It's All About Probability

As a chemistry major and, hopefully, an eventual teacher, I've tried to pay attention to how chemistry is taught.  One particular trend in organic chemistry confused me a little before I figured out what the truth really is.

Organic chemists, or maybe just those that teach, tend to describe reactions and chemicals as if they “prefer” or “like to do” certain reactions.  This isn't particularly accurate or quantitative in the scientific sense.  I can appreciate the use of these metaphors to make reactions understandable, but it’s just not correct and, I believe, hinders true understanding of chemical systems.  Here, as in many places, honesty may be the best policy.

So what’s the truth about what’s going on?  When a student asks about a potential alternate reaction and the teacher says “That doesn't really happen” what do they mean?  The truth dawned on me as I was finishing up our university’s two-semester o-chem course.  The fact of the matter is that every possible reaction occurs.  Every single one.  What a mess, huh.  Either I was zoned out too much or this was never explicitly explained.  Anyway, how do we get from this fact to “That doesn't happen?”  Simply enough, not every reaction occurs with the same frequency.  Some reactions require more energy to get them going, others require specific orientations of the molecules, and some do happen but then reverse and become the starting molecules again.  (Actually, theoretically every reaction reverses at least a little.)  With all these considerations, there are many reactions that do not happen often enough to warrant much consideration. However, some reactions dominate and provide a majority of the new molecules produced in a given chemical system.  These are the reactions that “like to happen.”  The best phrasing would probably be that these reactions “happen the most” or “are the most likely to occur at any given point in time.”  These are the reactions that students memorize, teachers recite, and career chemists exploit.

One last tidbit.  Sometimes the most likely reaction is actually not the desired reaction.  Sometimes you want to isolate another product from another, less likely reaction.  Try explaining to a bunch of students that you now want to focus on a reaction that doesn't “like to happen” and see if you don’t get some perplexed expressions.  After all, everything else they've studied “likes to happen,” so why is there this sour puss of a reaction that we care about.  They might be thinking “If its personality is so contrary, why bother?”  Obviously, anthropomorphizing some things just doesn't make sense.