The molecule is such:

My working is such:

But my supervisor said the molecule is actually R configuration. Could someone please explain this and tell me where I went wrong?

  • $\begingroup$ This is the typical question that I can never answer for complex cases about ghost atoms. Either the ghost atoms count less than proper atoms of the same type. In that case, isopropene has a higher priority (two proper one ghost over one proper two ghosts). Or ghost and proper atoms count the same, in which case we need to expand the first (proper) C to give (C) (C) H versus (C) H H where the second ghost C wins over H and thus alkyne wins. It’s nice that two different searching methods give different results (see below). $\endgroup$ – Jan Dec 11 '15 at 1:38

I think your working is correct, and the molecule should be assigned as (S), with the alkyne taking priority over the disubstituted alkene such that the priority goes:

  1. alcohol
  2. alkyne
  3. alkene
  4. hydrogen

This leads to the (S) configuration, as you propose.

There is a chance that your supervisor is confused because in IUPAC naming nomenclature, the alkene often (and indeed, in this case) takes priority over the alkyne, which would of course lead to assignment of the molecule as (R).

  • 1
    $\begingroup$ Chemdraw agrees with this analysis. $\endgroup$ – jerepierre Dec 10 '15 at 1:16
  • $\begingroup$ Chemdoodle disagrees and says the molecule is (3R)-2-Methyl-1-penten-4-yn-3-ol. $\endgroup$ – Curt F. Dec 10 '15 at 4:22

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