Quest for a Stable BOO

Is BOO stable?

A student in my chemistry class would be scribbling away an answer to the question. Everyone else might be wondering: “What kind of a question is that?”

Happy Halloween! If you are looking for the perfect BOO pumpkin, I suggest visiting The Pumpkin Lady’s carving patterns. I particularly like the one shown below from that website.

If instead you’re interested in a strange trip down the rabbit hole, with geeky chemical structures, then read on!

Two weeks ago, my first semester general chemistry class went through Lewis structures in great detail. I tell students that the most important thing they will learn in my class this semester is how to draw good Lewis structures. While Lewis structures may not provide the most accurate picture of what electrons are doing in chemical bonds, they are the epitome of practicality and usefulness to all chemists.

Drawing Lewis structures can tell us something about the stability, or conversely reactivity, of a molecule. A Lewis structure can tell us the shape of a molecule and its polarity; both are useful things to know when designing a molecule for a specific application. (The pharmaceutical industry is the business of designing drugs.)

In my class, students learn four guidelines to evaluate if a Lewis structure represents a stable structure.

·      Atoms try to reach an octet (especially C, N, O, F). While B can go below, and larger atoms can go above, the closer they get to the octet, the better. This is known as the octet rule, but it’s really a rule-of-thumb.

·      Small formal charges (+1 or -1) assigned to an atom are okay. Try to avoid large formal charges (+2, -2 or larger in magnitude).

·      If there are formal charges, try to have the -1 on the more electronegative atom and the +1 on the less electronegative atom.

·      The existence of equivalent resonance structures is stabilizing (a la Heisenberg).

Let’s apply these to the original question: Is BOO stable?

The first structure a student is likely to draw is shown below. I christen it Bent BOO.

While the oxygens have octets, boron goes severely below (with 5 rather than 8 in the “octet count”) and the unpaired electron on B makes the molecule more reactive. Also, there is a formal positive charge on an electronegative O, and the O–O peroxo-bond is also prone to react chemically.

We could potentially improve things with Triangle BOO. Oxygens still follow the octet rule. The problems on B remain, but we’ve eliminated the positive formal charge on O.

However, my students have learned that the triangular O₃ structure of ozone suffers from ring strain due to Valence Shell Electron Pair Repulsion theory (the Pauli Principle again at work!). Oxygens with four electron clouds prefer to have 109.5-degree bond angles, and not 60 degrees in the triangle. Repulsion breaks open the ring.

The next structure one might draw is Linear BOO. There is an improvement for B (which is 6 by the octet count). Most general chemistry textbooks will declare that boron typically goes under the octet rule and has 6 in the count. One of the oxygens is a little worse having gone down to 7 from the octet. But having an equivalent resonance structure helps. And there isn’t the problem of the positive formal charge on oxygen.

Perhaps we should call this OBO, but since it’s Halloween we’ll stick to BOO. Also, chemists often write the carboxylate group as COO-minus even though C is in the centre, so BOO seems reasonable. My students worked on a problem set two weeks ago themed with nitrogen oxides as the theme. They had to draw structures such as NO₂ and explain why it was reactive (i.e. less stable) and they found that Lewis structures with N in the centre are superior to N at one end. Students then drew the more stable nitrite anion (NO₂ with a -1 charge overall). By adding a single electron to NO₂, they could draw good Lewis structures that followed the four guidelines.

Hence, they might probably guess that a more stable BOO might exist as an anion. By adding an electron to the non-octet oxygen, you get Linear BOO Minus consisting of a pair of resonance structures.

This is better than Bent BOO Minus where B is down to 4 in the octet count, rather than 6 in the linear version.

But maybe there’s another way to avoid reactive unpaired electrons. Bring two molecules of BOO together!

Possibly the best structure might be Two-Square Double BOO. Oxygens follow the octet rule. Boron has its typical 6 in the octet count.

But squares with 90-degree bond angles are still strained rings, although they are not as strained as the triangles. The two-square structure might open up to a six-membered ring, Hex Double BOO.

The two radicals on B might still be spin-paired (dashed double-headed arrow) and Double BOO might be a fluxional molecule. The preceding sentence is not something my general chemistry students would write, but a student who took my physical chemistry class would have learned about spin-paired singlets/triplets and possibly fluxional molecules (time-permitting).

Could you do better with anions of Double Boo? That strategy worked well for single BOO.

Adding two electrons to the hexagonal structure gives us Hex Double BOO Minus. (Note this is a -2 anion.) There are no unpaired electrons. B is 6 in the octet count and all oxygens follow the octet rule. However, the negative formal charges are on the less electronegative B.

Also the structure looks ugly and it has the more reactive peroxo bonds. What about Star Double BOO Minus? It’s an aesthetically pleasing and creative structure!

While it might look beautiful, ring strain is likely to open up the bridgehead oxygens leading to the symmetric Square Double BOO Minus. It’s a decent structure and the two negative formal charges are kept as far apart as possible to reduce repulsion. However, it still has a strained square.

What else can we do? Can we triple BOO?

I proudly present to you today’s winner: Hex Triple BOO Minus. (Note it is a -3 anion.)

It has a boroxine (B₃O₃) core, a stable motif found in a number of chemical structures. It’s even possible to have all atoms obey the octet rule in the resonance structure on the right. While that structure has the formal negative charge on the less electronegative boron, this is also found on stable anions such as BF₄ with a -1 charge.

Compared to all previous structures, Hex Triple BOO Minus actually looks pretty good. Triple Hex BOO as a moniker would be more Halloween-like, but since the name tells us something about the structure, it really is a Hexagonal Triple BOO.

But is it really stable?

Hopefully if I posed this question my students would ask “stable with reference to what?” Good question. They’ll have to wait until next semester when we talk about thermodynamics! In the meantime, any of those unstable structures is likely to behave like Explosive BOO.

P.S. If you found this reasoning fun, check out this earlier post on C₂O coconut water.

P.P.S. I leave you with a picture and recipe for ectoplasmic drool from this week’s Chemical and Engineering News.