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Hydrogen bonding

  1. Apr 20, 2008 #1
    [SOLVED] Hydrogen bonding

    Which types of bonding are present in CH3CHO(=ethanal) in the liquid state?

    I. Single covalent bonding
    II. Double covalent bonding
    III. Hydrogen bonding

    A. I and II only
    B. I and III only
    C. II and III only
    D. I, II and III


    The solution is A but I can not understand why it is not D. Namely, why would there be no hydrogen bonding in ethanal? I think there is no intramolecular hydrogen bonding but there should be intermolecular hydrogen bonding. Can someone explain why A is correct and not D, please.
     
    Last edited: Apr 20, 2008
  2. jcsd
  3. Apr 20, 2008 #2

    GCT

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    This is a trick question - the hydrogen is not attached to the Oxygen it's attached to the Carbon - hydrogen bonds technically require significant dipole moments that are generated by two adjacent heteroatoms with large electronegativity differences.
     
  4. Apr 20, 2008 #3
    I accept your point that this is a trick question. Namely, the question asks bonding "in CH3CHO(=ethanal)" i.e. in one molecule. In that sense, A is the right answer.

    However, if you suppose that the question ask bonding in a sample of ethanal, then I think the right answer would be D. Firstly, dipole-dipole forces can form between ethanal and water because of the electronegativity difference between O and H in ethanal. Although ethanal-ethanal-bond is probably weak due to your argument of "adjacent heteroatoms", there is still water to which ethanal can form a hydrogen-bond. Secondly, even molecules such as ketones and carboxylic acids can form hydrogen bond to water so why not ethanal? Hence, I argue that the right answer is D if one reason the question to concern molecules of ethanal in water.
     
    Last edited: Apr 20, 2008
  5. Apr 20, 2008 #4

    GCT

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    The question is in regards to a pure liquid sample of ethanal if it were an aqueous solution it would have mentioned it either as aqueous or as a solution -it seems that this question can be technical also - I would be quite bitter with my professor having asked this question nevertheless it makes sure that you have a good understanding of just what hydrogen bonding is.
     
    Last edited: Apr 20, 2008
  6. Apr 20, 2008 #5
    Why would ethanal not bond to other ethanal in hydrogen bond if carboxylic acid can bond between themselves? Is the reason similar as with ketones that there is not sufficient dipole-dipole moment.
     
    Last edited: Apr 20, 2008
  7. Apr 20, 2008 #6

    GCT

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    You should read up on the definition of hydrogen bonding. In Carboxylic Acids there is both an Carbonyl Oxygen as well as an alcohol Oxygen which has its Hydrogen bonded directly to the Oxygen-this is not the case with pure Ketones as well as pure Aldehydes. Usually there is intermolecular Hydrogen bonding between species which have Hydrogen bonded directly to an Oxygen or Nitrogen-not quite sure about Fluorines though.
     
  8. Apr 20, 2008 #7
    Thanks!
     
  9. Apr 20, 2008 #8

    GCT

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    Sure glad to help.
     
  10. Apr 21, 2008 #9
    woa! thats amazing question really..definitely i would be tricked in a question like this, but what about the dipole-dipole interaction between the two carbonyle groups? not considering hydrogen here..just the normalelectric interaction between the two carbonyls.
     
    Last edited: Apr 21, 2008
  11. Apr 21, 2008 #10

    GCT

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    There should be some dipole attractions between the two carbonyl groups the carbon of this group has a partial positive charge and the oxygen has a partial negative charge so there's a dipole moment from the carbon to the oxygen.
     
  12. Sep 15, 2008 #11
    Re: [SOLVED] Hydrogen bonding

    Your are not wrong regarding the hydrogen bonding in ethanol
    Ethanol have C-H group acting as a probable hydrogen bond donor and carbonyl O acting as hydrogen bond acceptor. Certainly due to low polarity of C-H bond. A hydrogen bond involving C-H..O intermolecular interaction will be certainly weak. But such type of hydrogen bond are well known and falls under the category of weak non conventional hydrogen bonds(with hydrogen bond energy of around 3-5kcal/mol). Also presence of C=O group next to the CH3 group activates the C-H bond for such interaction by inductive effect. And such an interaction is observed in the high resolution neutron powder x-ray crystallography of ethanal.

    Reference
    Journal of Molecular Structure 520 (2000) 265–272
    R.M. Ibbersona,*, O. Yamamurob, T. Matsuob
     
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