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What is the specific configuration (R or S) of this compound?

propane-1,2-diol

My answer is R, because $\ce{H}$ is not in the back so I flipped the compound. Is my answer right?

My teacher is telling me it is S, though I'm pretty sure that it is R. That made me very confused.

What I did is that I got S at the beginning, but because $\ce{H}$ is not in the back, I flipped the compound and got the answer R. My teacher flipped the compound in a way that the answer was still S.

$\ce{OH}$ (1), $\ce{CH2OH}$ (2), $\ce{CH3}$ (3), $\ce{H}$ (4) from 1 to 3, anticlockwise which means it is S configuration. But $\ce{H}$ is not in the back, Therefore rotate the compound and the answer should be the opposite to S which means it is R.

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  • $\begingroup$ Well clearly one of you is wrong. Perhaps you could add some diagrams to your question to show what you mean. I have some difficulty in visualising what you are trying to say. $\endgroup$
    – bon
    Commented Feb 17, 2016 at 18:56
  • $\begingroup$ @saf, here's a hint. The H isn't in the front either... $\endgroup$
    – jerepierre
    Commented Feb 17, 2016 at 18:59
  • $\begingroup$ Words of caution about flipping structures: chemistry.stackexchange.com/questions/99721/… $\endgroup$
    – user55119
    Commented May 11, 2019 at 1:53

1 Answer 1

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Your comment is correct where you assign the priority.

Rather than "rotating" or "flipping" the molecule, consider "looking" at it from the angle which makes the H atom be at the back. In the graphic it is drawn to the left side, so place yourself on the right side and look down the bond.

Annotated original image of propan-1,2-diol

The priority groups 1-2-3 run anticlockwise, hence it has the S configuration.

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  • $\begingroup$ Thank you, I always thought that if number 4 was not in the back, I have to rotate the compound. $\endgroup$
    – saf
    Commented Feb 17, 2016 at 20:01
  • $\begingroup$ It depends. For example if you swap the H with the CH3 or CH2OH groups, the initial glance of the priorities still looks anticlockwise but when you look down the bond, you see the configuration is actually flipped! $\endgroup$
    – Spontification
    Commented Feb 17, 2016 at 20:02

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