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Acidity in alcohols

  1. Nov 1, 2015 #1
    • Please post this type of questions in HW section using the template.
    I have read from a book that from the following reaction
    H2O + R-O- - - - - > R-O-H + O--H
    We can conclude that water is a better proton donor (Bronsted acid) than alcohol.
    But how did they conclude so?
    I can figure out the reason on basis of electronegativity concepts but how to do so from the reaction.
    Please help.
     
  2. jcsd
  3. Nov 1, 2015 #2

    Borek

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    Add sodium ethoxide to water, observe what happens.
     
  4. Nov 1, 2015 #3

    Ygggdrasil

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    In the above reaction, there is an equilibrium going on. In the forward direction, water is acting as an acid and the alkoxide is acting as a base. In the reverse direction, the alcohol is acting as the acid and the hydroxide as the base. The fact that the equilibrium strongly favors the forward direction tells you about the relative acidity of water versus alcohols.
     
  5. Nov 3, 2015 #4
    Thanks
     
  6. Nov 3, 2015 #5
    Thankyou so much. I got my doubts cleared very well.
     
  7. Nov 3, 2015 #6

    epenguin

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    I liked that because it does relate to some experimental fact. Maybe the student doesn't have any sodium ethoxide to hand though so what does happen? I suppose it warms up as protons are lost from H2O and protonate the ethoxide ion. If instead you added ethanol to NaOH there is no disequilibrium and nothing much should happen.

    Anyway you would have to have some sodium ethoxide to start with. I also liked that because it stirred a very faded memory of my chemistry teacher showing adding sodium to ethanol - we could see it react like water but much more feebly!

    (I probably at the time would not have thought to ask about whether the alcohol was dry. A less faded memory is a year or two later in an industrial laboratory we kept solvents dry by extruding a wire of fresh sodium into them with a 'sodium press'. Never having been in a lab with that necessity since I don't know whether this is commonly done.)
     
    Last edited: Nov 3, 2015
  8. Nov 3, 2015 #7

    epenguin

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    Due to new computer I lost the first half of my reply which was to say you may well ask - you have read this in a book, I have read it in books and now on sites and it appears to me that if your question is about any experimental support or manifestation of ethanol acidity, amongst the many the books and sites with much to say about alcohol acidity, those that tell you anything about the answer to your question appear to me a very small minority. A book I have with a chapter about mostly organic compounds acidity/basicity, and many about their consequences, mentions ethanol's pK in water) of 15.9. Looking hard I find such pKs have to be determined 'by indirect methods'.

    Perhaps some rare book tells of these. I am not rarely struck by a contrast between textbooks' overslidings like this and their Sunday sermons on p.1 about the paramountcy of experiment and its evidence.


    *something weird happened, when I press Edit I can still see my 1st paragraph but otherwise not.
     
    Last edited: Nov 3, 2015
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