MHB Thioester Isomer Count: ${C}_{4}{H}_{8}OS$ - 4 Possibilities

[MermaidWonders]

How many isomers are there with the following description?
- Thioesters with the formula ${C}_{4}{H}_{8}OS$?

I was able to draw 2 of them, but apparently, the answer key showed and stated that there are 4. I am confused about why the following two are possibliities:

View attachment 8117

I thought that thioesters are the functional groups where a C double-bonded to O is attached to both an R group and S, with S itself being attached to an R group. So, I was wondering how the 2 structures in the above attachment could be thioesters when the C that is double-bonded to O isn't attached to an R group but is instead to an H?

 

[I like Serena]

How many isomers are there with the following description?
- Thioesters with the formula ${C}_{4}{H}_{8}OS$?

I was able to draw 2 of them, but apparently, the answer key showed and stated that there are 4. I am confused about why the following two are possibliities:
I thought that thioesters are the functional groups where a C double-bonded to O is attached to both an R group and S, with S itself being attached to an R group. So, I was wondering how the 2 structures in the above attachment could be thioesters when the C that is double-bonded to O isn't attached to an R group but is instead to an H?

Hi MermaidWonders!

From https://en.wikipedia.org/wiki/Thioester:

In chemistry thioesters are compounds with the functional group R–S–CO–R'.
View attachment 8118​

So we can have both an R-group at the double-bonded C, and we can have another R'-group at the S.

 

[MermaidWonders]

Hi MermaidWonders!

From https://en.wikipedia.org/wiki/Thioester:

In chemistry thioesters are compounds with the functional group R–S–CO–R'.
​

So we can have both an R-group at the double-bonded C, and we can have another R'-group a the S.

Right. So doesn't it mean that there must be an R group attached to the double-bonded C, which means that the above isomers in the attached image are not valid structures since C is attached to H in both?

 

[MermaidWonders]

Wait... does H count as an R group?

 

[I like Serena]

Yep. H counts as an R-group. (Nod)

 

[MermaidWonders]

Yep. H counts as an R-group. (Nod)

Makes sense now. Thanks! :)