Is Conjugation Stabilization Real?

It's an old organic chemistry tenet that conjugated polyenes, with their alternating double and single bonds, are more stable than their unconjugated isomers. An abundance of lab experiments and theoretical calculations leaves no question of this in the minds of chemists, who impart the concept to their beginning students.

Though there's been little exploration of triple-bonded examples, chemists assumed that polyynes, too, would be stabilized by conjugation--even more so than polyenes. But recently, a group of Long Island University chemists, using tried-and-true computational methods, came up with a startling value for the conjugative stabilization energy of butadiyne: zero.

The notion that polyynes aren't stabilized by conjugation flies in the face of data and provoked another study by chemists at the University of California, Los Angeles, and the University of Georgia. That group performed more calculations that generated the same zero value, but they say the effects of hyperconjugation account for the strange result. A controversy has ensued over the relative nature of "stability" and whether conjugative stabilization energy can be measured absolutely.

Back in the 1930s, eminent thermodynamicist George B. Kistiakowsky devised a strategy for quantifying the conjugative stabilization of dienes: Measure the difference between the heat of hydrogenation of butadiene to butene and the heat of hydrogenation of butene to butane. If butadiene is more stabilized, then the energy released in the former hydrogenation step should be less than in the latter. And indeed that is the case, the difference being about 3.7 kcal per mol.

Diynes are ubiquitous in the materials industry and are present in almost all optoelectronic acetylenic materials. Yet diynes' penchant for detonating in oxygen has made their combustion chemistry next to impossible to study in the lab. In fact, nobody has studied well their heats of hydrogenation.

But in recent years, computational methods have evolved to the point where they can be trusted to accurately predict these energies. So a year and a half ago, emeritus chemistry professor Donald W. Rogers and chemistry professor Andreas A. Zavitsas at Long Island University and their colleagues there and at the University of Maryland calculated the heats of hydrogenation of 1,3-butadiyne to 1-butyne, and then from 1-butyne to butane.

They expected the difference between the former and latter processes to be about twice as big as the value for dienes. Instead, that difference was zero [Org. Lett., 5, 2373 (2003)]. "I looked at that and thought, 'Oh, that's a mistake,' " Rogers says.

As it turns out, the calculations were no mistake. So the group extended their study to larger conjugated polyynes. Again, they found almost no conjugative stabilization [J. Org. Chem., 69, 7143 (2004)]..

Enter chemistry professors Kendall N. Houk at UCLA, Paul v. R. Schleyer at Georgia, and their colleagues, including UCLA graduate student Peter D. Jarowski. Their calculations generated the same numbers as those from Rogers' group, but their interpretation is very different [J. Am. Chem. Soc., 126, 15036 (2004)].

They say the problem resolved itself when they considered the effects of hyperconjugation, in which a p bond interacts with a neighboring C–H bond. It turns out that the stabilization from hyperconjugation is twice as large in butyne as in butene. This makes sense because there are two p bonds in the alkyne. "Kistiakowsky's method shows the difference between the stabilization of an acetylene group by a second acetylene, and by an ethyl group. The conjugative and hyperconjugative effects are about the same, and are both large," Houk says.

THE RESEARCH groups, as well as onlooking chemists, acknowledge that this debate is largely intellectual, and even semantic. Molecules can't be identified as stable or unstable on an absolute scale--they must have a reference, notes Barry K. Carpenter, chemistry professor at Cornell University. "One would be hard-pressed to defend a claim that either side was right or wrong," he says. Houk notes, however, that it's well known that conjugated diynes are more stable than isomeric nonconjugated diynes. "More stable implies conjugative stabilization, doesn't it?" he asks.

The issue bears resemblance to the controversy over the resonance energy of benzene, which differed depending on whether chemists used the hydrogenation energies of cyclohexatriene or hexatriene for comparison.

"There is no single definition of 'conjugative stabilization energy' because this quantity, like 'aromatic stabilization energy,' depends on the choice of model system," notes Weston T. Borden, chemistry professor at the University of North Texas, Denton. "It's not an experimental question; it's a question of interpretation."

Most observers, including Borden, acknowledge that, if given a choice, they would pick the reference compound that generates a chemically useful model.

"There is so much overwhelming experimental evidence for stabilization arising from conjugation," notes chemistry professor François Diederich of the Swiss Federal Institute of Technology, Zurich.

Zavitsas says his group is continuing to study the phenomenon, particularly in free radicals.

"Hopefully, other chemists will bring attention to this problem," Zavitsas says. "We're looking forward to having other people jump into this."