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PKa of Phenol

  1. Mar 21, 2007 #1
    please help!!!! questions on pKa of Phenol

    Hi, I have some questions regarding the pKa of Phenol lab that I am working on right now.

    1) What would cause the absorbance curves to not go through the isosbestic point?

    For this one, I know that isosbestic point is not pH dependent so pH won't affect, and definitely true if there is a change in the total concentration of phenol or ratio of phenol to phenolate. But what about a change in the position of the equilibrium?

    2) The wavelength of the transition depends on the exact energy difference between either π or n orbitals and the π* orbitals -- in other words, the HOMO-LUMO gap or the n-LUMO gap. When the energy gap becomes smaller, for example by increasing the size of the conjugated system, the absorbance wavelength increases. The question is what kind of factors affect this energy gap (HOMO-LUMO gap or the n-LUMO gap) for aromatic compounds.

    3) Why was the rinsing of glassware with acetone not allowed in this experiment?

    I know that acetone is UV active so it will give interences also acetone will dissolve the phenol, but will the acetone react with phenol?

    Thank you so much for your help =)
    Last edited: Mar 21, 2007
  2. jcsd
  3. Mar 21, 2007 #2


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    Consider the source of the absorbing moiety at the wavelength of the isosbestic point. You have already pointed out that it is not pH dependent. This absorbance is not dependent on the protonation or deprotonation of the phenol. Therefore, the equilibrium point, which only affects the relative concentrations of anion and parent compound, cannot affect isosbestic point.

    Only the number of total equivalents of the protonated and deprotonated species will affect the absorptivity. Any change in total concentration of the sum of these two species, by any mechanism, will change the absorptivity at this point.

    Can you posit any mechanism that might change the total concentration?
  4. Mar 21, 2007 #3
    what about the ratio of phenol and phenoate? is that mean the ratio of concentration?
  5. Mar 22, 2007 #4


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    By definition, the isosbestic point is only dependent on the SUM of the concentrations of the protonated and deprotonated species, not the individual concentrations.
  6. Mar 22, 2007 #5


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    Homo Lumo

    I also have the same question about HOMO and LUMO gap. I know that adding a electron donating substituent and adding a double bond which is conjugated will increase the wavelength of absorption and adding electron withdrawing substituents will decrease the wavelength of absorption.

    But what about adding alkyl chains, will it decrease or increase the wavelength of absorption??

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