Looking along the C2-C3 bond of 3-chlorobutan-2-ol, the back carbon is C3 and its groups are Cl at 6 o'clock, $\mathrm{CH}{}_3$ at 2 o'clock and H at 10 o'clock. The front carbon C2 points toward the viewer. What is the CIP descriptor of C3?

The priorities at C3 are Cl, then C2 (which carries O), then $\mathrm{CH}{}_3$, then H. The front carbon C2 points toward the viewer, so exchange H and C2: H is then toward the viewer and C2 is at 10 o\'clock. Cl (6) to C2 (10) to $\mathrm{CH}{}_3$ (2) runs clockwise, which would be R, but with H toward the viewer it is reversed to S for the exchanged structure. Undoing the exchange gives R for the real C3.