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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 $\ce{S-O}$ bond in sulfoxides is best considered as a single covalent bond with positive charge on $\ce{S}$ and negative 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.

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

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  • $\begingroup$ Or, you could say the bonding is complicated and not easily described as a simple single or double bond. Simple models don't always do a good job of describing complex bonds. $\endgroup$ – matt_black Aug 21 '20 at 20:08

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