The Student Room Group
H and CH3 are in the plane of the paper. The other two groups are pointing out and into the plane. You imagine CHO (the one coming out towards) you as a handle which you grab and then you pull it up to occupy the position where H was before. This then causes H to occupy the position of the one going away from you (as you wanted) and now Cl is pointing towards you. CH3 is in the plane and remains in the same position.

Now assign priorities and you will see that it is R (clockwise).
Reply 2
i would suggest imagine looking at the molecule as a tetrahedron and looking at it from the base with the lowest priority group (H) pointing away
however, I disagree with the isomer - I think it is S - the highest priority is Cl, then CHO, then CH3

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