Thiocarboxylic acids, protonation

Unlike carboxylic esters, open-chained S-alkyl thiocarboxylates cannot be obtained by direct proton-catalyzed esterification. Whereas thiocarboxylic acids react with alcohols to form esters as expected, thiol esters are formed on reaction of carboxylic acids with thiols, but the equilibrium is shifted towards the educts and the reaction is not useful for synthetic purposes. Therefore, activation is necessary and several methods have been developed to meet this requirement. 

Thiols and sulfides are protonated on sulfur in superacid media and give mono- and dialkylsulfonium ions, respectively49. Thiocarboxylic acids, S-alkyl esters, thio-esters, dithioesters and thiocarbonates in similar media also form stable protonated ions179 such as... 

After irradiation, the HABI molecules decompose into lophyl radicals which subsequently abstract protons from a donor, e.g. thiols [BER 14] (Figure 4.9), heteroaromatic thiols, benzothiazoles, benzooxazoles, tertiary amines, alcohols, thiocarboxylic acids [BLA 09] or dyes [BEN 02], If used in a dental composite, they lead to improved compressive, yield and flexural strengths compared to conventional composites with CQ/DMABE. Furthermore, the sensitivity to ambient light could be decreased [BLA 09]. 

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