In the October 2018 PTC Reaction of the Month, we asked for suggestions to explain the mechanism for the tetramethylammonium chloride catalyzed esterification of methacrylic acid with epichlorohydrin. Peter Kapferer, a process chemist at a well-known manufacturer in Switzerland, proposed the following mechanism that is quite plausible.
Peter wrote: Opening of the epoxide by the Quat-chloride would, in the absence of any inorganic cations, lead to Quat-1,3-dichloro-2-propanolate, a strong base. This could take the proton from methacrylic acid, and then Quat-methacrylate reacts with Quat-1,3-dichloro-2-propanol to form the ester and Quat-Cl, closing the catalytic cycle. There are a few examples in the literature where tertiary amines and an epoxide form strong alkoxide bases. The advantage of the two-stage process could thus be to carry out the esterification under more or less neutral conditions (with only traces of a base formed in-situ), avoiding the formation of oligomers by repeated addition of epichlorohydrin. As the ester forms, the base “disappears”.
We thank Dr. Kapferer for this contribution and continued interest in the PTC Tip of the Month for more than a decade.
We also corrected the name of the inhibitor shown in the diagram in the October 2018 PTC Reaction of the Month,
About Marc Halpern
Dr. Halpern is founder and president of PTC Organics, Inc., the only company dedicated exclusively to developing low-cost high-performance green chemistry processes for the manufacture of organic chemicals using Phase Transfer Catalysis. Dr. Halpern has innovated PTC breakthroughs for pharmaceuticals, agrochemicals, petrochemicals, monomers, polymers, flavors & fragrances, dyes & pigments and solvents. Dr. Halpern has provided PTC services on-site at more than 260 industrial process R&D departments in 37 countries and has helped chemical companies save > $200 million. Dr. Halpern co-authored five books including the best-selling “Phase-Transfer Catalysis: Fundamentals, Applications and Industrial Perspectives” and has presented the 2-day course “Practical Phase-Transfer Catalysis” at 50 locations in the US, Europe and Asia.
Dr. Halpern founded the journal “Industrial Phase-Transfer Catalysis” and “The PTC Tip of the Month” enjoyed by 2,100 qualified subscribers, now beyond 130 issues. In 2014, Dr. Halpern is celebrating his 30th year in the chemical industry, including serving as a process chemist at Dow Chemical, a supervisor of process chemistry at ICI, Director of R&D at Sybron Chemicals and founder and president of PTC Organics Inc. (15 years) and PTC Communications Inc. (20 years). Dr. Halpern also co-founded PTC Interface Inc. in 1989 and PTC Value Recovery Inc. in 1999. His academic breakthroughs include the PTC pKa Guidelines, the q-value for quat accessibility and he has achieved industrial PTC breakthroughs for a dozen strong base reactions as well as esterifications, transesterifications, epoxidations and chloromethylations plus contributed to more than 100 other industrial PTC process development projects.
Dr. Halpern has dedicated his adult life to his family and to phase-transfer catalysis (in that order!).
