Help with PKa of Phenol Lab: Q&A for 1,2,3

[jkh4]

Homework Statement

I am working on a lab about pKa of phenol, and I don't understand how to do the following questions. Can someone please kindly help me??

1)
Why was the rinsing of glassware with acetone not allowed in this experiment?
T/F Acetone has a UV absorption itself and causes errors in the absorption measurement.
T/F Acetone would dilute the solution, thus causing errors in the measurement.
T/F Acetone would react with the buffer and skew the pH of the solution.
T/F Acetone will react with the phenol and cause the experiment to fail.

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 for aromatic compounds. Please select T/F for the following options:
T/F addition of a double bond which is conjugated to the ring will decrease the wavelength of absorbtion?
T/F addition of electron-withdrawing substituents like keto or nitro groups will increase the wavelength of absorbtion?
T/F addition of alkyl chains like methyl or ethyl groups will increase the wavelength of absorption?
T/F addition of electron-donating substituents like methoxy or hydroxy groups will decrease the wavelength of absorbtion?

3) What would cause the absorbance curves to not go through the isosbestic point?
T/F A change in the position of the equilibrium.
T/F A change in pH.
T/F A change in the total concentration of phenol.
T/F A change in the ratio of phenol to phenolate.



Thank you !

Homework Equations

The Attempt at a Solution

These are the answer I think it's right so please correct me if i got anything wrong. I have totally no idea how to do #2 if someone could kindly explain, thank you.
#1 (T,T,F,T)

#3 (F,F,T,F)

 

[nate808]

Hello, for question #2, you are correct in your answers for #1 and #3. For question #2, the correct answers are: (T,T,T,T). The addition of a double bond which is conjugated to the ring will decrease the energy gap, resulting in a shorter wavelength of absorption. Addition of electron-withdrawing substituents like keto or nitro groups will also decrease the energy gap, resulting in a longer wavelength of absorption. Addition of alkyl chains like methyl or ethyl groups will increase the energy gap, resulting in a shorter wavelength of absorption. And finally, addition of electron-donating substituents like methoxy or hydroxy groups will decrease the energy gap, resulting in a longer wavelength of absorption. Hope this helps! If you have any other questions, please let me know. Good luck with your lab!

 

[Blueshift5]

Hello,

For #1, your answers are correct. Acetone has a strong UV absorption, which can interfere with the absorption measurement in this experiment. Additionally, it can also dilute the solution and react with the buffer or phenol, causing errors in the measurement and potentially causing the experiment to fail.

For #2, the correct answers are T, T, T, F. The energy gap between the HOMO and LUMO orbitals can be affected by the conjugation of double bonds, electron-withdrawing substituents, and alkyl chains. The addition of a double bond that is conjugated to the ring will decrease the energy gap, resulting in a smaller wavelength of absorption. Similarly, the addition of electron-withdrawing substituents like keto or nitro groups will also decrease the energy gap and result in a smaller wavelength of absorption. However, the addition of alkyl chains like methyl or ethyl groups will increase the energy gap and result in a larger wavelength of absorption. The addition of electron-donating substituents like methoxy or hydroxy groups will not have a significant effect on the energy gap and therefore will not significantly change the wavelength of absorption.

For #3, the correct answer is T. A change in the position of the equilibrium between phenol and phenolate ions can cause the absorbance curves to not go through the isosbestic point. This could be due to changes in temperature, pH, or concentration of the solution. A change in pH or the total concentration of phenol would not necessarily cause the curves to not go through the isosbestic point, but it could affect the overall shape of the curves. A change in the ratio of phenol to phenolate ions, however, would not affect the absorbance curves.

I hope this helps! Let me know if you have any further questions. Good luck with your lab!