Is Thiophene Aromatic?
We teach students that benzene is more
stable than expected . . . that it is "aromatic". The aromaticity
of benzene can be quantified by comparing energies for successive hydrogenation
Addition of H2 to
benzene "trades" an H-H bond and a C-C p bond for two C-H bonds,
but destroys the aromaticity, whereas H2 addition to either
cyclohexadiene or cyclohexene "trades" the same bonds but does
not result in any loss of aromaticity. Therefore, the difference in the
heats of hydrogenation (134-142 kJ/mol) is a measure of the aromaticity
First, see if Hartree-Fock calculations are able to properly account for
the difference in hydrogenation energies of benzene and cyclohexadiene
. . . and hence for the aromaticity of benzene. You will need to perform
calculations on these two molecules as well as on cyclohexene and on H2.
First build benzene. Click on .
Select Benzene from the Rings menu, click anywhere
on screen and click on .
Next, select New Molecule from the File menu and build cyclohexadiene.
Start from cyclohexane (Rings menu) and make two double bonds (click
and then click on the appropriate pairs of free valences). Click
Select New Molecule and build and minimize cyclohexene again starting
from cyclohexane and making a double bond. Finally, select New Molecule
one last time and build H2. Click on
and then on .