typical run, the readion mixture in a 100 ml Dewar vessel consisted of 75 ml of water, 12.5 g of the acid, 8 g of KOH, small amounts of K&Os added successively for buffering until p H = 8.5 and 0.6 g of charcoal-10% Pd as a catalyst. If the reaction is conducted in a Dewar vessel equipped with a thermometer having a scale division to a tenth of a degree C the temperature rise, 5-10' C, can be measured accurately. The enthalpy change per male of reaetant, Ah, can be calculated from Ah = -(A&-c,/C) [(8, - en)/(%, - €).)I, where Afl, is the temperature rise due to the reaction, corrected for heat exchange with the surroundings, ep is the specific heat capacity of the solution, 3.85 J g-' 'C-I (0.92 cal g-' Y-'1, C is the number of moles of reactant per mass unit of solution (the mass itself is not required), 8, and 80are the temperatures of the hot solution just before and just after, respectively, it is poured into the Dewar vessel of temperature 8. (ambient). No additional calibration is required. The resonance energy, corrected far the 1,3-dime interaction3 (+2 x 7.5 kJ mole-I) was found (-153 7) kJ m o W L ((-36.8 1.7) kcal mole-'). The uncertainty given is five times the standard deviation of a series of three measurements. A literature value3 for benzene is (-149 i 2) kJ molecL((-35.6 i 0.5) kcal mole-'). A detailed information sheet "Resonance Experiment" can be ohtained from author C. M. by private correspondence. The authors wish to thank Dr. 0. S. Akkerman and Mr. E.A.I.M. Evers, Department of Organic Chemistry of the Free University, for making and interpreting the glc and mass spectrometric analyses, and Mr. H. Dekker of the Department of Physical Chemistry for useful advice.
*
'Linstead, R. P., Braude, E. A., Mitchell, P. W. D., Wooldridge, K. R. H., and Jackman, L. M.. Nature, 169. 100 (19621. . . 'Vugt, W. H. van, and Mosselman, C., unpublished results. Wheland, G. W., "Resonance in Organic Chemistry," 2nd ed., John Wiley & Sons Inc., New York, 1961, p. 80. ~~
Laboratory of Physical Chemistry Free University d e Lairessestrsat 174, Amsterdam. T h e Netherlands
746 / Journal of ChemicalEducation
W. H. van Vugt C. Mosselman
