Master Organic Chemistry Reaction Guide

Elimination of alcohols to give alkenes using POCl3

Description: When treated with phosphorus oxychloride (POCl3) alcohols are converted to alkenes via an elimination reaction.

1-header

Notes: Note that the more substituted alkene is formed (Zaitsev product)

Examples:

2-examples

Notes:  The purpose of pyridine is to perform the key elimination step. Note in the second and third examples that the most substituted alkene is formed (Zaitsev rule).

Mechanism: 1,2-addition of the alcohol oxygen to the phosphorus (Step 1, arrows A and B) is followed by 1,2-elimination of chloride (Step 2, arrows C and D). Finally, deprotonation of the carbon  β to the OPOCl2 leads to an elimination (E2) to give the alkene (Step 3, arrows E, F, and G).

arrow pushing mechanism for formation of alkenes from alcohols using pocl3 and base pyridine to give alkenes

Notes:  Pyridine is used to do the key elimination step.  It would also be reasonable to show a proton transfer step from OH to O in between steps 1 and 2.


(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 POCl3-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).

Comments

Comment section

6 thoughts on “Elimination of alcohols to give alkenes using POCl3

  1. Please forgive if this is an elementary question, but I am trying to extract information that was last used when a 128k Macintosh was 1600.00. The POCl3 : the electronegativities sandwich the poor little P between a rock and a hard place. The Cl wants the density in 3 directions but the O wants it more by .97. Without getting into the vector analysis, will the Cl’s be pulled upward in the same manner but different direction, as H’s in H2O are bent away the electrons on the O.

    Sorry, for what I think Brother Adrian called, Illigitimus Parenthetica.

    1. Electronegativity: O=3.44, Cl=3.16. As I understand, the chlorines will minimize repulsive forces by settling into the optimal angle from the oxygen. The Cl-Cl angles should be the same, and the Cl-O angle should be slightly larger than Cl-Cl. Exact numbers? Try scientific literature!

    1. Pyridine would significantly improve the rate of reaction. I’d put it in. Need to update this page to show that. Thanks!

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