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The Single Swap Rule

There aren’t that many “hacks” or “tricks” or “shortcuts” in ochem, but there are a few. Here’s a little one that comes in handy. It’s called the Single Swap Rule.

If you have a stereocenter – let’s say it’s R – and swap any two groups on it, the result will be the S.

Try it for yourself:

Why is this useful?

  • If you’re trying to determine R or S for a molecule, but the #4 substituent is in the front, do a single swap to put it in the back and determine whether the 1-2-3 substituents rotate clockwise or counter-clockwise from there. [Dont forget though, that the stereocenter will be the opposite of what you obtain (since you did a single-swap)].
  • It makes drawing the enantiomer of any molecule a cinch. All you need to do is do a single swap on each stereocenter and you’re done.
  • It will also come in handy with the SN2 reaction (coming up) which inverts stereochemistry. To draw the product, just redraw the starting material (with the new bond attached) and do a single swap.

Tomorrow: how to figure out how molecules are related.

Thanks for reading! James

PS The Single Swap Rule