energy of a compound is a
measure of the extra stability of the conjugated system compared to the
corresponding number of isolated double bonds. This can be
from experimental measurements.
|The diagram shows
the experimental heats of hydrogenation, ΔHh,
for three molecules, benzene, 1,3-cyclohexadiene and cyclohexene. These
are related in that under appropriate conditions that can all be
to the same product, cyclohexane.
The ΔHh for "cyclohexatriene", a hypothetical molecule in which the double bonds are assumed to be isolated from each other, is calculated to be 3 times the value for cyclohexene. This value reflects the energy we could expect to be released from 3 isolated C=C.
By comparing this value with the experimental value for benzene, we can conclude that benzene is 152 kJ or 36 kcal / mol more stable than the hypothetical system. This is the resonance energy for benzene.
What is the resonance energy of 1,3-cyclohexadiene ? ANSWER
|In principle, resonance energies can be calculated for any π systems. The following table contains data on a selection of systems, and some comments about them in relation to benzene or about their aromaticity.|
|© Dr. Ian Hunt, Department of Chemistry, University of Calgary|