Some stereochemistry confusion...

[LetsGo352]

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1.) When you assign R/S configuration, what is the priority of the secondary connection based on? I know the basics, but when one subsitituient is a tertiary carbon and when another is a carbon with 2 H's and a Nitrogen, why does the one with Nitrogen get higher priority? I understand that Nitrogen is higher amu/atomic # than Carbon, but that same carbon also has 2 H's coming off of it. It would seem to me that the tertiary Carbon would get higher priority because, overall, it is bulkier than the other subsitituient.

2.) Also, is it possible to determine which way a molecule rotates polarized light (+/-) just by knowing its configuration (R/S)? Or does we have to be giving more information like (l) and (d).

 

[loveoforganic]

Retracted, the below is correct.

 

[unique135]

1. In R/S configuration (absolute configuration), the substituent priority is simply based on atomic weight. I think configuration based on bulkier sustituents wouldn't simplify rather make naming complex. However, there is a preference for one isomer formation over other due to steric effect/hindrance/bulkiness called stereoselectivity.



2.

• R/S configuration is based on the atomic weight
• Dextrorotatory (d/+)/Levorotatory (l/-) configuration is based on the polarization of light due to isomers
• D/L configuration is based on chirality of glyceraldehyde. Notice D/L configuration is different from d/l configuration.

All configurations are different - none can be determined from another.

 

[LetsGo352]

yeah, I understand that part. But I'm talking about when two atoms are the same, you go to what's connected to those atoms. So, let's say one substituient is a Carbon with 3 methyl groups off of it...and the other substituient is a Carbon with 2 Hydrogens and a NH2 group. The substituient with the 3 methyl groups are bulkier than the other substituient in terms of overall amu. But EK says otherwise...why is that?

 

[MDub1]

Because we're only concerned about the atomic number of the highest number substituent. Thus, the Nitrogen has an atomic number of 14 and the three C's have numbers of 12 so the N gets higher priority. If there were a tie in atomic number, then we look at number of substituents. For example, if one substituent was a tertiary C and the other was a secondary C than the tertiary C would be assigned higher priority.

One thing to remember is that R/S designation is simply a convention used to unambiguously identify stereoisomers. It says nothing about the properties of the molecule, including the direction that it rotates plane-polarized light. Thus even though it may not make sense to give higher priority to the N instead of the 2 C's, it's the convention and that's just the way it's done.

Hope this helps and good luck studying 🙂

 

[LetsGo352]

So as long as there is just one atom with a higher atomic # (in this case nitrogen), then it automatically becomes the higher priority, correct?

 

[MDub1]

Right

 

[unique135]

Agreed. I think the idea was to simplify the naming process of chiral molecules. Atomic weight/atomic number serve best for this purpose. I don't think there is any cause/effect type of reasoning behind this.