Hello, my name is Mark and I've been refreshing my organic chemistry knowledge lately and after reading about carboxylic acids derivatives and their reactions i stumbled upon something that got my attention. So basically the most reactive species of carboxylic acid derivatives are obviously their acyl halides. So lets focus on the most common one - The chloride. There are plenty of ways to form them from which the most common are reactions with thionyl chloride (SOCl2
), phosphorus(III) chloride (PCl3
), phosphorus(V) chloride (PCl5
) or little bit more exotic oxalyl chloride (C2
). The reaction with SOCl2
is quite simple reaction in which SO2
and HCl are evolved as side products. In the reaction with PCl5
, the POCl3
and HCl are evolved, and in the reaction with PCl3
is evolved. The oxalyl chloride reaction is also catalyzed by addition of little bit of DMF that forms imidolyl chloride intermediate products like in the Vilsmeier–Haack reaction reaction with POCl3
. But in some of the articles that i've read i saw that POCl3
can be also used as a chlorinating agent in carboxylic acid conversion to acyl chlorides. From what i know, POCl3
is used mainly in alcohol dehydratation reactions and generally in the E2 elimination reactions, not to mention that it is also a side product of using PCl5
, but in those paperworks, to be exact:
Christian A. G. N. Montalbetti* and Virginie Falque Amide bond formation and peptide coupling
Tetrahedron report number 740
ROGER ADAMS,O. KAMM, C. S. MARVEL Organic Chemical Reagents
UNIVERSITY OF ILLINOIS Vol. XVI June 23, 1919 No. 43
is mentioned as a chlorinating agent. There is really no other paperworks that i could find that mention POCl3
as a chlorinating agent.
So my wonder is what is the mechanism of the reaction and when would it be favoured over the common ones.
Moreover in this paperwork from 1919 there is a quite interesting result. The synthesis of acetyl chloride was carried by refluxing sodium acetate with POCl3
The most interesting thing is that in the article there is mentioned that
The acetyl chloride which is thus obtained, is
very much superior to the acetyl chloride formed by the action of
phosphorus trichloride upon acetic acid.
The acetyl chloride as it is formed is so pure that by using a
fractionating column on the original reaction mixture flask, pure
acetyl chloride may be distilled directly and no redistillation is
I couldnt find any reaction mechanism for the reaction of chlorinatin carboxylic acids with POCl3
so i made one myself (analogue to the SOCl2
Is this a valid mechanism ? If not then what is the real mechanism ? Is it similiar to the imidolyl chloride intermediate formation ? As it was mentioned in the Amide bond formation and peptide coupling
paperwork, so maybe it forms intermediate product with amines and acyl chloride forms only in the presence of amines ?