How Simplicity Breeds Complexity in Organic Chemistry

by James

in Organic Chem Study Tips, Uncategorized

Learning rules is relatively easy.

Looking backward from a situation and understanding how those rules led to a specific consequence is a little harder.

Predicting the consequences of those rules in the forward direction is hard. Incredibly hard.

Let’s illustrate with a chess example. If you’ve played chess, you know that there aren’t really a lot of rules, but the game gets very complex very quickly. Let’s talk about three principles in particular.

  1. Knights move in an L-shape and can “hop” over pieces in between.
  2. When a king is in check, you must get him out of check.
  3. All things considered, knights are less valuable than rooks and queens.

Those rules can be taught in a few minutes, but learning how to play chess well can take a lifetime.

My cousin taught me chess when I was seven. I still remember the look of grim satisfaction on his face when he introduced me to a most unpleasant situation:  to be on the receiving end of a “fork”.

1_976594_f520

The idea behind the fork is that the knight threatens two pieces at once: your king and another valuable piece. But you have to move your king because he’s in check. Therefore, your other valuable piece (in this case, the queen) is toast. So are your chances for winning the game.

Once you arrive at this situation,  it’s easy to see how it flows out of those three rules, but it would be extremely difficult to imagine this situation from first principles. Our imaginations are just not good enough. However, once you’ve been on the receiving end of a fork a few times, it becomes a pattern that you recognize and learn to avoid (and use against newbies).

The point of this story is to say that while learning the key concepts is a vitally important first step, it isn’t enough. One must experience examples of how they combine. In learning organic chemistry, the place to do this is through working problems. Then, after doing each problem, it’s useful to break it down again and look at it from a conceptual perspective, to see how they fit together.

Here’s a chemistry example.

1) Electronegativity differences lead to certain bonds being polarized, with one end being more electron rich (delta negative) and the other being electron-poor (delta positive).

2) The electrons of π-bonds can attack electron-poor species.

3) Carbocations increase in stability with increasing substitution.

In the addition of an acid like HCl to an alkene, we end up with the “more substituted” product because the chloride (electronegativity of 3.0) is the “delta negative” end and the hydrogen (electronegativity 2.2)  is delta positive. The selectivity for the product is governed by the increased stability of positive charge on the more substituted carbon. This is what we call Markovnikoff addition, and we see this pattern in the addition of all kinds of different acids to alkenes.

1-Markov

Well, what about the addition of BH3 to alkenes?

2-antiMarkov It looks weird, but it’s the same three principles at work. Due to electronegativity differences, it’s the H (electronegativity 2.2) which is delta minus (electron-rich), and the B (electronegativity of 2.0) which is delta plus (electron poor). So this time it’s the hydrogen that will end up on the more substituted carbon, and the boron ends up on the less substituted carbon. We call this “anti-Markovnikoff addition”, since the position of the new hydrogen is different (there’s an extra wrinkle here because the addition doesn’t involve a free carbocation, but the point stands)

Again – it’s hard to predict the consequences of these concepts when they combine.

“OK”, you might say, “I see what you’re saying, but maybe that’s just me – I suck at orgo. Surely, the egghead genius chemists who do this for a living have the foresight to predict these reactions from first principles, right?”

Wrong. H.C. Brown, who discovered the hydroboration reaction, screwed around with BH3 for twenty years before one of his coworkers discovered that something unusual was happening in reactions containing alkenes. Twenty fricking years. We’re talking about a Nobel-prize winning chemist here.

 

Looking back, it seems obvious that it works – all the principles were known at the time. But he didn’t predict it in advance.

To taste something is to know it. Problems may taste terrible to you at the moment, but they really are the best way to understand how the key concepts interact. It sounds masochistic, but you really do need to get “forked” a couple of times before you become better at orgo.

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