What do we know about the molecular mechanism of 3‐MCPD ester formation?

The 3-monochloropropane-1,2-diol (3-MCPD) esters belong to a group of well known process-induced contaminants derived from mono- and di-chlorinated glycerols. The 3-MCPD was first identified in 1978 in acid-hydrolysed vegetable proteins and its corresponding esters in 1980 and since their discovery almost 30 years later the exact mechanism of their formation is still unknown. The recent renewed interest in their formation mechanism can be attributed to the realization that the ester content in real food systems often exceeds that of free 3-MCPD content by many orders of magnitude. Presently, there are four proposed mechanisms all involving SN2 nucleophilic attack by chloride ions but differing from each other based on either the nature of the substrate or the leaving group. Two of the proposed mechanisms involve direct nucleophilic attack by the chloride ion at the glycerol carbon atoms carrying either an ester group or a protonated hydroxyl group. The other two pathways propose the formation of reactive intermediates such as acyloxonium ion or an epoxide ring in the form of glycidol prior to the nucleophilic attack by chloride ions.