Order of dissociation

Marvin Beans, Calculator 5.8

 

I have executed calculation pKa for three hydroxyl groups: 7.53; 8.76; 12.21. 

The first should dissociate the hydroxyl group from 7.53, then - 8.76 !

And in drawing it turns out so: the molecular form, structure with рК 8.76, but only then - with рК 7.53 ???

I attach a file under name Forum

Dear Alexandr,

 

Your molecule was deleted from  this forum topic .If it is not confidental, then  please attach it once more.

Thanks.

 

Jozsi

Dear Yozsi,

Of course, my molecule is not confidential.
Once I went to the forum and saw that in my post two identicalmolecules.
This is wrong, I attached the molecule.
I wanted to remove one molecule.
Both have disappeared. I'm sorry.
Attach a molecule again.

Alexandr

Hi,

 

And in drawing it turns out so: the molecular form, structure with рК 8.76, but only then - with рК 7.53 ???

Instead of the pKa = 7.53 I would predict a little stronger  acidic pKa value. 

Probably the pKa = 6.0  would be more realistic value. If you have experimental value please let me know. 

I try to improve the pKa calculator further in this respect. 

 

Jozsi

Hi, Jozsi

For practical values ​​of the constants, which calculates the calculator,the first constant is important.
This is the dissociation constant of the protonated carbonyl group pK₀ = 2.3
I conducted experiments and quantum-chemical calculations. The order of the dissociation of the hydroxyl groups are.
First, the hydroxyl group dissociates at position 6 (pK₁ = 6.1). The second hydroxyl group dissociates at position 7 (pK₂ = 10.0). Finally, the dissociation of the hydroxyl group at position 2 (pK₃ = 11.6).
This macro constants. They do not take into account the presence of other processes, with another order of dissociation. It can be estimated using microconstants your calculator.
If you can configure the calculator, so he showed all four of these macroconstants, and then we get microconstants. This is an important scientific work. I invite you to write together several scientific articles.

With regards,

Alexandr

Hi,

On the attached figure the "blue colored pKa value"  refers to the basic groups. The carbonyl is a very weak basic group according to our pKa calculator . It is half protonated at pH= -8.64. The pKa value of all other oxygen atoms are  outlined on the atteched figure as well.

Your molecule has an unusually  large  carbonyl  pKa value, pKa0=2.3. Probably the protonated carbonyl group is stabilized by resonance contributors.

Is this pKa0 value based on your experimental results  or it is taken from a "literature source" ?

Is this molecule a commercially  available  acid - base indicator?

 

The common publication is a very good idea!  

Let's continue this part via e-mail. My e-mail address given in my profile.

 

Jozsi