Week 6 of 52: Dehydration using phosporus oxychloride and pyridine

This one is late. My apologies. But that means you'll get two reactions this week, and that's cool, right? Well, in this case, I'm going to have to make the post a brief one. I know, short and late. It's almost as though I'm not doing a very good job or something. But this is just going to be a specialized reaction. More important reactions should follow soon. And those posts will be longer. This one is really pretty much the same as last week. Well, now I guess it would be the week before last week, because I failed to update this blog last week. Whatever.

Last time, I showed how a strong acid can turn a bad leaving group (hydroxide) in to a good one (water), facilitating a specific type of elimination reaction in which water is eliminated by either an E2 or E1 mechanism, ultimately forming a double bond. Essentially, this can be used to turn an alcohol into an alkene. If the rest of a molecule won't react with the strong acid, it can even be used on a more complicated molecule forming a C=C bond at the area of interest, perhaps as one component of a series of reactions to form a desired product. But there's that caveat: this is only possible if the molecule will behave for us once we put it in a highly acidic environment. Many organic molecules will simply not do this. And that's where this reaction comes in. Phosphorus oxychloride and pyridine offer a way to dehydrate an alcohol by an E2 mechanism under basic conditions.

I will depict the mechanism for the first step of a reaction that converts cyclohexanol to cyclohexene. First, one of the lone electron pairs on cychohexanol's oxygen attacks the phosphorus in POCl₃, breaking chloride off to float away and never come back...

Next, pyridine reacts with the exposed proton from the alcohol: an acid base reaction...
Now, we have a good leaving group on that α-carbon for our E2 reaction. Note that there's already a base (pyridine) present. So it will remove a proton from a β-carbon...
And we have cyclohexene! Formed by the dehydration of cyclohexanol. Amazing, I know.