[puffincatz23]

Hey everyone,

I recently came across this issue regarding anti-aromatic vs non-aromatic species while TA-ing for the summer orgo I class here at my university. During lecture, the professor said that antiaromatic compounds are slightly more stable than their nonaromatic counterparts, and that there are only a few cases in which antiaromatic compounds are actually more unstable.

My previous professors have taught me that antiaromatic compounds are special because of their unexpected INSTABILITY, which is why I'm slightly confused. Could anyone please offer some input? Thank you!

[phth]

When its 4n (antiaromatic), the energy levels can be degenerate, but the charge is delocalized differently.  For example,  antiaromatic compounds between magnets are paramagnetic; aromatic compounds are diamagnetic. cyclobutadiene will form 4 energy levels which one of the double bonds is in the singlet state.  Cyclobutadiene is an archetype of antiaromatic compounds, and maybe that is what they mean by instability versus hydrocarbon; dimerization has zero activation energy (diffusion controlled because of the radicals).  I think you are asking an ambiguous question, and it really depends on what specific compounds you are talking about.

[Corribus]

I recently came across this issue regarding anti-aromatic vs non-aromatic species while TA-ing for the summer orgo I class here at my university. During lecture, the professor said that antiaromatic compounds are slightly more stable than their nonaromatic counterparts, and that there are only a few cases in which antiaromatic compounds are actually more unstable.

From time to time people come here and say something to the extent of, "My professor said this but my textbook says that - is my professor wrong?" One must be careful arbitrating such disputes because we aren't always given the full context in which said professor made the statement in question.

That said:

In general, antiaromatic compounds are thermodynamically and/or kinetically unstable - in fact they generally aren't observed in nature: usually they relax to a more stable, nonaromatic state by buckling or making some other structural modification.  The classic example of this is cyclo-octatetraene (COT), which forms a boat- or tub-like nonaromatic structure in preference to a less stable planar (and paramagnetic) antiaromatic structure. This has been sometimes called a Jahn-Teller-type distortion, and you can read more about it here: Senn, J. Chem. Educ., 1992, 69 (10), p 819; http://pubs.acs.org/doi/pdf/10.1021/ed069p819, this example deals with cyclobutadiene, though, and since the distortion in COT is out-of-plane, it may not be an apples-to-apples comparison; a paper that discussed the specific example of COT is Klarner, Angew. Chem. Int. Ed. 2001, 40, No. 21, 3977.  Moreover, COT undergoes reactions like a polyene rather than like benzene. Notably, when it is reduced by two electrons, COT planarizes and becomes "aromatic" - supposedly by virtue of the fact that the molecule is no longer open-shell and paramagnetic.

Honestly, I may be in the minority when I say that I regard aromaticity and antiaromaticity as rather contrived concepts. They derive from Huckel's rules, which themselves derive from a somewhat simplistic LCAO-MO treatment of pi-molecular orbitals. Specifically you will find that, for cyclic polyenes, when the total number of pi-electrons is equal to 4n, the Huckel treatment predicts that the two highest energy electrons will be located in nonbonding orbitals and - importantly - will remain unpaired. This was from the beginning regarded as likely to be unfavorable/unstable, at least compared to other possible configurations. (Although asking why this should lead defacto to instability doesn't lead to any easy answers other than that chemists seem to find open shells more inherently unsettling than nature does.) Contrast this to the 4n + 2 case, in which the highest energy electrons are predicted to be in stabilized, delocalized orbitals, all in pairs, thus closed-shell. Hence the basis for Huckel's rule.

Despite all this, if you perform a simple Huckel treatment on COT, you may be surprised to find that it is still predicted to have a resonance stabilization energy. Certainly it is smaller than benzene, but definitely nonzero. It depends a little on what you compare the total pi-energy to as a reference point to determine what the resonance energy is, but even so, the fact remains that a simply Huckel treatment predicts that COT should be slightly more stable than 4 electronically isolated ethene molecules - that is, it is predicted that the planar COT should have a lower energy than a nonaromatic, nonplanar version where the four double-bonds are electronically isolated. Perhaps this is what your professor meant when he said that antiaromatic conformations can be (predicted to be) more stable than nonaromatic versions? Either way, the fact of the matter is that COT does not persist in planar, antiaromatic form - it exists at equilibrium in tub-like nonaromatic form, so this must be the more stable state.

So what is going on, then?

The answer is that the Huckel method does not take into account spin-spin interaction energies, or the energies of the sigma-bond framework, which certainly must contribute to the overall energetic landscape of any molecule. Also, the output energies are dependent on assumptions made about the structure of the molecule and resulting matrix elements, and so important effects like symmetry breaking are only incorporated into the model in an ad hoc way. [10]Annulene, for example, should be aromatic according to Huckel, but the angle strain is so large that it is not a stable molecule. Another pertinent example is the Peierls distortion in infinitely long polyacetylene chains; a basic Huckel treatment, in which all adjacent electronic coupling matrix elements are assumed to be the same, predicts that polyacetylene is a metal and that all bond lengths are the same, whereas in reality the polymer is a semiconductor with a pronounced alternation in single and double bond lengths. The energy stabilization that results from such symmetry breaking can be incorporated into the Huckel model, of course, but when you start refining the model in this way, you start to muddy the meaning of terms like "aromatic" and "antiaromatic" that were originally brought about by predictions made from the basic model.

In the case of COT, I suspect what happens is that the planar symmetry breaks in order to lower the HOMOs, and then even planarity is distorted because the loss in pi-stabilization energy is probably more than made up for in energy gains by relaxing the sigma-framework angle strain. There is probably some pi-energy stabilization remaining, but without some sophisticated calculations, it'd be hard to say how much there is. Compare this to benzene, which has much greater pi-stabilization energy (it's "aromatic" after all, with no nonbonding HOMO electrons, so symmetry breaking isn't necessary) AND the hexagonal structure has much less sigma bond strain, so that isn't a motivating factor for structural reorganization either. I suspect the actual open shell nature of "antiaromatic" structures actually has very little to do with their latent instability, and it is more the result of the nonbonding electrons, which contribute nothing to the overall energy stabilization. This is all just speculation on my part - maybe the literature has something more to say on the matter, but the literature has long since abandoned basic Huckel calculations in the wake of far more sophisticated models, leaving classic terms like "aromatic" and "antiaromatic" with vague meanings to say the least.

Well, I hope that wasn't more information than you needed.

[puffincatz23]

Thank you both for answering! Definitely love the thoroughness of the answers.