Notices
Results 1 to 4 of 4

Thread: NMR and double bonds

  1. #1 NMR and double bonds 
    Forum Masters Degree organic god's Avatar
    Join Date
    Feb 2008
    Location
    London
    Posts
    567
    when you do an NMR spec of a compund with a double bond.
    for example if i cut out a small part. and say the two hydrogen atoms are in different environments in the molecule so it causes two peaks

    H(a)----C=C---H(b)


    when you do the spectrum, will the hydrogen (a) effect hydrogen (b) and so cause the peak to split. or does it have no effect?

    another question.

    if 2 hydrogens are on the same carbon in a C=C double bond.
    does the fact they cant rotate mean they have different environments therefore causing 2 different peaks


    everything is mathematical.
    Reply With Quote  
     

  2.  
     

  3. #2  
    Forum Freshman
    Join Date
    Mar 2008
    Location
    UK
    Posts
    5
    Good question,

    firstly, the compound that you mention doesn't exist unless it is some type of radical. Symmetry means that all h-bonds in either an ethylene or acetylene are identical in NMR.

    If two H atoms are on the same C atom then they will have different shifts in NMR depending on what is on the other C atom - if one is attached.

    When looking at spectroscopy in chemistry, always look at the symmetry, that's what determines everything.


    Reply With Quote  
     

  4. #3  
    Forum Masters Degree organic god's Avatar
    Join Date
    Feb 2008
    Location
    London
    Posts
    567
    the compound i mentioned doesnt exist because it is a cut out from a compound, i was just comparing two hydrogens A and B.

    so say we had CH3CH2CH=CH2

    and we take out this part

    CH=CH2.

    do all 3 hydrogens have different chemical shifts or are there any similair parts?
    everything is mathematical.
    Reply With Quote  
     

  5. #4  
    Forum Sophomore Matt Lacey's Avatar
    Join Date
    Sep 2006
    Location
    Southampton, England
    Posts
    119
    Quote Originally Posted by organic god
    the compound i mentioned doesnt exist because it is a cut out from a compound, i was just comparing two hydrogens A and B.

    so say we had CH3CH2CH=CH2

    and we take out this part

    CH=CH2.

    do all 3 hydrogens have different chemical shifts or are there any similair parts?
    I believe all 3 hydrogens will have different chemical shifts, but the terminal hydrogens will have similar shifts. However, you can tell them apart because their coupling constants will be quite different.

    I think that's right, someone with a better knowledge of organic chemistry can probably confirm/rubbish this...
    Reply With Quote  
     

Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •