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

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Answer 1 · 2015-07-16T21:40:56

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.