Constitutional isomers for four carbon thioesters

Question:

How many isomers are there for thioesters with the formula $\ce{C4H8OS}$?

I can see that the following two are possible constitutional isomers:

But what are the other two?

• When I tell you, that dithioesters exist ... will this be a hint? – pH13 - Yet another Philipp Jul 24 '15 at 10:07
• Here's my hint: Formic acid exists. – jerepierre Jul 24 '15 at 12:36
• @jerepierre Didn't thought of that. But then there are more possibilities than 4. – pH13 - Yet another Philipp Jul 24 '15 at 15:09
• @PH13 How you can have a dithioester with only one sulfur? – jerepierre Jul 24 '15 at 15:56

One way to break things down in by the number of carbon atoms on either "side" of the thioester.

There are 4 carbon atoms to work with, of which one will be taken up by the thioester moiety. Thus there are three left to work with.

On the carbonyl "side" of the thioester, between zero and three carbons are possible. However, on the sulfur "side" of the thioester, at least one carbon atom is required; otherwise the compound is not a thioester but a thioacid. Thus the possibilities for carbon atom distribution are:

• one carbon on S side, two on CO side.
• two carbons on S side, one on CO side.
• three carbons on S side, zero on CO side.

You have drawn the only structures for the first two possibilities.

For the last possibility, there are two possible ways to attach a three-carbon alkyl chain to the thioester sulfur:

• as a $n$-propyl group
• as an isopropyl group

Thus, the four possible thioesters are:

1. methyl thiopropanoate
2. ethyl thioacetate
3. n-propyl thioformate
4. isopropyl thioformate
• Actually, it looks like due to haphazard naming of thionoesters (where the O and S atoms of the thioester moiety are reversed), it is less ambiguous to say S-propyl thioformate and S-isopropyl thioformate, etc., at least for formates. People apparently use the name O-propyl thioformate for the thionoester, confusingly. – Curt F. Jul 24 '15 at 19:10
• What people mean by R varies by context. In this case, according my understanding of your question and my understanding of the definition of thioester, the R bonded to the carbonyl carbon could be H, but the R' bonded to the sulfur could not. Unfortunately there isn't a general answer for what folks mean by R in organic chemistry. – Curt F. Jul 24 '15 at 20:46
• Thanks for you input. Just to make sure, is there some link you can share where I could verify what you said and read more about thioesters? I haven't been able to find one by myself. – TheProgrammer Jul 24 '15 at 21:02