How To Tell Enantiomers From Diastereomers

by James

Bottom line for today: you can tell if molecules are enantiomers or diastereomers by looking at their (R,S) designations.

Enantiomers are non-superimposable mirror images of each other. Pardon me while I push the Caps Lock button:

ENANTIOMERS ALWAYS HAVE OPPOSITE R,S DESIGNATIONS.

Buy “opposite” I mean they have the same names, but their R’s and S’s are reversed.

The enantiomer of (S)-2-chlorobutane? It’s  (R)-2-chlorobutane.

The enantiomer of (R)-2-butanol? It’s (S)-2-butanol.

  • Let’s say you’ve got a molecule with two stereocenters and the configuration is (R,R). The enantiomer will be the molecule with the exact same name except it’s (S,S). For instance, the enantiomer of (2R,3R)2-bromo-3-iodobutane is (2S,3S)2-bromo-3-iodobutane.
  • If you’ve got a chiral molecule with two stereocenters and the configuration is (R,S), the enantiomer will have the (S,R) configuration.
  • Taking it even further, if we had a molecule that was (R,R,S,R)-(followed by long chemical name), what would its enantiomer be? It would be (S,S,R,S). Get the idea?

This is why learning to figure out R/S designations is such a key skill! You can figure out whether two molecules are enantiomers (or not) simply by examining their names and their (R,S) designations!

Now, what if we have a molecule with the exact same name, except their (R,S) designations are not opposite, but not identical either?

Like (R,R) and (R,S)…. or (S,S) and (S,R) ?

Or: (R,R,R) and (R,S,R)
Or: (S,S,R,S) and (S,S,S,S)
These molecules will be stereoisomers…. but not enantiomers. So they’re  diastereomers! Caps lock time agein…

DIASTEREOMERS WILL ALWAYS HAVE NON-IDENTICAL (BUT NON-OPPOSITE) R,S DESIGNATIONS. Like (2S,3R)-2,3-butanediol and (2R,3R)-2,3-butanediol.

OK! Quiz time.

1. Maybe you’ve heard of pseudoephedrine (Sudafed)? It’s (S,S)-2-methylamino-1-phenylpropan-1-ol. Guess what its enantiomer is? (Answer below)

2. As you might have guessed,  there’s also a molecule called ephedrine. It’s (R,S)-2-(methylamino)-1-phenylpropan-1-ol. Guess what its enantiomer is?

3. Now guess how ephedrine and pseudoephedrine are related to each other?

Tomorrow: let’s sum up the week.

Thanks for reading! James

P.S.

  1. The enantiomer would be (R,R)-2-methylamino-1-phenylpropan-1-ol
  2. The enantiomer would be (S,R)-2-methylamino-1-phenylpropan-1-ol
  3. Pseudoephedrine (S,S) and ephedrine (R,S) are diastereomers of each other.

P.P.S. You might ask if there are any exceptions to this. Yes and no. Enantiomers always have opposite R,S designations. BUT (and this is important) not all molecules with opposite R,S designations are enantiomers! As we’ll see later, it’s possible for a molecule to have chiral centers but be an achiral molecule due to a plane of symmetry. These are called meso compounds, and we’ll talk about them next week.