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I have learnt that:

Our absolute configuration is R(if clockwise) or S(if anticlockwise) when we rank the priorities, when the hydrogen (or other lowest priority group) is at the rear i.e. into the page. If the hydrogen is coming out of the page (i.e. as a wedge) we swap our answer from R to S or vice versa (to get the hydrogen going into the page).

What do we do when the hydrogen is flat and not a wedge or dash?

E.g.

enter image description here

The priorities are

  1. Cl
  2. Carbon chain on left
  3. Carbon chain on right

This gives an anticlockwise (S) configuration. But the answer is given as clockwise or (R) configuration.

Why is this the case? The hydrogen isn't coming out of the page. Is there something else that has to be done when the hydrogen is flat or in the plane of the page? If I visualise it rotating to bring the H to the back, I still end up with an (S) configuration.

Thank you.

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You had the right idea when you said if the hydrogen is coming out of the page (i.e. as a wedge) we swap our answer from R to S or vice versa (to get the hydrogen going into the page).

The difficulty seems to have came with your manipulation of the drawing. The diagram you have implies that the hydrogen is coming out of the page towards us (i.e. anticlockwise = R).

You could also flip the entire molecule through by 180 to give you the chlorine pointing out of the page towards you.

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  • $\begingroup$ Thanks for the answer @NotWoodard Why does my drawing "imply the hydrogen is coming out of the page"? Is this some sort of convention? I know the Cl was going into the page due to the dashed line, but the hydrogen isn't shown. How did you know it was out of the page? I assumed it was just flat in the plane of the page (like the carbon chains on either side) and not inward or outward. Also, do you mind telling me which drawing software you used for the diagrams please? $\endgroup$ – K-Feldspar Jul 27 '16 at 21:51
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    $\begingroup$ The carbon of interest is tetrahedral. As you drew it, you had the methyl and the propyl group in the plane of the paper, with the chlorine going behind the paper therefore the hydrogen must be coming towards you in order for the carbon to indeed be tetrahedral. The hydrogen is implicit. The software is called ChemDraw, but unless you're at a university it's prohibitively expensive— ChemDoodle is a much cheaper alternative that will do you well throughout your high school/college studies :) $\endgroup$ – NotEvans. Jul 27 '16 at 21:57
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    $\begingroup$ I see. I didn't even think of the tetrahedral shape. Cheers :) $\endgroup$ – K-Feldspar Jul 27 '16 at 22:04

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