Aromaticity

AROMATICITY

A special type of orbital interaction occurs when a conjugated π system is in a ring. The π system of benzene is a classic example of this behavior. In benzene, the carbons of the six-membered ring are sp² hybridized, and so each has a singly filled 2p orbital to interact with the others of the conjugated system. The six 2p orbitals interact, giving rise to six new MOs. Three are bonding MOs and three are antibonding MOs. Because of the symmetry properties of the six-membered ring, the six MOs are distributed energetically, as shown below. The six available π electrons completely fill the bonding levels, leading to an enhanced stability of the π system, which is termed aromatic stabilization or aromaticity.

This “extra” stability of benzene and other aromatic compounds was a well-known phenomenon. In fact, aromaticity was first described as chemical stability (unreactivity) toward reagents that normally attack double bonds and π systems. Moreover, reagents which did attack the aromatic ring gave substitution products in which the aromatic ring was retained; the same reagents usually give addition products with typical double bonds and conjugated π systems.

Since stability refers to energy level, aromaticity was later defined as the energy difference between an aromatic π system and a model π system in which there is no aromatic stabilization. The aromatic stabilization of benzene was taken as the difference between the heat of hydrogenation of benzene (ΔH _hyd = −49.8 kcal/mol) and the heat of hydrogenation of the hypothetical molecule cyclohexatriene (ΔH_hyd = −85.8 kcal/mol), which has three noninteracting double bonds in a six-membered ring. The heat of hydrogenation of cyclohexatriene was estimated as being three times the heat of hydrogenation of cyclohexene. Since both give cyclohexane upon hydrogenation, a difference in the heats of hydrogenation must be due to a difference in the energies of the starting mate-rials. This difference amounts to 36 kcal/mol (it is termed the RE of benzene), and it corresponds to the extra stability of benzene due to aromatic stabiliza-tion. The same approach can be used to estimate the resonance energy of other aromatic molecules.

A physical distinction between benzene and the hypothetical model compound is that benzene has equal bond lengths and bond angles and is planar, whereas the hypothetical model would have localized bonds and unequal bond lengths (double bonds are shorter than single bonds). Thus the resonance energy determination is only as good as the model system that is used.

Aromaticity was found to be a general property of many (but not all) cyclic, conjugated π systems. Moreover, it was found that aromaticity in molecules can be predicted by Huckel’s rule. The structural requirements implicit in Huckel’s rule are that there be 4n + 2 (n is an integer) π electrons in a cyclic, conjugated π system. Obviously benzene, which has six π electrons (4n + 2, n = 1) in a conju-gated π system, is aromatic. However, Huckel’s rule predicts that molecules such as cyclodecapentaene 4n + 2 = 10 (n = 2) and [18]-annulene 4n + 2 = 18 (n = 4) should be aromatic, have equal bond lengths, and be planar — and they are.

A further manifestation of aromaticity is the presence of ring current in aro-matic molecules. When aromatic compounds are placed in the magnetic field of an NMR instrument, a ring current is induced in the π system. The ring current results in an induced magnetic field which causes the protons attached to the aromatic ring to absorb nearly 2 ppm downfield from simple olefinic protons. Aromatic character can thus be detected by a downfield shift of protons attached to the aromatic ring.

In contrast to aromatic molecules which have 4n + 2 π electrons, cyclobuta-diene and cyclooctatetraene do not have 4n + 2 π electrons and are not aromatic. In fact, these molecules, which contain 4n π electrons (n is an integer), are less stable than the planar model compounds and are termed antiaromatic. Both of these molecules adopt shapes that minimize interactions of the π orbitals.

Cyclobutadiene is an antiaromatic 4n = 4 (n = 1) system, and it is quite unstable and can only be observed at very low temperatures. Although it must be planar (accounting for its instability), it distorts to a rectangular geometry with unequal bond lengths to minimize π -bond interactions. Planar cyclooctatetraene would be an antiaromatic 4n = 8 (n = 2) system, and thus it adopts a boat shape so that the π bonds are orthogonal and cannot interact!

Huckel’s rule is more than an operational way to identify aromatic molecules. Its origins are in MO theory and its applicability is general, regardless of ring size or charge. In terms of Huckel’s rule, the requirement for aromatic stabilization is that there is a cyclic system with all atoms having a p orbital available for interaction. The array of MOs produced from this interaction is populated by the total number of electrons that are present in the interacting p orbitals. If that number of electrons is 4n + 2, then the molecule will have aromatic stabilization. It turns out that the above requirements lead to a situation where the bonding MOs are completely filled, the nonbonding orbitals are either completely filled or completely empty, and antibonding levels are unfilled.

As seen above in the MO description of benzene, there are three bonding MOs that are filled by the six electrons of the π system. In another example, the tropylium ion is known to be aromatic. The interaction of seven 2p orbitals leads to an MO array with three bonding MOs and four antibonding MOs. The six electrons fill the bonding MOs and give an aromatic system, irrespective of the fact that, to do so, one of the seven interacting p orbitals must be unfilled, leading to a net positive charge on the delocalized aromatic ion.

It is also clear why cyclooctatetraene is not aromatic. The interaction of eight contiguous 2p AOs in a planar ring gives rise to an MO array which has three occupied bonding MOs and two nonbonding MOs which are degenerate and thus singly occupied. Since this is an unstable bonding situation, the molecule distorts to the shape of a boat so that interactions are avoided and four isolated π bonds can form. It is clear that either by removing two electrons (8 → 6 π electrons) or by adding two electrons (8 → 10 π electrons), one could reach an aromatic system. It turns out that cyclooctatetraene is easily reduced by the addition of two electrons which fill the nonbonding MOs and give a planar, aromatic dianion.

Examples of simple aromatic molecules and ions which have been studied are shown below.

Other elements can also participate in the formation of aromatic species. Furan, pyrrole, and thiophene are all aromatic molecules. This is due to the fact that if the heteroatom is sp² hybridized, then a doubly occupied p orbital interacts with the carbon 2p orbitals to give an MO array which contains six π electrons and is aromatic. Note that in the development of the MO diagram for these systems the identity of the heteroatom is not important. It is only important in determining the magnitude of the aromatic stabilization.

The added stability of an aromatic system is a significant energetic feature of molecules. Reactions which occur with the formation of an aromatic system are generally facile, while reactions in which an aromatic system is disrupted are generally very difficult. Thus aromaticity can dramatically influence the reactivity of compounds and should be kept in mind.