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I am doing some research on propanethial S-oxide, a chemical produced by onions. I am trying to find out whether they have a single or double bond between the sulfur and oxygen atom. I have seen both from different sources, simply search it up on Google Images. Is this because there is resonance going on? Also is this considered a sulfinyl functional group (I am guessing no because it cannot be on the end of a chain)?

In addition, is the C double-bond S part in this molecule considered a thial functional group? A Google search indicates that the atom bonded to the sulfur must be a hydrogen for it to be a thial. If not, what type of functional group (if any) exists between the C, S, or O?

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The S-O bond in sulfoxides is best consdered as a single covalent bond with +ve charge on S and -ve on Oxygen. The Oxygen is significantly nucleophilic (see for example the Swern Oxidation mechanism 1 ) and the Sulfur will stabilise an anion on the alpha carbon.

C=S has significant enol content and is better considered HS-C=CR. The S is significantly nucleophilic and is easily oxidised to the ene-disulfide RC=C-S-S-C=CR

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