Elimination of Alcohols To Alkenes With POCl3

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

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:

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

Notes

(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
   DOI: 10.1021/ja01527a028
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
   DOI: 10.1021/jo00263a020
   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
   DOI: 10.1021/ja00272a046
   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).