How to assign R and S, solved step by step

Problem. Is the chiral carbon of 2-bromobutane R or S? In the drawing, Br is on a wedge (toward you), H is on a dash (away from you), CH₃ is on the left and CH₂CH₃ is on the right.

Step by step

1. Assign R or S to the chiral carbon of 2-bromobutane.

   R or S? 2-bromobutane

2. Here's the stereocenter in the middle, with methyl on the left and ethyl on the right.

3. Bromine comes out toward us on a wedge.

4. Hydrogen goes back, away from us, on a dash.

5. Step one: rank by atomic number. Bromine is biggest, so it's priority one.

6. Hydrogen is smallest, so it's priority four.

7. Methyl and ethyl are both carbon, so compare what's attached to each carbon.

8. Ethyl carries carbon, hydrogen, hydrogen, which beats hydrogen, hydrogen, hydrogen. Ethyl is two, methyl is three.

9. Now trace one, two, three: that sweeps clockwise.

10. Hydrogen, the lowest priority, points away on the dash, so we read the direction as drawn.

11. Clockwise means R.

12. Rule to remember: lowest priority away, then clockwise is R, counterclockwise is S.

Answer: R. As drawn, this is (R)-2-bromobutane.