Structure and Resonance Energy of Benzene A First Look at Aromaticity

STRUCTURE AND RESONANCE ENERGY OF BENZENE A FIRST LOOK AT AROMATICITY 

The benzene ring contains six equally overlapping p orbitals 

A way to establish the relative stability of a series of alkenes is to measure their heats of hydrogenation (Sections 11-5 and 14-5). We may carry out a similar experiment with benzene, relating its heat of hydrogenation to those of 1,3-cyclohexadiene and cyclohexene. These molecules are conveniently compared because hydrogenation changes all three into 

The hydrogenation of cyclohexene is exothermic by -28.6 kcal mol , a valne expected for the hydrogenation of a cis double bond (Section 11-5). The heat of hydrogenation of 1,3-cyclohexadiene (A//° = -54.9 kcal mol ) is slightly less than double that of cyclohexene because of the resonance stabilization in a conjugated diene (Section 14-5) the energy of that stabilization is (2 X 28.6) - 54.9 = 2.3 kcal mol (9.6 kJ mol ). 

Armed with these numbers, we can calculate the expected heat of hydrogenation of benzene, as though it were simply composed of three double bonds like that of cyclohexene, but with the extra resonance stabilization of conjugation as in 1,3-cyclohexadiene. 

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