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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: tolerant on July 23, 2014, 08:04:57 AM

Title: isomer naming question
Post by: tolerant on July 23, 2014, 08:04:57 AM

I have a question about representation in writing of molecule isomers.

Take for example the following: (2S,6S;2R,6R)-hydroxynorketamine.

Does this formula:

1. Represent a mixture of two isomers, namely (2S,6S)-HNK and (2R,6R)-HNK? Does it tell us anything about the proportions of each isomer in the mixture?

2. Represent a separate compound different from both of the abovementioned isomers?

3. Represent a short-hand for listing the two isomers, i.e. it is given instead of saying "(2S,6S)-HNK and (2R,6R)-HNK", but does not represent a mixture of these compounds.

Thanks!

Title: Re: isomer naming question
Post by: Dan on July 23, 2014, 08:42:43 AM

I assume it refers to a racemate of the cis* diastereoisomer.

*with respect to the OH and NH₂

Title: Re: isomer naming question
Post by: sjb on July 23, 2014, 09:33:25 AM

Quote from: tolerant on July 23, 2014, 08:04:57 AM

I have a question about representation in writing of molecule isomers.

Take for example the following: (2S,6S;2R,6R)-hydroxynorketamine.

Does this formula:

1. Represent a mixture of two isomers, namely (2S,6S)-HNK and (2R,6R)-HNK? Does it tell us anything about the proportions of each isomer in the mixture?

2. Represent a separate compound different from both of the abovementioned isomers?

3. Represent a short-hand for listing the two isomers, i.e. it is given instead of saying "(2S,6S)-HNK and (2R,6R)-HNK", but does not represent a mixture of these compounds.

Thanks!

My understanding would be most akin to example one. i.e. we know that this is the Z isomer pair but not how much of each enantiomer is present.

Title: Re: isomer naming question
Post by: Dan on July 23, 2014, 10:05:05 AM

Quote from: sjb on July 23, 2014, 09:33:25 AM

we know that this is the Z isomer pair

I don't think there's a double bond. It's this one right?

Title: Re: isomer naming question
Post by: sjb on July 24, 2014, 09:55:57 AM

Sorry, had in my mind Z as a synonym for cis in all cases; agreed that cis is probably better but as you've said we need to be clear to what it refers to.

Title: Re: isomer naming question
Post by: tolerant on July 25, 2014, 06:12:19 AM

Hi guys!

Thanks for replying. I come from a non-chemistry background, so this is all new to me. I came across this terminology in a medical journal. I had it confirmed on another forum that it indeed represents option (1) identified in my original post. But judging from your responses, this is not standard chemical notation and doesn't reveal everything about the compound (i.e. is not a substitute for a chemical formula). Would that be right?

Title: Re: isomer naming question
Post by: Dan on July 25, 2014, 10:32:20 AM

Quote from: tolerant on July 25, 2014, 06:12:19 AM

this is not standard chemical notation and doesn't reveal everything about the compound (i.e. is not a substitute for a chemical formula). Would that be right?

No, R/S nomenclature is the IUPAC standard. What it tells you is that you have a mixture of the R,R and S,S isomers, but no R,S or S,R. The name you gave does not explicitly give the ratio.

i.e. you have these (the O,N-cis isomers):



but not these (the O,N-trans isomers):

Title: Re: isomer naming question
Post by: Vidya on November 21, 2016, 03:44:44 AM

Quote from: tolerant on July 23, 2014, 08:04:57 AM

I have a question about representation in writing of molecule isomers.

Take for example the following: (2S,6S;2R,6R)-hydroxynorketamine.

Thanks!

These are enantiomers and in racemic mixture are present in 50:50 ratio.
https://en.wikipedia.org/wiki/Racemic_mixture