1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: R.S Cahn/Ingold Nomenclature Help

  1. Sep 25, 2012 #1
    Here is the problem: http://i.imgur.com/Eltwj.png

    I understand the nomenclature rules but this problem has been confusing me. From my understanding, the CH2NH2 would have a higher priority than the CN, because of the hydrogens. This wold mean that if I rotated the lowest priority back (the #4 hydrogen shown in the pic below), it would be S. But the answer key says that the CN has higher priority than the CH2NH2, and that the configuration is R.

    Here is my attempt: http://i.imgur.com/gbxnA.png

    thank you
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Sep 25, 2012 #2
    Last edited: Sep 25, 2012
  4. Sep 25, 2012 #3
    I'm going to assume we have not learned yet (It's not even mentioned in the chapter). Is this common in an organic chem class?
  5. Sep 25, 2012 #4
  6. Sep 26, 2012 #5
    There is a rule about assigning priority when there are double and triple bonds in the substituents. Ever heard of that?
  7. Sep 26, 2012 #6
    yes, but there are only single bonds in that problem
  8. Sep 26, 2012 #7
    CN has multiple bonds.
  9. Sep 26, 2012 #8
    is there a way to determine that without going through formal charges?
  10. Sep 26, 2012 #9
    Formal charges? Why are you talking about them here? See the structure of CN and use the rule i said a couple of posts ago.
  11. Sep 26, 2012 #10
    how can you easily tell if the bonds in the diagram are single double or triple (C=N and not C-N)?
  12. Sep 27, 2012 #11


    User Avatar
    Gold Member

    Just try to satisfy the valencies of the respective atoms + A little textbook wisdom will do. Most of the bonds can easily be determined just by valency, but some may deceive, especially the co-ordinate bonds. That is where you jot them down and learn them.

    In chemistry, life can be much simpler if you just rote learn some exceptional cases. Saves you the trouble.
  13. Sep 28, 2012 #12
    i thought multiple bonds would be given, and to just assume they are all sigma if not.

    but i guess not
  14. Sep 28, 2012 #13
    O Chem is a good place not to make assumptions, especially in the early stages. The rules are semi intuitive but are still made by man. Ask yourself questions about what you are looking at. Learning to recognize functional groups as they are written in the -COOH and -OH form or carboxylic acid and hydroxyl *might* help you see what you are looking at without having to count electrons and formal charges. Will you be taking the second semester?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook