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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.

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 on and then click on the appropriate pairs of free valences). Click on . 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 .

 
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.

To see to what extent thiophene is aromatic, calculate energies for the successive hydrogenation reactions.


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.

The calculated difference in hydrogenation energies is 77 kJ/mol.


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