[tenchan]

Hi ^^

I had to synthesize monochloroacetic acid for our laboratory class, and the procedure I found said that the rate of chlorination would be dependent on the amount of illumination.  I read that radiant energy causes homolytic cleavage of the Cl-Cl bonds which initiates a free radical reaction.  However, I used phosphorus as halogen carrier in the experiment and I was wondering if sunlight is still required for the reaction between phosphorus and chlorine to occur?  That is, is sunlight needed for the two to form the phosphorus trichloride involved in the Hell-Volhard-Zelinsky reaction?  I didn't read anything about sunlight being needed for P and Cl2 to react so I thought maybe sunlight would have some other role...

Thanks very much for any ideas ^^

[HP]

No you dont need the sun light for this reaction because the mechanism is different. The phospphorus here play intermediate role as forming PCl3 with Cl2. Three moles of the carboacid react with 1 mole of PCl3  to form an acylchloride and H3PO3. The acylcloride readily exist in its enole form,
and this tautomer is rapidly chlorinated at the ?-carbon.The monochlorinated compound is much less nucleophilic, so the reaction stops at this stage..As a very young chemist i like the name of this reaction best and alredy new the mechanism of the reaction  

[tenchan]

Thanks for the reply ^^

That is why I was wondering why the synthesis had to be carried out in the presence of sunlight.  If I don't need it for the Hell-Volhard-Zelinsky reaction, then what is the radiant energy for?  All the procedures I read stated that the reaction had to be done in the presence of sunlight.  And this has been puzzling me...

Is it also possible that another mechanism other than the HVZ reaction occurs?  That is, can acetic acid be chlorinated at the alpha position via a free radical reaction?

Again, thanks for any ideas ^^

[HP]

Well sunlight activate chlorine in the initial stage of its reaction with the red phosphorum to form PCl3(exsothermic reaction) but later it dont affect the reaction i think . If Cl2 is in big excess and you have sunlight in the reactor all the time its possible to obtain di and may be trichloroacetic acid by free radical machanism i suppose.  Again on the mechanism reaction: a key role there play the acylchloride mediate which enol form is readily chlorinate(Ae mechanism)  at the ?-carbon. This acyl intermediate allow the catalitic cycle  to continue until the conversion is complete. So for this reaction phosphorous  really play the role of catalizator for the initiation of the reaction-obtaining the acylchloride and later reaction proceed between Cl2,acetic acid and the intermediates i mentioned. If you use acetic acid and catalitic amounts of acylchloride the result should be simmilar i think but may be kinetic will be different.In the industry chloroacetic acid is obtain by chlorination of mixture of acetic acid and acetic anhydride which with chlorine in situ generate acylchloride and so on and so on

[tenchan]

Would the activation of chlorine involve breaking Cl-Cl bonds to form Cl free radicals?  And uhm, what exactly is the mechanism for the formation of PCl3? Is it a simple redox rxn or would free radicals also be involved?

Also, since you mentioned that the formation of PCl3 is exothermic...the procedure said that I was supposed to heat the reaction vessel containing P and Cl2...That's kinda confusing  

sorry for the many questions ^^;  Thanks for the reply!