Week 8 of 52: Cleaving ethers with hydrohalic acids

Well, first we had reactions of alkyl halides, then a reaction of an alcohol to an alkyl halide. Alkyl halides are so much fun that you definitely want to learn more reactions involving them. I know I would, if I didn't already know all of them (that last phrase may not actually be true).

As you may have guessed, cleaving an ether means breaking one or both of the bonds to the oxygen, which also breaks the chain at that point. Don't think of ethers as particularly unstable, because most of the time they are not. But in the right environment, that oxygen can be the weak link in a chain (and when cleavage occurs, that's where it happens). Hydrobromic and hydroiodic acid are one way to provide that environment, protonating the oxygen in an acid-base reaction. Did I mention that this reaction involves a nucleophilic substitution mechanism? That should be a big hint.

Still don't get it? Well, I haven't talked about ethers much, so you probably just aren't used to them. But remember how we can turn a bad leaving group into a good one? Of course you do. Well, that's what happens here. Twice. The oxygen is protonated, and a bromide or iodide reacts with one of the α-carbons by nucleophilic substitution. I'll emphasize that, yet again, this is S_N1 in the case or secondary or tertiary α-carbons and S_N2 in the case of primary or methyl α-carbons. It's important and I don't think I've been emphasizing it enough so far, but now it's in bold, so you are not allowed to ever forget it.

Conveniently enough, this leaves us with one alkyl halide (with the carbon chain on the side that underwent nucleophilic substitution) and one alcohol (the other carbon chain keeps the oxygen, which is now bonded to hydrogen). Also conveniently, the alcohol undergoes nucleophilic substitution by the reaction we covered last week.

And that's it. In conclusion, we go from R—C—O—C—R' to R—C—X and R'—C—X (with water as a byproduct). Keep in mind that this reaction works because the acid provides protonation of the oxygen, which creates a leaving group, and also because the acid provides a halide to act as a nucleophile.

Ugh, and we're still a week behind.