Nomenclature of stereoisomers

[nancy9494]

 

Hello!
 I don't understand why it is (S) rather than (R). The circled numbers are the rankings of priority according to Cahn-Ingold-Prelog Rules.  I thought it would be clockwise. The molecule seems to be oriented correctly with the H in the back because of its low priority. My answer key says it should be (S)-1-chloro-3,4-dimethylpentane. Sorry for the sideways pics. I couldn't find a way to rotate them. Thanks in advance!

[hypervalent_iodine]

You haven't assigned your priorities correctly. Remember that you need to work out only one atom at a time. In your case, the first atoms out from the stereocentre are C, C, C, and H (atoms in red). Obviously the H is the lowest priority here, but you cannot distinguish between three C's. You therefore need to go one further to see if you can differentiate. To do this, you consider the atoms bonded to each atom connect to the stereo centre, and rank them. In the image below, these are the blue atoms. 

 

a. C, H, H

b. C, C, H

c. H, H, H

 

 

You then look at where they first differ. Atom c. is different from a. and b. in that its highest ranked atom is a H. This group is therefore the lowest priority of the three. Atom a. and b. first differ at the second highest ranked atom. In this case, atom b. has a C, where a. has a H. Since carbon is higher priority, atom b. is considered higher priority than a. The order is thus, b., a., c., then the H, making it an (S) isomer. 

[nancy9494]

Thank you so much!!!  Love the chart. It's so helpful! I really appreciate it you have no idea

[hypervalent_iodine]

No problems!