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By James Ashenhurst

Ephedrine and Pseudoephedrine

Last updated: March 27th, 2019 |

These molecules make the news quite a bit.

1-pseudoephedrine

Some questions to think about:

  • What’s the (R),(S) designation for the stereocenters of ephedrine and pseudoephedrine, as drawn?
  • What’s best way to describe the relationship between these two compounds? Enantiomers? Diastereomers?
  • Let’s say you removed the hydroxyl groups of ephedrine and pseudoephedrine and replaced them with hydrogen. Would the two compounds be enantiomers, diastereomers, or the same?

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Comments

Comment section

11 thoughts on “Ephedrine and Pseudoephedrine

  1. Hmmm. I’m thinking they’re not enantiomers — my memory is that every stereo center would need to be flipped. My orgo skills are rusty but my guess is (R,R) for ephedrine and (S, R) or pseudoephedrine (but I can remember where i’d start numbering carbons so maybe that’s flipped).

    I say they’re disasteriomers and they’d be the same compound if you replaced the hydroxyl groups with hydrogen.

  2. right on all counts! [EDIT] – Whoops. actually ephedrine is (S,R) and pseudoephedrine is (S,S). They are diastereomers though.

    especially the last one… meth makers are equally happy to obtain ephedrine or psuedoephedrine, since removal of the oxygen converts either one to methamphetamine.

      1. not incorrect…. they are diastereomers…but there is no flipping of anything…as james said, you end up with the same molecule when the hydroxyls are replaced by hydrogen.

        1. I get just as Ryan, R,S for Ephedrine and S,S for pseudoephedrine.

          Can you explain me what is wrong in my way of thinking? My reasoning is the following Ephedrine is 2-(methylamino)-1-phenylpropan-1-ol and the first carbon is 1R because the priority is 1) -OH; 2) -CNR; 3) -phenyl and the second carbon is 2S because the priority is 1) -NR; 2) -CHOHC6H5; 3) -CH3.

    1. For the images above I get 1R, 2S and 1S, 2S. But there are two configurations missing. Ephedrine has two enantiomers, 1R,2S and 1S,2R and pseudoephedrine has two, 1S,2S and 1R,2R. But most meth cooks are only interested in the -2S- versions, as they result in dextro-methamphetamine. Levo-methamphetamine (2R versions) is not desired on the street and is actually available as an over the counter decongestant in the USA.

  3. Perhaps this is a dumb question. But after looking at those two molecules, they look EXACTLY the same. Except that one OH is angled differently that the other. Will there be any difference in molecular properties because of this?

    1. I think only in the rotation of polarized light. But I do remember reading something about each pair of enantiomers having slightly different melting points. I assumed it was human error.

    1. From scratch? First, put the hydrogen atoms in the heart of a star, and wait for them to combine into helium. Then wait a little longer until the star has consumed its nuclear fuel, whereupon it will start fusing helium into higher nuclei, including carbon and nitrogen. Then wait for the star to explode, spreading its atoms far and wide throughout the galaxy…

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