¹³ C NMR study of the acid-catalyzed carbonylation of methyl tert-butyl ether (MTBE)

Methyl tert-butyl ether (MTBE) is a widely used additive in oxygenated gasoline that has recently been identified as a potential health threat to the drinking water supply due to leaking underground storage tanks. One alternate use for MTBE is the production of methyl 2,2-dimethylpropanoate (methyl privalate) via Koch carbonylation chemistry. BF ₃ /H ₂ O catalysts are employed in industrial applications of Koch chemistry, but cannot be used for direct ester production because the presence of water in the system leads to the formation of carboxylic acids and lowers the selectivity to esters. Therefore, a BF ₃ /CH ₃ OH complex was investigated for the carbonylation of MTBE to avoid this loss in selectivity. This study used ¹³ C NMR spectroscopy and ab initio calculations to investigate this carbonylation reaction. NMR results and ab initio calculations suggest a structure for the BF ₃ /CH ₃ OH acid which is in agreement with previous studies, and a Hammett acidity value of -4.2 was calculated for BF ₃ -2.19CH ₃ OH using the Δδ method. It is believed that these are the first reported ab initio calculations on the BF ₃ /CH ₃ OH system. NMR results also show that MTBE begins to react between 50°C and 75°C to produce oligomers of isobutene when no CO is present and carbonylated species when CO is present.