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I have learnt that the absolute configuration of a chiral centre can be labelled as (R) or (S) according to the Cahn–Ingold–Prelog rules, when the hydrogen (or other lowest priority group) is facing backwards, i.e. going into into the page, and the remaining groups decrease in priority clockwise or anticlockwise respectively. On the other hand, 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).

But what do we do when the hydrogen is flat and not a wedge or dash, as is the case here?

2-chloropentane

The priorities are

  1. Chlorine atom
  2. Carbon chain on left (propyl group)
  3. Carbon chain on right (methyl group)

This gives an anticlockwise or (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.

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  • $\begingroup$ One of my favorite ways to figure out the absolute configuration is to imagine the tetrahedral chiral center as a steering wheel. Make the lowest priority atom the "axle" and the three other atoms the "wheel". Mentally visualize which way the wheel would have to rotate to achieve 1-2-3. Right is R and left is S. $\endgroup$
    – Eli Jones
    Commented Dec 23, 2020 at 3:16
  • $\begingroup$ Here is my favorite method: chemistry.stackexchange.com/questions/99721/… $\endgroup$
    – user55119
    Commented Dec 23, 2020 at 14:58

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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
    Commented Jul 27, 2016 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.
    Commented Jul 27, 2016 at 21:57
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    $\begingroup$ I see. I didn't even think of the tetrahedral shape. Cheers :) $\endgroup$
    – K-Feldspar
    Commented Jul 27, 2016 at 22:04

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