Finding R and S for Tricky Examples

Key Questions

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 $\text{C-1}$, draw a wedge, a dash, and a solid bond.

Step 4. Put the $\text{H}$ on the dashed bond. Then add the $\text{Br}$ and $\text{Cl}$ 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.

$\text{Br}$ = 1; $\text{Cl}$ = 2; $\text{C-2}$ = 3; $\text{H}$ = 4.

Step 6. Assign stereochemistry

Then $\text{Br}$$\text{Cl}$$\text{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 $\text{Cl}$ and $\text{Br}$ atoms converts the ($S$) isomer to the ($R$) isomer.

The structure is below...

Explanation:

Both $C l$ groups should be pointing backward. After assigning priorities, the rotation will be counter-clock wise (which means $S$) however because the highest priority group is pointing backward, the $S$ becomes $R$.

A video on labeling $R$ and $S$ is here: