In my former life I taught organic chemistry — sometimes to classes of 300. A significant fraction of the students were ‘pre-med’ which for those of you who don’t know, roughly translates as, ‘I couldn’t give a crap about chemistry, but I need an A in this class so that I can go to a decent medical school’. I realise that’s a horrible stereotype… some pre-meds did actually find chemistry fascinating and really wanted to learn and understand things. In my experience, however, the majority didn’t.
I was teaching in the US system having myself been educated in the UK. I came from a land where (at least when I was going through the system) the average score on an exam was not typically in the 70-80% range, unlike in some of the courses my US students had taken — including chemistry ones. If you scored that well in the UK over the course of your degree, you generally received the top classification.
So, I took great pleasure in setting exams that would give a class average of roughly 50%. I also wrote questions that tested understanding rather than memorization. This, in the short term, made me somewhat unpopular. It only ever resulted in the one death threat*, however. (*Note – I’m not making this up). After time passed and wounds healed, and my students went on to the next classes in the organic sequence, I would get messages from some of them telling me just how much my course had prepared them for what came next.
Anyway, I digress. The main point here is not to bang on about how to teach, but I plan to recycle on this blog some of the exam questions I devised to torment challenge my students. So, here’s the first one. Pretty simple, but makes you think a bit:
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).
And before you begin, don’t fall into the trap of one of my poor students. Firstly, they either missed or misunderstood ‘hydrocarbon’. And secondly, although they were — in some ways — quite analytical in their approach to the question, they failed on their understanding of some basic principles of chemical bonding. They drew a tetrahedral carbon (not a bad start) and then must have thought the following: right, what are the four lightest things I can stitch on to this chiral centre? — and after looking at the obligatory periodic table in the lecture theatre, they drew a hydrogen (good), and then a helium, a lithium and a beryllium… (bad, so very bad).
Right — answers in the comments section. No prizes, but hey, why not give it a go?
Chemical connections 
trans-1,2-dimethylcyclopropane
Oddly enough, that was the answer to the question as it was phrased in the exam I set a few years back, but that’s because I originally specified ‘saturated hydrocarbon’. After putting the blog post live last night, I realised that I’d forgotten to include the word ‘saturated’… and then figured out that trans-1,2-dimethylcyclopropane is no longer the correct answer without that requirement. There are at least four other compounds with a lower molecular weight that are chiral.
This is the same question Stuart asked me on my first day at Nature Chemistry some 3.5 years ago. And he forgot to include ‘saturated’ qualifier then as well! which allowed me to outsmart him – always satisfying, because it doesn’t happen often. So I’m fairly sure I know the answer but I’ll DM it so as not to spoil anyone’s fun.
Um, how about a disubstituted allene? Make it one methyl and one lithium; (CAS# 88723-10-8) MW = 60.02
That one is at lest in theory chiral, no?
Please, I’m dying to know if I’m close!
Err… “at least” (where is the edit button?)
Hmm, can commenters usually edit their own posts… is that a setting I can change?
Anyway, nice try! But, I’m going to say that you’d have to count the lithium, and so that no longer makes it a hydrocarbon! And the hydrocarbon should be neutral and closed shell (no radicals)… but you are close with the allene idea…
(Background music: “Little Johnny Malko-pee, wet his pants so all can see.”)
Ouch. 15 years of training down the drain, just like that. Swear I never saw it coming, the definition of a hydrocarbon being my downfall. I could REALLY use that edit button now :) (No idea if there is a setting for it; probably pops up for logged in users.)
I hereby retract my previous answer. My thesis. My publications and patents. My blog. Whatever it takes to redeem my name!
On topic: How about the the first first answer -H2? (The cyclopropene.) Or could there possibly be a chiral hydrocarbon with MW < 68?
So it seems my guess was right – and after some prompting I got two more compounds. Stu finally put me out of my misery on the fourth (I read this as ‘Do some real work’). My final suggestion beats Stu for molecular weight (MW=52) but I’m certain that it (cyclobuta-1,2-diene) doesn’t exist in anything other than my head. Even if it does, it ought to be about as reactive as benzyne on speed!
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