[villageidiot]

I hope this is the right forum - the question arose through study of an organic chem textbook though it is probably a high school level question (dumb).

When assigning priority to atoms bonded to an asymmetrical carbon atom to determine whether the enantiomer is in the R or S configuration, can you do it by atomic mass?  Let me try to explain:

          H
           |
           C
 CHCl / |  \(CH2)2 CH3

       CH2CH3

Hydrogen is obviously lowest priority, then next you have to choose between the propyl and ethyl groups and the CHCl.  Since they all start with carbon, can you simply take the atomic mass of the three groups and then the one with the highest atomic mass has highest priority, next highest has second highest priority, and so on?  

In this case it would mean the CHCl, then the propyl, then the ethyl.  Am I wrong here?

All the explanations I found for determining priority seemed to be confusing (like this one: http://members.aol.com/logan20/configur.html) and used atomic number.  Will simply taking the atomic mass work (given you have two or more substituents beginning with the same atom)?

Hopefully this question was not too confusing or basic...it just threw me a curve and I hope you all could help.  Thanks a lot.

[Albert]

Well, I'm not sure this is the best way of determining priorities. I mean, for what concerns the molecule you drew, you rightly gave the priorities.

I was taught to give priorities (sorry for the many repetitions ) this way: once you determine the chiral centre, you consider every atom directly bonded to it and, due to its weight. You have 3 C and 1 H.
Then, you look at the second positions, the groups bonded to those aforementioned atoms, that make the groups different from each other. Their atomic masses tell you their priorities. You have: Cl, 2CH₃ and 1CH₃.
And so on...

[villageidiot]

Excellent...that makes perfect sense now.  I was just missing how to compare similar atoms that were bonded to the asymmetric carbon (such as CH2CH3 and CH3).  So simply if they both begin in C, take the mass of H2CH3 (~17g) and H3 (~3g) and then the greater mass (ethyl group) is higher priority.  

Thanks a lot!