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 reactions.
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 of benzene.
|Enter the Calculations dialog (Setup menu) and request calculation of equilibrium geometry using the Hartree-Fock 3-21G model. Click on Submit ("benzene hydrogenation"). When completed (a few minutes), bring up the spreadsheet (Display menu) and enter the energies. Click inside the header cell for a blank column in the spreadsheet, then click on Add at the bottom of the spreadsheet, select "E" and click on OK. Calculate hydrogenation energies for benzene and cyclohexadiene.|
The calculated difference in hydrogenation
energies is 163 kJ/mol.
Build thiophene, dihydrothiophene
and tetrahydrothiophene and put into a list with H2. Start
from cyclopentane (Rings menu) and to substitute sulfur for one
of the carbons by first selecting sp3 sulfur
from the model kit and then double clicking on one of the carbons
(not on the attached free valences). Setup for equilibrium geometry
calculations using the 3-21G(*) Hartree-Fock model
and submit ("thiophene hydrogenation"). When completed,
compute the hydrogenation energies for thiophene and dihydrothiophene.