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?
The priorities are
- Carbon chain on left
- 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.