1. Not finding help here? Sign up for a free 30min 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!

Gauche and anti of n-butane

  1. Oct 26, 2008 #1
    The difference in energy between gauche and anti of n-butane is 3.6kJ/mol. For the molecule iodobutane, I found the differnece in energy between the gauche and anti to be about 0.62KJ/mol...
    What accounts for this difference???

    and also, when you're asked to find the energy barriers to the rotation from anti to gauche and you have a graph of energy vs dihedral angle, you just find the difference in energy between anti and gauche right??
     
  2. jcsd
  3. Oct 27, 2008 #2

    GCT

    User Avatar
    Science Advisor
    Homework Helper

    Re: conformations??

    Well what role does Iodine play here? You need to propose an explanation. Your second portion sounds correct if I am interpreting it correctly.
     
  4. Oct 27, 2008 #3
    Re: conformations??

    ok to clarify i'm comparing the energy difference between gauche and anti of n-butane and 1-iodopropane not butane...- -;; what was i thinking...
    anyways, I just think that there'd be more energy difference when there is Iodine because it's a bulkier group... hence more steric strain... but it turns out it's the opposite...! > . <
    very confusedddd
     
  5. Oct 27, 2008 #4

    chemisttree

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Re: conformations??

    You need to also consider that the bond length between carbon and iodine is about 2 angstroms while the bond length between carbons in butane is about 1.54 angstroms.

    BTW... are you sure that iodine is bulkier than a methyl group?
     
  6. Oct 28, 2008 #5
    Re: conformations??

    so since they're farther apart, there's less energy..??
    oh... I just thought iodine was bulkier... cuz.. it has a higher atomic number.... nope... maybe not... lol ohh if methyl groupd is bulkier, it makes sense that it has a higher energy difference...
    please clarify for me..!! > . <
     
  7. Oct 28, 2008 #6

    chemisttree

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Re: conformations??

    Atomic diameters (covalent):

    Carbon - 1.52 A
    Hydrogen - 0.72 A (and there's three of 'em) Only about half of this diameter adds to the total diameter of the methyl group.

    Iodine - 2.66 A. So the methyl group is about 1.8 A in diameter which is smaller than iodine at about 2.7 A.

    Oops! My reference shows the difference between gauche and anti in n-butane to be about 0.6 kcal/mol, not 3.6 kcal/mol. 3.4 kcal/mol is the energy difference between eclipsed and gauche... and it's kcal/mole not kJ/mole.
     
    Last edited: Oct 28, 2008
  8. Oct 28, 2008 #7
    Re: conformations??

    no... the question stated that the difference between gauche and anti of n-butane is 3.6kJ/mol... not kcal/mol...
     
  9. Oct 28, 2008 #8

    chemisttree

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Re: conformations??

    OK, my bad. I should have looked at that more carefully.
     
  10. Oct 28, 2008 #9
    Re: conformations??


    so the reason why there's less energy difference for 1-iodopropane... is that I is a less bulkier group, and the bond length with C is longer??
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?