Last week I posed the following question:
What is the lowest molecular weight hydrocarbon that is chiral? (A couple of restrictions — isotopes don’t count, so no deuterium or 13C, and no conformational stereoisomers, I’m looking for stable enantiomers at room temperature).
This all came about because of a question I used on an exam I wrote a few years ago for an undergraduate organic chemistry class. My intention was to ask the same question on the blog but, of course, I accidentally missed out a few details — which ended up making the question a bit more interesting. The original question asked the students to identify the lowest molecular mass cyclic alkane (the previous question on the exam had asked about the corresponding acyclic alkane). I don’t really remember how well the class did on the question, but the structure I gave on the exam’s answer key was trans-1,2-dimethylcyclopropane (1). It turns out that this might not — technically speaking — have been the best answer… sorry kids.
Well, if you don’t specify ‘alkane’ and you use the word ‘hydrocarbon’ instead, that means unsaturated systems are fair game — and this means that there are better answers than 1. There seem to be four fairly reasonable isomers of C5H8 that fit the bill — compounds 2–5 below. I was quite happy with this set of answers until a few clever people on Twitter suggested some others. @MatToddChem came up with the bicyclobutane structure (6), although this does violate Bredt’s rule. Then @zhorakovsky followed up with the bicyclopentane (7) and bicyclopentene (8) compounds — but these structures are probably far too strained to exist with those trans-fused ring junctions. Finally, @stephengdavey pointed out that 1,2-cyclobutadiene (9) would also be a valid answer, if it existed.
So, while all of these answers are OK on paper, the blue ones are much more likely to exist than the red ones. Nevertheless, from what seems to be a very simple question, you can start thinking about structure, bonding, isomerism, stereochemistry, and strain. And if it floats your boat*, nomenclature too (*it doesn’t mine, and this might be the topic of a future post). And that bit above where I mentioned not giving the correct answer on the original exam? Well, that bicyclopentane structure (7) is definitely an alkane and has a lower molecular mass than the trans-dimethylcyclopropane (1)… in my defence, however, 7 probably doesn’t exist.
Thanks to everyone else who suggested answers on Twitter or in the comments of the original post!