# Determining absolute configuration when the lowest priority group is not facing backwards

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?

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.

• 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. Dec 23, 2020 at 3:16
• Here is my favorite method: chemistry.stackexchange.com/questions/99721/… Dec 23, 2020 at 14:58