One of the common complaints you hear a lot from people learning English is that there are just so many exceptions. The plural of “goose” is “geese”, but the plural of “moose” is “moose”. Why is that? Who knows.
You just have to memorize it. (Edit: well, when learning English, you do have to memorize it. But that’s not going to be the point of this post. Maybe this wasn’t the best example to choose).
Organic chemistry is also rife with many exceptions (real and apparent) to the beginning student. It isn’t long after you learn about Markovnikoff’s rule, for instance, that you learn when this “rule” is broken. And that’s just the first of several exceptions that come up in the course.
So one common approach is to simply memorize these exceptions, the way you might memorize “i before e, except after c”. While a memorization approach could be effective for exams that test the ability to regurgitate book knowledge, however, any instructor looking to test problem-solving ability could easily design a test that will render such pure memorization efforts ineffective.
Behind every exception there is a deep reason why things occur the way they do, and these reasons illustrate deeper principles of organic chemistry. The purpose of today’s post is to illustrate the key concepts behind some of the most common “exceptions” in Org 1.
Annoying exception #1 : Hydroboration. Not long after you learn about Markovnkoff’s rule, you learn that when you add a borane to an alkene, it adds the “opposite way”. What’s going on here? Interestingly, it isn’t as much of an exception as it seems. Things make a lot more sense if you examine the relative electronegativities of the atoms being added. In all cases – whether adding HBr, HCl, H3O(+), or BH3, the most electronegative atom always adds to the most substituted carbon, because that’s the carbon that best stabilizes positive charge. In BH3, it just so happens that the most electronegative atom is hydrogen. It’s not really that weird after all.
Annoying exception #2 – “Peroxides”. HBr by itself does Markovnikoff addition to alkenes but if peroxides are present, it adds the opposite way. Again, one approach here is to just memorize that if you see “peroxides”, it goes the other way. However, it isn’t much more effort to understand that what is going on in both cases is exactly the same: just as in the above example, an electron-deficient intermediate (carbocation or radical) ends up on the most substituted carbon.
Annoying exception #3 – the nucleophilicity of halides in polar protic vs. polar aprotic solvents. Iodine is a better nucleophile than F(-) in polar protic solvents, but fluoride is a better nucleophile than I(-) in polar aprotic solvents.
What’s the key lesson here? There are really three key trends here. 1) nucleophilicity is decreased by hydrogen bonding – and a nucleophile in a protic solvent will be surrounded by solvent molecules it is hydrogen-bonded to 2) hydrogen bonding ability decreases as one goes down a column in the periodic table 3) in the absence of hydrogen bonding, nucleophilicity increases with basicity.
In fairness, thiis example is tough, because the key trends oppose each other and the relative nucleophilicities are not something that could have easily been predicted from first principles, but result from actual measurement of reaction rates. Furthermore, basicity and nucleophilicity are measured by different yardsticks – one (basicity) is measured according to equilibrium, and the other (nucleophilicity) is measured according to reaction rate. There’s no getting around having to memorize these trends, but understanding why they operate is key.
Annoying exception #4 – Primary carbocations and radicals are unstable, unless they are stabilized by resonance. This example illustrates the importance of resonance, which is one of the key stabilizing factors in organic chemistry. In general, any factors which allow a charge (or unpaired electron, in the case of the radical) to be distributed over a larger area tends to be stabilizing, which makes up for the otherwise unstable situation of having a carbocation on a primary carbon.
That takes care of some of the common “electronic” effects. Next post, we’ll look at some of the “steric” effects that lead to weird exceptions in Org 1.