The Determination of the Resonance Gerald R. Stevenson
Energy of Benzene
University of Puerto Rico Rio Piedros, Puerto Rico 00931
A physical chemistry laboratory experiment
M o s t undergraduate physical chemistry laboratory curriculums include a bomb calorimetry experiment, which involves the measurement of the heat combustion of some simple hydrocarbon. I n our experiment we have used this technique to measure the resonancc energy of benzene, which is a useful way to relatc the concepts of ar~maticit~y and resonance energy to experimental thermodynamics. Our students, working in pairs, have obtaincd excellent results in t u ~ o3-hr laboratory pcriods using the procedure described below. Outline and Theory
Cyclooctatetraene (cot) is known to cxist in a tub conformation (shown below), and it lacks conjugation stabilization that planar aromatic molecules possess.'
Benzene on t h r othcr hand, is known to be planar and fully conjugated. Since cot consists simply of a benzene ring expanded by one ethylene group with the elimination of the aromaticity, the resonance encrgy of benzene can be estimated by comparing the heat of combustion of cot minus one ethylene moiety with that of benzene.
--
+ 15/202 + 1001
CBHs
CsHs
+ 3Hs0 +4 8 0
6C02
8C02
AE,
(1)
AE2
(2)
A& and AEz represent the energies of combustion. Since cot is effectively unconjugated and contains four methylene groups, the energy of combustion of three of these ethylene functions would he AEa = (3/dAE2. CsHs (uncanjugated)
+ 15/20a
+ 6C0s
+ 3Hp0
AE,
(3)
The difference between AE1 and AE3 represents the resonance energy of the. benzene ring assuming zero ' K a ~ z ,T. J. AND STRAUSS, H. L., J. Chem. Phys., 32, 1873 (1960). 2 PAULING, LINUS,"The Nature of the Chemicd Bond'' (3rd ed.) Cornell University Press, Ithaca, 1960, p. 196. R o n s n ~ s ,J. D., "Notes on Moleculnr Orbital Calculations," W. A. Benjamin, Inc., New York, 1962, p. 48. *DANNIELS,F., WILLIAMS,J. W., BENDER,P., ~ R E R T Y , R. A,, AND COBNWELL, C. D. "Experimental Physical Chemistry," (6th ed.) McGraw-Hill Book Co., New York, 1962, pp. 15-21.
resonance energy in the cot ring. Subtracting eqn. (3) from eqn. (1)we obtain cqn. (4).
The value obtained for the resonance energy of benzene (AIL) using this technique is about 24 kcal/mole. Actually, heats of hydrogruation slio~l-that thc cot ring contains 5 kcal/mole of resonance ~ n e r g yso , ~our valnr of 24 kcal/mole is too low by 3 / 4 X 5 or about 4 kcal/ mole. This leads to a predicted value for the rrsonancc energy of benzene from this experiment of 2s kcal/molr. Estimates of the "true" resonant? cnrrgy of t h r benzene ring vary from 10 to 70 k c a l / m ~ l e . ~ The energies of reaction are converted directly to enthalpies of reaction by applying the folloaing expression. AH = AE + RT(An) (5) where An is the number of moles of products in the gas phase minus that for the reactants. Since An for reaction (4) is zero, the energy and rnthalpy of rraction are identical. Howcvrr, to obtain the enthalpies of rombustion of cot and bcnzrnr rqn. (5) must be used. The experimental enthalpies can be corrected for trmprratures diffrrcnt from 29S°C using thc Iiirchoff equation in the standard manner. Experimental
Basically the experiment is carried out as any bomb calorimetry heat of combustion expcriment4 except that modifications must be made for the handling of liquid samples. Since cot and benzene are relatively volatile liquids, they must be weighed and handled in cithcr gclatin capsules or in a Pam volatile samplr holder. If the gelatin capsules are used a correction must be made for the heat of combustion of the gelatin. The heat of combustion of a single capule can be determined by firing an empty capsule. The fusc wire is wrapped around the capsule, and the capsulc will ignite when the fusc r ~ i r cis heated. The gelatin capsules scal 1x11 enough so that there is no evaporation of the cot or bcnzrnr from them. This also allows the bomb to bc purged with oxygen bcfore firing. This climinates error caused by the formation of nitric acid from the nitrogen in the bomb. Excellent combustions are obtained when thc bomb is Volume 49, Number 1 1, November 1972
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to 30 atm with oxygen. Between 0.3 and 0.7 g can be efficiently fired in these capsules. Comments
I n this experiment the heats of combustion for cot and benzene are found to be 1089 and 789 kcal/mole, respectively. The resonance energy of benzene comes
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from the difference between two large numbers t o give a relatively small number, and care must be taken to avoid introducing errors in the measured heats of cornbustion. If the student is careful, he can measure the heats of combustion well within the accuracy needed. Heat of combustion measured by bomb calorimetry is one of the most exact of physical measurements.
