[tenchan]

Hi ^^

I would just like to confirm some things about this mechanism... I couldn't find the exact mech for this reaction, but I found the mechanisms for the formation of acyl halides from acids and SOCl2 and the formation of alkyl halides from alcohols and PBr3.  So I'm thinking the mech for acid & Pcl3 would just be slightly different from both..

Would the mechanism end with the formation of HOPCl2 initially, and would a 2nd and 3rd reaction with more acid convert it to H3PO3?

Also, I'd be very grateful if anyone has the exact mechanism.

Thanks very much ^^

[lolly_1116]

The mechanism for the reaction of acid and PCl3 should be the same as that for the reaction of acid and PBr3, since PCl3 and PBr3 have similar properties, both are halides of P.

[tenchan]

I didn't find the mech for acid & PBr3, it was the mech for alcohol and PBr3.  But would it be the same, except for the delocalization of the carbonyl group in the acid, and would it end with HOPCl2?

[movies]

I'm not certain of what the final phosphorous species would be, but my guess would be H₂PO₃.

The mechanism is similar to the alcohol --> halide reaction, although not exactly the same.

[tenchan]

Thanks for the reply ^^

In the alcohol to halide reaction, the mechanism I found showed that the lone electron pair on oxygen attacked the P of PCl3.  Why is that?  I'm confused coz PCl3 has a lone electron pair as well...would the cl atoms give it a slightly positive charge due to their electron withdrawing effects?  

[movies]

Atoms in the third row behave a little bit differently than those in the second row.  So, while phosphorus does have a lone pair of electrons, it is also electropositive enough to be attacked by an electron rich atom like oxygen in this case.  You will lose a Cl from the PCl₃ however, so the oxidation state of phosphorous doesn't change in that step.

[laotree]

Take a look! The Oxgen has very stong affinity with P derivatives.

[Winga]

I think it is better to illustrate that using a lone pair of oxygen to attack P instead of using O-H bond pair.

[movies]

Winga makes a good point about mechanistic formalism.  When I draw mechansims like this I usually invoke the lone pair through resonance to the carbonyl group, since the carbonyl oxygen is more basic and probably more nucleophilic.  (See the attached picture)

It's kinda nit-picky...

[tenchan]

Thanks so much for the replies ^^  I think I'm sticking with the lone e pair on O of the hydroxyl attacking P though.

I have another question howver...I'm not sure if I should start a new topic or if I should just add this here...

This mechanism is actually part of the Hell-Volhard-Zelinskii reaction and there's a part there where the enol form of the acyl halide is halogenated at the alpha position.  My question is, since the acid can undergo tautomerism as well, why the need to convert it to the acyl halide?  I read that acyl halides are more reactive, but I think that reactivity has to do mostly with the carbonyl carbon and not the alpha carbon. Would the presence of the chlorine atom have an effect on keto-enol tautomerism? And would that effect make it easier to halogenate the alpha carbon?    Thanks so much for any ideas.

[movies]

Well, to make the enol tautomer you have to remove a proton that has a pKa of about 25 or so.  In the case of the carboxylic acid, the proton on the carboxylic acid has a pKa of about 5, so it will definitely be the first proton pulled off.  Once the acid is deprotonated then you have a carboxylate anion with two resonance structures.  Think about what effect this would have on the acidity of the protons alpha to the carbonyl group.  Those protons are acidic because the carbonyl carbon is electron deficient.  Once you have a carboxylate anion there is a lot more electron density floating around that carbon (the carboxylate is negatively charged), so the carbon isn't nearly as electron deficient and the alpha protons aren't as acidic.  In the case of the acid chloride, however, you make the carbonyl carbon somewhat more acidic because you introduce an inductive electron withdrawing group next to an already electron deficient carbon atom.  Plus, you can't make a carboxylate anion then.

You should also be aware that a common side reaction of deprotonating an acid chloride is the formation of a ketene by elimination of chloride.