“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?