If I look up the IUPAC name of ascorbic acid, I find this: (5R)-[(1S)-1,2-Dihydroxyethyl]-3,4-dihydroxyfuran-2(5H)-one.

I am used to numbering of molecules in a way like the 3,4-dihydroxyfuran part in this molecule name. But now what I want to know is: what do the 5R, 1S and 5H parts signify?

For the 5H I came up with this: furan has two double bonds, so the H might signify that this is not the case any more, because if there is an extra H at the fifth position, this would be impossible. This can be seen in the IUPAC name of 2-furanone: 5H-furan-2-one. I don't know though if this assumption is correct, also because in 5H-furan-2-one the 5H is before the furan part and in 3,4-dihydroxyfuran-2(5H)-one it is after (shouldn't it be 2(3,4-dihydroxy-5H)-furanone anyway?)

Fact is that I can't find an explanation for the 5R, nor the 1S (as I see no sulfur). Could anyone explain me these weird molecule numberings?


The numbering of (R)-5-[(S)-1,2-dihydroxyethyl]-3,4-dihydroxyfuran-2(5​H)-one has four parts:

1) Numbering of the furan ring

enter image description here

2) Numbering of the ethyl side chain

enter image description here

3) Stereodescriptors R and S

enter image description here

4) Italicized element symbols

The italic element symbol H denotes indicated or added hydrogen.

| improve this answer | |
  • $\begingroup$ Using the PIN mentioned in the question would be much nicer ;) $\endgroup$ – mykhal Oct 4 '18 at 8:33

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.