Alcohols, Epoxides and Ethers
Elimination of Alcohols To Alkenes With POCl3
Last updated: November 9th, 2022 |
POCl3 For The Elimination Of Alcohols To Alkenes
Phosphorus oxychloride (POCl3) is a useful reagent for cleanly performing elimination reactions on alcohols.
- POCl3 converts the OH group into a good leaving group
- A base such as pyridine is generally added, which performs an E2 elimination to give the more substituted double bond
- Elimination occurs without rearrangements.
Table Of Contents
- Two Ways To Convert Alcohols To Alkenes We’ve Learned Previously
- Direct Elimination of Alcohols To Alkenes With POCl3: The Mechanism
- The Reaction Works For Primary, Secondary, and Tertiary Alcohols
- Summary: Direct Elimination of Alcohols With POCl3
- (Advanced) References and Further Reading
1. Two Ways To Convert Alcohols To Alkenes We’ve Learned Previously
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 a website 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.
2. Direct Elimination of Alcohols To Alkenes With Phosphorus Oxychloride (POCl3)
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.
3. The Reaction Works For Primary, Secondary, and Tertiary Alcohols
It works for primary, secondary, and tertiary alcohols.
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!
4. Summary: Direct Elimination of Alcohols To Alkenes With POCl3
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!
Next Post: Alcohol Oxidation – “Strong” and “Weak” Oxidants
(Advanced) References and Further Reading
- The Effect of Structure on the Course of Phosphoryl Chloride-Pyridine Dehydration of Tertiary Alcohols
Ronald R. Sauers
Journal of the American Chemical Society 1959 81(18), 4873-4876
- Stereospecificity and regiospecificity of the phosphorus oxychloride dehydration of sterol side chain alcohols
Jose Luis Giner, Christian Margot, and Carl Djerassi
The Journal of Organic Chemistry 1989 54(2), 369-373
This article by the legendary chemist Carl Djerassi (who developed norethindrone, the first female contraceptive) describes the selectivity of POCl3-pyridine dehydration conditions in steroid synthesis. It also has a general procedure for POCl3-pyridine dehydration in the experimental section.
- A general approach to linearly fused triquinane natural products. Total syntheses of (.+-.)-hirsutene, (.+-.)-coriolin, and (.+-.)-capnellene
Goverdhan Mehta, A. Narayana. Murthy, D. Sivakumar. Reddy, and A. Veera. Reddy
Journal of the American Chemical Society 1986108(12), 3443-3452
This paper by Prof. Goverdhan Mehta demonstrates the applicability of the POCl-pyridine dehydration in natural product total synthesis.
- The 3-methylcholestanols and their derivatives
D. H. R. Barton, A. da S. Campos-Neves and R. C. Cookson
J. Chem. Soc., 1956, 3500-3506
This paper by Nobel Laureate Prof. Derek H. R. Barton has a POCl3-pyridine dehydration (see p. 3504-3505 in the experimental section).
20 thoughts on “Elimination of Alcohols To Alkenes With POCl3”
I’ve seen some texts show SN2 at P of POCl3 and S of SOCl2. Clayden 2E (p 729 and p 214) are examples of this. Is there any evidence for the presence of pentacoordinate phosphorus or sulfur intermediates in these reactions? What are the pros and cons of each depiction?
The addition of the alcohol to POCl3 is denoted as forming a chlorophosphite. Should this be a chlorophosphate? If not, why not?
Shoot, you are right. It should read, “chlorophosphate”.
I love you. Best teacher EVER.
Why POCl3 gives elimination and SOCl2 gives substitution? (Both reactions with same reagents..for example a secondary alcohol and pyridine). In case of SOCl2, p
Can pyridine attack a beta H anyway?
I read somewhere that thorium dioxide also does the same thing(E2) Is it true?
Thorium oxide? That’s a new one to me. Although there is a reaction called the Ruzicka cyclization which involves heating dicarboxylic acids with ThO2 to very high temperatures. https://onlinelibrary.wiley.com/doi/abs/10.1002/hlca.19260090130
sir,what happens if POCl3 REACTS WITH ALKENE IN PRESENCE OF P2O5
POCl3 doesn’t react with alkenes. Did you mean alcohol?
WILL SHIFT HAPPEN IN POCl3 ELIMINATION?
THIS IS MY FIRST COMMENT. WHY IS IT SHOWING I ALREADY SENT IT BEFORE?
No, hydride shifts do not occur in POCl3 eliminations, since it does not go through a free carbocation.
whoops, I think you meant to include the mechanism, but instead posted the same ChemDraw graphic on “works with primary, secondary, and tertiary alcohols” twice.
Oh dear, you are 100% correct. Fixed. Thanks Tim
Well, I’m not so sure this is correct.
While there are a few examples of 2o alcohols being dehydrated to give alkenes, a SciFinder search for 2o ROH –POCl3–>RCl returns over 750 examples, many of which are 90+% yield and many of which use pyridine as solvent or co-reactant.
POCl3 elimination of ROH only works generally with tertiary alcohols. 1o and 2o alcohols generally give substitution — same as for PCl3 — in very high yield.
It appears that this idea that POCl3/pyr gives elimination with secondary alcohols has gotten into some textbooks and is being propagated as fact with just these minimally supportive examples.
That’s a great point to look into, Bob. Thank you.
Is pyridine a large base or a small base?
Certainly not a large base. A more sterically hindered base related to pyridine would be 2,4,6-collidine, or even better, 2,6-di-t-butyl 4 methyl pyridine.
Can we react alkanols with strong acids to give an ester product
If by alkanol you mean, alcohol, no. They are at the wrong oxidation state.
Typo: in the second step of the mechanism, pyridine removes a proton (arrow shifted).