Week 1 of 52: Bimolecular nucleophilic substitution

I think this was the first reaction I learned in an organic chemistry class. Regardless of whether I'm right, it will be the first reaction in this series. Now, I know the name seems intimidating to you, because you're so pathetic. But I'll confess something: I didn't remember that name when I set out to write this post. At least I don't think so. It didn't really cross my mind. I'm used to just calling this reaction by the same name everyone else calls it. The more common name for this reaction is S_N2.

"Bimolecular" in this case refers to the fact that the reaction involves a collision of two molecules. Unlike some other reactions I'll be dazzling you with, this entire reaction happens in one step. One bond breaks at the same time as another bond is formed. Consequently, the thermodynamics of this follow the "second-order rate equation." I'll be gleefully ignoring that for now, so you can too, if you want. The important thing about it is that the rate at which this reaction occurs depends on the concentrations of both molecules involved (increase the amount of either in a system, and the rate of reaction speeds up).

"Nucleophilic" refers to the fact that one of the two reactants is, well, a nucleophile. Nucleophiles are attracted to positive charge. Remember: nucleii of atoms are positively charged. Here's the part where I could elaborate on the intricacies of nucleophilicity as a property, which molecules make good nucleophiles and which ones do not and why, but it turns out that I've been procrastinating on writing this post, so we're pretty much skipping that. Anyway, I will tell you that nucleophiles are often negatively charged particles, which should be obvious anyway.

"Substitution" means that the nucleophile replaces another group. The other group is aptly known as a leaving group. You know, because it leaves. To be a leaving group, an atom or group of atoms must be able to accept electron density. This leaves less electron density on the other side of the bond (which is to a carbon atom) and more exposed nucleus for the nucleophile to do its thing. The most popular leaving groups are halogen atoms, especially bromine and iodine (they're bigger, so the electron density is spread over a larger space). One thing that I didn't remember, but that my textbook deemed noteworthy is that "all good leaving groups are weak bases with strong conjugate acids having low pK_a values."

Another fun fact that can sometimes matter is that this reaction happens by "backside attack." As you may have noted, the nucleophile reacts with the carbon atom, not the leaving group and it wouldn't make sense for it to form a bond in the same spot where the bond to the leaving group is simultaneously breaking. This means that the stereochemistry of the carbon can be completely changed. It also allows for the pickup line: "Baby, if I were a reaction, I'd be S_N2, so I could attack your backside."