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What's the difference between H3C and CH3?

  1. Feb 3, 2012 #1

    tony873004

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    When looking at the "shiny ball" models of a caffeine molecule, I notice that it terminates in 3 places with a carbon atom bonded to 3 hydrogens. But diagrams from different sources list two of them as CH3 and one of them as H3C. What's the difference?

    Also, what are the spacings in H3C / CH3? It looks like each bond is 120 degrees from the other bonds. But somewhere I recall hearing 105 degrees. I'm probably mixing this up with something else.

    Thanks in advance!
     
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  3. Feb 3, 2012 #2

    DaveC426913

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    I noticed that too, when I was looking at theobromine (almost identical to caffeine). I assumed it was just to visually indicate that the N (or C in your case) is the one with the bond, not the H's, but I don't really know.
     
  4. Feb 3, 2012 #3

    Ygggdrasil

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    There's no difference. Sometimes it's more convenient to write the three hydrogens on the left side of the carbon and sometimes its more convenient to write the hydrogens on the right side of the carbon.
     
  5. Feb 4, 2012 #4

    Borek

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    I tend to write them in the form H3C-CH3 - that is, leaving carbons on the bond side (as opposed to CH3H3C - which suggests to me carbons being bonded through hydrogen; sometimes that would make sense though, like in HBH2BH - not that I would ever write diborane this way). But that's just my approach, there is no generally accepted rule/convention here.

    Just like some people write -CO2H and others write -COOH for carboxyl group.
     
  6. Feb 4, 2012 #5
    As others said you both CH3 and H3C are notations for the same methyl group -CH3.

    Regarding your other question the ideal bond angle H-C-H in methyl is 109.5º. Small variations may be waited in function of the nature of the fragment (R) to which is linked this group R-CH3

    If you do a 2D projection of the group it must look as 120º, but this is fictitious.
     
    Last edited: Feb 4, 2012
  7. Feb 4, 2012 #6
    A 120º bond angle is formed when there are just three pair of electron (lone pairs or bond pairs) around an atom. One example being BF3.

    -CH3 on the other hand has 4 electron pairs around it, and hence forms a 3D geometric figure, a tetrahedral, to ensure minimum repulsion between electrons. This structure has an angle of 109º28' (But can change slightly in case of unlike bonds)
     
  8. Feb 4, 2012 #7

    tony873004

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    Thanks for the replies!
     
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