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?