Question

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

Answer 1

Here's how I do it.

Explanation:

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.