1H and 13C N.M.R. studies on the positional isomers of methyl thialaurate and methyl thiastearate

Abstract

¹ H N.m.r. analysis of methyl thialaurate and methyl thiastearate shows that the sulphur atom in the alkyl chain has a significant deshielding effect (α-effect) on the chemical shift of the protons of the adjacent methylene and methyl groups, and also on the methyl protons of the methoxycarbonyl (ester) function. It was possible to identify seven of the positional isomers in each series by examining the chemical shifts and the splitting pattern of the signals in the ¹H n.m.r. spectra of these fatty acid ester analogues. The ¹³C n.m.r. results showed that this S atom has an interesting effect (α-, β-, and γ-effect) on the chemical shift of the carbonyl carbon atom of the methoxycarbonyl (ester) function: the sulphur atom exerts a significant shielding effect in the case of the 2-, 3-, and 4-thia isomers. In the remaining isomers the sulphur atom causes a strong deshielding effect on the methylene or methyl carbon atoms on either side of the sulphide linkage. These results permitted all positional isomers of methyl thialaurate to be identified by this technique, while 10 out of the 16 positional isomers of methyl thiastearate could be characterized.