Here’s what we talk about today: more eliminations of alcohols! Note that this reagent isn’t covered in all courses, but I’ll include it here for completeness’ sake.
We’ve talked about 2 ways to convert alcohols to alkenes so far:
Option #1: Convert the alcohol to an alkyl halide [with SOCl2, PBr3, or a hydrohalic acid] and then treat with a strong base like NaOEt or similar to produce the alkene through an E2 process [2 operations]
Option #2: Heat the alcohol with a strong non-nucleophilic acid like H2SO4 or H3PO4 [1 operation]. [see post]
So which is better? Well, actually they both have their drawbacks.
• Converting an alcohol to an alkyl halide followed by treatment with base is two separate operations. This is OK, but it would be nice to be able to do this in one step.
• Heating alcohols with strong acids is a one-step process, but can lead to carbocation rearrangements. Ideally we’d like to have better control of the products of these reactions, and avoid byproducts that come from hydride or alkyl shifts.
Before we go any further, you might think this is nitpicky. You might think, “two steps! Who cares!! What’s the big deal?” .
The big deal is – we CARE about our time – a lot! Think about how much you hate it when Facebook takes more than 3 seconds to load. People will walk through a nicely manicured garden to shave five seconds off their journey. Chemists are no different. If there’s a way to do something in one step instead of two, we’ll take it! So yes, one step instead of two matters to us.
Today we talk about a process that gives us the best of both worlds – a one-step process that proceeds under much milder conditions than heating with acid.
It doesn’t get covered in all introductory organic chemistry courses, but for completeness, we’ll cover it here.
Direct Elimination of Alcohols To Alkenes With Phosphorus Oxychloride (POCl3)
As we’ve discussed before, hydroxide (HO- ) is a very poor leaving group. In order for alcohols to participate in substitution and elimination reactions, it’s best to modify the oxygen in some way so as to be able to stabilize the negative charge generated when the C-O bond breaks.
One way we’ve seen how to do this is by converting alcohols to alkyl sulfonates, such as tosylates or mesylates.
It would also work if we converted an alcohol to an alkyl phosphate [itself a good leaving group], but as it turns out the OH groups on phosphate are acidic and can interfere with the basic reagents we typically use for elimination. So a compromise is to use the reagent phosphorus oxychloride (POCl3), a derivative of phosphoric acid. When POCl3 is added to an alcohol, we form a new O-P bond [the oxygen phosphorus bond is strong] and break a P-Cl bond to form what we could call a “chlorophosphate ester”.
This is now a good leaving group! If we have a decent base around [such as pyridine] we can then get elimination of this good leaving group to form a new alkene [via E2].
In practice an excess of pyridine is used here, or even use pyridine as the solvent.
Here’s how it works:
This process proceeds in on operation, is much milder than heating an alcohol with strong acid and doesn’t result in rearrangements.
Like I said it doesn’t appear in all introductory courses but it’s important to know that when you see it, think “elimination”. Importantly, don’t confuse this reagent with PBr3 or PCl3 –> those will convert an alcohol to an alkyl halide, which is not the same reaction at all!
This is all we’ll have to say about substitution and elimination reactions of alcohols, for now. In the next few posts, we’ll go through a special property of alcohols – the ability of certain reagents to lead to their “oxidation” to species such as aldehydes, ketones, and carboxylic acids. More next time!