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Topic: R/S configuration of cyclopentanes  (Read 4003 times)

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Offline cardene

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R/S configuration of cyclopentanes
« on: May 11, 2012, 06:07:35 PM »
I think there must be a deficiency in my understanding of R/S Cahn-Ingold-Prelog naming system.  The following cyclopentane is from the MIT Open Courseware problem sets answer key (Problem Set 3, Spring 05).



They classify both chiral carbons as being "S" and "S."  This baffles me, when I took the first carbon, flipped the "H" to the back, and assigned priorities, it seemed to me that it was "R."  Have attached a sketch of my priorities:



Would appreciate feedback as to where I'm going wrong. Awfully confused over here... ???

Offline wynn

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Re: R/S configuration of cyclopentanes
« Reply #1 on: May 11, 2012, 06:16:08 PM »
The actual configuration is opposite to whatever you see after you flip the H. In this case, after you flip H to the back, it seems the molecule has a R configuration, but the actual configuration is S.

Offline cardene

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Re: R/S configuration of cyclopentanes
« Reply #2 on: May 11, 2012, 06:31:05 PM »
What other structures, besides cycloalkanes, does this rule of thumb apply to?

Offline wynn

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Re: R/S configuration of cyclopentanes
« Reply #3 on: May 11, 2012, 07:26:42 PM »
What other structures, besides cycloalkanes, does this rule of thumb apply to?

This rule applies to all (I haven't seem an exception yet) structures.

Offline Doc Oc

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Re: R/S configuration of cyclopentanes
« Reply #4 on: May 11, 2012, 08:13:51 PM »
You can't just arbitrarily invert stereocenters as you please, you have to maintain absolute configuration.  Look at molecule 1.  If you imagine that looks like a pancake, and you flip it over, that's how you can get the hydrogen to point backwards as you've drawn.  However, you also have to move the carbon with the chlorine over to the left, it doesn't stay on the right.  That's why your configuration is off, you didn't flip the whole structure over, you only flipped that one carbon.

Offline orgopete

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Re: R/S configuration of cyclopentanes
« Reply #5 on: May 12, 2012, 12:34:35 AM »
I agree with the Doc on this. This is where molecular models are useful. You can look at the carbon from any angle you wish and its absolute stereochemistry will remain constant. If you have to break the molecular apart, then you know you have changed the molecule.
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