Boxing Gloves. The Non-Classical Carbocation Debate

The Non-Classical Carbocation Debate

There is a tendency to interpret the phrase “hotly debate” in academic matters as a rather sedate sort of thing. Which is absurd, since scientists are, contrary to popular belief, still people. And that makes them just as prone to absurd drama as knitting enthusiasts and science fiction fans. Though maybe not quite as much as Star Wars fans. No one does drama like Star Wars fans. But to illustrate how absurd academic controversy can get, I bring you the non-classical carbocation debate.

Boxing Gloves. The Non-Classical Carbocation Debate

Debate is the polite term

When I was in grad school I actually took a class about academic controversy. Officially it was on scientific revolution but given that our case study was the non-classical carbocation debate, I often referred to it as my scientific catfights class. It was a fantastic class.

The Proposal

Norbornyl structure

Thank goodness for ChemDraw templates

Behold the basic norbornyl structure. A bit of a pain to draw, but a simple enough thing. You wouldn’t think these little lines would cause a lot of grief. The funny thing is, the first time a non-classical structure was proposed in reactions involving this structure, it actually looks to have been a bit of an afterthought. The alternative structure is squished down in a corner, most likely because it was added after typesetting was mostly done and they were not redoing the entire page dangit.

And… nothing was really said on the matter until a decade later when Saul Winstein was trying to explain some reaction rates. He decided there needed to be a stable intermediate and proposed that the most sensible structure would involve a novel structure where one carbon was basically bridging across two others using only a total of two electrons.

Norbornyl cation resonance structures

Yes this is basically about whether the line should go all dotty when it comes down to it

The Challenger

Enter H. C. Brown, who did not care for this proposal at all. He was quite insistent that everything about the molecule’s reactivity could be explained by two separate structures with the traditional two electrons for two atoms bonding them. The bond essentially went back and forth between the two other carbons like a windshield wiper.

For a while in the fifties and sixties, every new technique was quickly applied to the non-classical carbocation question. George Olah was doing work on other carbocation species, including studying them using an exciting new technique called NMR spectroscopy (always serious business). Both Winstein and Brown asked him to wade into the fray (naturally each hoping that he’d take their side).

The Drama

That’s when the debate got heated. It eventually reached the point where there was no point scheduling anything at the same time as a talk on the matter because everyone was going to go watch to see if this would be the ACS meeting where it actually came to blows. Though other reports indicate that if it came up at other times people would just leave the room because no one was going to get anywhere now. They would each sit in the front row of the other’s talks and stand to ask their questions. Which were never terribly surprising questions but did neatly eat up the entire question period.

In a lot of ways, the debate was downright philosophical. With every new study that pointed to the bridged structure, Brown could always argue that the two classical structures were interconverting faster than the detection limit of the technique.  Which was noted in my favourite footnote of all time. Reviewers are of course anonymous, but that doesn’t stop them from being occasionally really obvious. And in this case, the reviewer basically had to be Brown. As the techniques got faster, it really did start to call into question how fast something can be interconverting without being better described as the average structure. Which would be, well, a bridged structure that gets a little dotty in the bonds.

And while Brown was most definitely a stubborn mofo, he did have a point on several other strucutres proposed during the discourse. For a while, organic chemists became a bit too smitten with bridged structures and started proposing them for everything. Well, except methane. No one ever did figure out a way to draw a bridged structure for methane.

The Resolution

Finally, in 2013, Scholz et al were able to obtain a crystal structure of the disputed cation. It was… exactly what everyone not named H. C. Brown had been saying it was for decades. Unfortunately, since Brown had died in 2004, neenering was not possible. Then again, many of his contemporaries figured that he still would have found a way to argue that it didn’t reflect conditions in solution.

I feel it worth noting that both Olah and Brown would eventually win the Nobel Prize for Chemistry. Olah won it for his work on carbocations. Brown won it for something else entirely (boron reagents in organic synthesis). So any time you start thinking Nobel Laureates are all high minded, just remember that these two got just as vicious as any fandom ship war.

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