We're aware that ketones and alcohols both react with phosphorus pentachloride, to produce geminal dichlorides and alkyl chlorides respectively.

What happens in the compound shown below, when treated with $\ce{PCl5}$? Which group reacts first? (which is to say, what happens if I were to add just one equivalent of $\ce{PCl5}$?)

enter image description here

Please help me out here, I'm not able to qualitatively predict an order.


1 Answer 1


Reaction of alcohols with PCl5 and PCl3 (for reaction with alcohol) Mechanism for conversion of ketone to dichloride with phosphorus pentachloride (for reaction with ketone) Now as per the question as it is not given in excess so it will react with any one of the alcohol or ketone. It's quite hard to tell which one of them will react first because both of them show totally different kinds of reactions. Like ketones prefer neucleophile attack and alcohols prefer substitution and elimination. But if you look at the mechanism quite closely you would find all the favourability of the reaction depends on the lone pair donation tendency of oxygen. Now as you know, in sp³ hybridization state the s -character is nearly 25% and in sp² hybridization state s-character is nearly 33% and as s-character increases electronegativity increases Why a higher s character increases a carbon atom's electronegativity?

and the tendency to hold the electron pair increases so for ketone it is quite difficult to donate it's electron pair to PCl5. So in that perspective I can say alcohol is quite more reactive than ketone in this case.

  • $\begingroup$ @arya_stark, I think from a thermodynamic perspective, ROH would be more reactive. In both cases the same product POCl3 is being formed but with the ketone a strong C=O is being broken which is unfavourable. $\endgroup$ Commented Jan 1, 2020 at 2:44

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