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Steric Effects on Basicity

  1. Jul 31, 2014 #1

    Qube

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

    How do sterics affect basicity?

    2. Relevant equations

    Sterics has to do with kinetics.

    Basicity is a thermodynamic property.

    Nucleophilicity is a kinetic property.

    3. The attempt at a solution

    So is the answer "sterics don't affect basicity"?

    I can't see why sterics would affect basicity. Sure, steric hindrance might make things harder for the hydrogen proton to reach the site of basicity. But thermo is just about the beginning and end on the energy coordinate diagram. We don't care if it takes a trillion years for the hydrogen proton to reach the site of basicity. Time doesn't affect basicity. Time affects nucleophilicity.

    What do you think? Is this a trick question?
     
  2. jcsd
  3. Aug 1, 2014 #2

    epenguin

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    Well that sounded to me at first a somewhat reasoned answer.
    The question for what you are saying would be whether steric or any other effect on reaching the site is different from its effect on the proton leaving the site, so what you wrote doesn't convince yet as an answer, maybe what you thought could have been in the right direction.

    However I have to say that IMHO what you should be looking for for this type of problem is Solvation Solvation Solvation! (British catchphrase).

    Though from what I see e.g. here this has not fully penetrated chemical education, education, education.

    I thought there was a significant, not huge, steric effect on acidity, looked it up and found this explanation (I don't think it is worth trying to lead someone there through hints):

    Entropy: The numbers show that the enthalpies of dissociation are relatively constant, which argues against any electron-releasing arguments:

    Thus, in aqueous solution, the decrease in acidity with substituent size should be interpreted as follows. The anion must be hydrogen bonded for solvation. However, the steric demands of the alkyl groups mean that a relatively small number of orientations of the water molecule are possible. Thus, the entropy cost of solvation is increased.

    http://isites.harvard.edu/fs/docs/icb.topic776365.files/lecture%2017.pdfq [Broken] pp106, 105

    He gives the examples of less and more crowded carboxylic acids and alcohols.

    Make sure you are OK with converting between ΔG0 s and pK s (or Δ(ΔG0) s and ΔpK s); you will find this very useful!
     
    Last edited by a moderator: May 6, 2017
  4. Aug 2, 2014 #3

    Qube

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    Yep, I realized that solvation is a factor. I mentioned this to my professor and although he did not outright disagree with me about solvation, he dismissed it as being basically too advanced for the students.

    I could definitely see sterics having an effect on solvation of the conjugate base. A bulky conjugate base with long carbon side-chains could be poorly solvated (in water) as opposed to a compact, polar conjugate base.

    On the other hand I revise my original stance; if one looks at the IUPAC definition of steric effects, the definition itself lists sterics as having an effect on equilibrium constants.

    If that isn't an endorsement of the idea that sterics can affect thermodynamics, then I don't know what is.
     
  5. Aug 4, 2014 #4

    epenguin

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    There may be a teaching issue; it would be good to hear the physical chemists.

    To me it seems a bit unreal to treat a molecule in solution as thought it were in the gas phase

    I have the impressions that:

    at some point a 'physical organic chemistry' developed a bit separate from real physical chemistry and that some of it, not all of it right, got too fixed into curriculums;

    for real explanations of tendencies you really need to know enthalpic and entropic contributions, but in the cases cited it is entropic one that explain the tendencies;

    enthalpies of acid dissociation in the gas phase can be typically two orders of magnitude different from in water

    and the tendencies (and physical organic chem explanations) do not hold up.

    "The order of decreasing acid strength in water (of MeOH, EtOH, Me2CHOH, Me3CHOH was thought at one time to be due to a molecular electronic (electron donating inductive) effect...
    The gas phase acidities are in the opposite order however.... must be due to solvation.

    ("The Physical basis of organic chemistry. H. Maskill pp 195-6)



    I don't understand your problem of your paras 2 and 4. The dissociation of the proton is favoured by hydrogen bonding or other dipole reorientation towards the resulting anion which is less when it is bulky - that is a thermodynamic effect.
     
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