Pyrrole, furan, and thiophene are five-membered-ring heterocycles. Each has three
pairs of delocalized π electrons: Two of the pairs are shown as π bonds, and one pair
is shown as a lone pair on the heteroatom. Pyrrole, furan, and thiophene are aromatic because they are cyclic and planar,
every carbon in the ring has 3 sp2 orbitals and a p orbital perpendicular to the ring plane and parallel to one another, and the π cloud contains three pairs of π
electrons filling three bonding π MO in accordance with Huckel's rule of aromaticity.
We knew that the more stable and more nearly equivalent the resonance
contributors, the greater is the resonance energy. The resonance energies of
pyrrole, furan, and thiophene are not as great as the resonance energies of benzene which has equivalent resonance contributors. Thiophene, with the least electronegative heteroatom, has the greatest
resonance energy of these five-membered heterocycles; and furan, with the most electronegative
heteroatom, has the smallest resonance energy.
Because these heterocycles are aromatic, they can undergo electrophilic substitution reaction. However, you should distinguish between these two concepts: aromaticity and reactivity to electrophilic reagents.Pyrrole, furan, and thiophene are all more reactive than benzene toward electrophilic
substitution because they are better able to stabilize the positive charge on the
carbocation intermediate, since the lone pair on the hetereoatom can donate electrons
into the ring by resonance.
Furan is not as reactive as pyrrole in electrophilic aromatic substitution reactions.
The oxygen of furan is more electronegative than the nitrogen of pyrrole, so the oxygen
is not as effective as nitrogen in stabilizing the carbocation. Thiophene is less
reactive than furan toward electrophilic substitution because sulfur’s electrons are in
a 3p orbital, which overlaps less effectively than the 2p orbital of nitrogen or oxygen
with the 2p orbital of carbon (because large atomic radius of sulfur results in large distance between overlapping orbitals although energy level of sulfur's 3p orbital is as comparably high as that of carbon's 2p orbitals and this leads to great resonance energy of thiophene) . The electrostatic potential maps illustrate the different
electron densities of the three rings.(see the attachment below)
What I mentioned above is based on out-of-date theories of molecular structure. We could also explain the problem in the view point of modern MO theory.
I used Hyperchem software to compute and display HOMOs of these three heterocycles by semi-empirical AM1 method. You should realize that the HOMO of thiophene differs from that of pyrrole and furan. It consists of two separate partitions and you can see that the bigger one has form of 3p orbital of Sulfur. It mean that this 3p orbital overlaps ineffectively with π electron system of carbon atoms and therefore donates electrons less effectively, too. This results in less reactivity to electrophile reagents of thiophene than furan and pyrrole.
I also used Hyperchem to display electrostatic potential maps of these three rings. Do you agree with me that pyrrole is the most reactive because of greatest π electron density in the ring system?Furan is less reactive because of the presence of the electron withdrawing oxygen heteroatom. And thiophene with sulfur's lone pair electrons partially localized in 3p orbital is also illustrated.
Do you find it all somewhat interesting?
tính thơm và khả năng PƯ của dị vòng
bluemonster
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