Elimination of alcohols to give alkenes using POCl3

by James

Description: When treated with phosphorus oxychloride (POCl3) alcohols are converted to alkenes via an elimination reaction.
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Notes: Note that the more substituted alkene is formed (Zaitsev product)

Examples:

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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).

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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.

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{ 6 comments… read them below or add one }

Joseph Blair

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.

Reply

Quinn Haynie

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!

Reply

navjot

Does this elimination involves an intermediate carbocation? Please if u could tell me !!

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james

No, I believe this is essentially an E2 reaction.

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Alexander Peltekoff

We were taught that pyridine has to be present for this reaction to take place. Is it not necessary?

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James Ashenhurst

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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