Meditations on Mechanism Determination

In my first class today the professor said something very interesting along these lines:

“When students are given a reaction and asked to find the mechanism, they tend look at the reactants and products, start numbering carbons and try to figure out what groups are supposed to end up where. But think about it–can a real molecule know what it’s going to turn into? No way! A molecule undergoes a series of elementary steps during a reaction, none of which depend on the identity of the products. Instead of looking at reactants and products (which might tempt you to find some quick-and-easy but wrong mechanism anyway), look at the reactants and other reagents. Take the right steps and you’ll end up with the right products. All you have to do is be smarter than a molecule!

“Is anyone in here smarter than a molecule?”

Crickets after that, but you get the idea. I found it intriguing because I find myself falling into that trap all the time, particularly while reading journal articles. Writing a good mechanism is all about understanding what the reactants do to each other, not where things are supposed to “end up” after some arbitrary period of time. The reactants taken together are what make the reaction actually go, so why even look at the products at first?

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7 Comments

  1. I believe it! Grossman can be an easy target, though. You should have been at the introductions for the new grad students…oh man, he screwed THAT up.

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  2. I like Grossman. He’s a tad awkward, sometimes, but if you apply certain Zen techniques, he isn’t bad. I just ignored anything demeaning or stupid that he said.

    On the other hand, when I first visited UK, my wife was with me. He pissed her off royally in the first 10 seconds we met him. She still curls her lip if I mention his name.

    Bob knows a lot about metal-mediated cyclizations, and I found him really helpful once I filtered out all his odd harmonics. I’m sufficiently reptilian that I can ignore everything but direct digs or threats, and even most of those, and most of Bob’s gaffes seem to be unintentional.

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  3. “A molecule undergoes a series of elementary steps during a reaction, none of which depend on the identity of the products”

    However, this is not entirely general. For example, think about “product controlled” reactions, especially controlled by the conformation of the product (eg. hydride additions to cyclohexanone)

    Reply

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