homework - Absolute configuration when hydrogen (lowest priority group) is in the plane of the page (and not a wedge or dash)

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.

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.

Answer

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.