Alcohols, Epoxides and Ethers

By James Ashenhurst

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.

phosphorus oxychloride pocl3 is a dehydration reagent for alcohols leads to formation of alkenes when pyridine is added

Table Of Contents

  1. Two Ways To Convert Alcohols  To Alkenes We’ve Learned Previously
  2. Direct Elimination of Alcohols To Alkenes With POCl3:  The Mechanism
  3. The Reaction Works For Primary, Secondary, and Tertiary Alcohols
  4. Summary: Direct Elimination of Alcohols With POCl3
  5. (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:

mechanism for reaction of alcohols with pocl3 in the presence of pyridine to give alkene generally gives zaitsev product-1

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.
pocl3 to give elimination reactions of alcohols examples of alkenes that are formed zaitsev rule applies trans alkenes favored over cis alkenes

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

  1. 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
  2. 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.
  3. 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.
  4. 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
    DOI: 10.1039/JR9560003500
    This paper by Nobel Laureate Prof. Derek H. R. Barton has a POCl3-pyridine dehydration (see p. 3504-3505 in the experimental section).


Comment section

25 thoughts on “Elimination of Alcohols To Alkenes With POCl3

  1. My instructor says that there is no such reaction of alcohols with POCL3!
    Can you please tell an international reference book in which this reaction might be given.

  2. When treating 3 degree alcohol with POCL3, we get a 3 degree alkyl chlorophosphateester , and if we are adding pyridine which we would consider a weak base ( according to Quick N Dirty Guide for SN1/SN2/E1/E2 , because it is neutral),so should E1 pathway not dominate in this case?

    1. Pyridine isn’t a particularly strong base, but it will perform elimination (E2) reactions when the leaving group is good enough, such as OMs or OTs. This includes the leaving created when POCl3 reacts with alcohols.

  3. 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.

  4. 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.

  5. 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?

  6. The addition of the alcohol to POCl3 is denoted as forming a chlorophosphite. Should this be a chlorophosphate? If not, why not?


  7. 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?

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