R1R2COSSR3R4

R1R2COSSR3R4, also known as thioesters, are a class of organic compounds characterized by a carbonyl group (C=O) bonded to a sulfur atom, which in turn is bonded to an alkyl or aryl group. The general formula R1R2COSSR3R4 indicates that the carbonyl carbon is attached to two organic substituents (R1 and R2) which can be the same or different. The sulfur atom is bonded to a third organic substituent (R3), and the sulfur atom is also part of a thioester linkage. Often, the R3 group represents the "thio" part of the ester, meaning the sulfur atom replaces an oxygen atom in a conventional ester. In some contexts, R4 might represent a hydrogen atom or another simple substituent on the sulfur, but the core functional group is the -CO-S- linkage.

Thioesters are important in biochemistry and organic chemistry. In biological systems, they play crucial roles in metabolism, particularly in the transfer of acyl groups. For example, acetyl-CoA is a vital thioester involved in the citric acid cycle and fatty acid metabolism. The carbon-sulfur bond in thioesters is generally weaker and more polarizable than the carbon-oxygen bond in regular esters, making the carbonyl carbon more susceptible to nucleophilic attack. This increased reactivity is key to their function as acyl group donors.

In organic synthesis, thioesters can be prepared through various methods, including the reaction of carboxylic acids with thiols in the presence of coupling agents, or the reaction of acid chlorides with thiols. They can then be used as intermediates in a variety of transformations, such as acylation reactions and the synthesis of other sulfur-containing compounds. Their distinct chemical properties differentiate them from their oxygen ester counterparts, offering unique reactivity profiles for chemists.