How do you draw ( S )-1-bromo-1-chlorobutane?

Step 1. Draw the structure of 1-bromo-1-chlorobutane.

Step 2. To get the stereochemical structure, start with the structure of butane.

Step 3. At #"C-1"#, draw a wedge, a dash, and a solid bond.

Step 4. Put the #"H"# on the dashed bond. Then add the #"Br"# and #"Cl"# to the remaining bonds in any order.

We have a 50:50 chance of getting it right. Here's one possibility.

Step 5. Assign priorities to the groups.

#"Br"# = 1; #"Cl"# = 2; #"C-2"# = 3; #"H"# = 4.

Step 6. Assign stereochemistry

Then #"Br"# → #"Cl"# → #"C-2"# = 1 → 2 → 3 goes in a counterclockwise direction (#S#).

We got it right!

So this really is (#S#)-1-bromo-1-chlorobutane.

If we had gotten it wrong, we would have interchanged any two of the groups.

For example, in the diagram above, interchanging the #"Cl"# and #"Br"# atoms converts the (#S#) isomer to the (#R#) isomer.