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A Carbon skeleton with 2 diff functional groups

  1. Jan 12, 2005 #1
    Lets say for example a hydrocarbon skeleton has two diff functional groups branched to the skeleton.. Lets say one of the functional groups pH is less than 9 to 10, and the other functional group's pH is greater than 2 to 4

    that means that molecule to be electrically charged, the pH of the molecule has the be true for both of the pH of the functional groups right?

    so pH will be (6 to 7) < pH < (9 to 10)

    correct me if im wrong

    thanks
     
  2. jcsd
  3. Jan 12, 2005 #2

    siddharth

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    What do you mean by Ph of a functional group? Ph is used to refer to the activity of available H+ ions. As the activity of H+ differs in each solution depending on concentration there can be no Ph of a functional group.
     
  4. Jan 15, 2005 #3
    First of all, a molecule by definition is uncharged. A functional group does not have a characteristic pH. As a concrete example, ethanol and propanol have different values of the dissociation constant because of a difference in their structure even though both have an identical hydroxyl group. There are countless other examples, which necessitate a correct explanation.

    Secondly, pH has nothing to do with electrical neutrality here.

    If you have a molecule which has multiple functional groups, its behavior in solution where it can ionize will depend on

    (a) temperature
    (b) nature of solvent
    (c) concentration of the molecule (the solvent for practical purposes)

    Generally the more acidic group (or more basic group as the case may be) predominates (suppose some molecule is a hydroxy acid...the hydrogen of the carboxyl group will be more easily lost due to subsequent stabilization of the anionic species). Note that pH in solution still depends on the total hydronium ion concentration in solution and not on just one functional group. (The acid dissociation constant, concentration and ion product of water at the particular temperature are some of the things you need to know to mathematically compute pH to a certain degree of theoretical accuracy.)

    Cheers
    Vivek
     
  5. Jan 15, 2005 #4
    He may be talking about Zwitter ions.Say i amino acids,it may be +vely charged -vely charged or may be neutral according to pH.
     
  6. Jan 15, 2005 #5
    Yes I am sorry I didn't consider that.
     
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