Acidic strength of phenol with isotopic oxygen i.e O 18 and just phenol

[Sat-P]

I have a doubt between the acidic strength of phenol with isotopic oxygen i.e O 18 and just phenol

 

[Borek]

What kind of doubt do you have?

 

[Sat-P]

Acidic strength comparison between phenol and phenol with O18

 

[Sat-P]

I don't know how to compare between

 

[Sat-P]

O18 has 2 extra proton so conjugate base must be more stable than that in O??

 

[Mayhem]

O18 has the same number of protons as O16.

 

[Sat-P]

It has two extra neutrons

 

[Sat-P]

I searched it somewhere and they said something about zero point energy. But I dont understand what it is!!

 

[TeethWhitener]

I searched it somewhere and they said something about zero point energy. But I dont understand what it is!!

It’s an example of the isotope effect. This is a really complex topic, but a simplified explanation goes like this: the zero point energy of an oscillator decreases with increasing mass, so the O-18 phenol OH vibration will have a lower ZPE than O-16 H. In addition, the transition state ZPE to break the OH bond will be different. In general these two ZPEs don’t decrease by the same amount, so one dissociation pathway will have a lower activation energy than the other. Predicting the magnitude and sign of this difference can be very complicated, but the magnitude is generally proportional to the mass difference between the isotopes. So O-18/O-16 isotope effects will be quite a bit smaller than H/D isotope effects. This page has a pretty good explanation with pictures and examples:
https://chem.libretexts.org/Bookshe...l_Harmonic_Oscillator/Kinetic_Isotope_Effects