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

Here's how I do it.

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.