There are times when it would be helpful to have an estimate of the pK
a for HO
- O
2- + H
+. I have never seen even an estimated value, but I once offered the value of 42 as a first (and very simplistic) approximation. To obtain an estimated pK
a of 42, I simply looked at the difference in the pK
a values of ammonium ion (9) and ammonia (35). and added this difference to the pK
a of water. Does anyone know of a better value?
The reason it would be helpful to have such a number is that one sometimes explains the need for certain transformations in organic chemistry or biochemistry as trading out a bad leaving group and replacing it with a good one. One can find pK
a values for the conjugate acids of some actual (good) to compare with hypothetical (bad) leaving groups: for example in the bacterial conversion of serine to cysteine, the conjugate acid of the hypothetical leaving group hydroxide ion is water. But sometimes in the absence of a transformation of one leaving group into another, the leaving group would be just the oxygen atom of a carboxylate group without a hydrogen: for example in the conversion of glutamate to glutamine, the leaving group is one of the oxygen atoms of the carboxylate group. It would be nice to be able to specify just how bad a leaving group the oxygen dianion is. I am not trying to imply that pKa is everything in determining whether a leaving group is good or bad, but I don't believe that it controversial to claim that pK
a is a factor. BTW, in these two transformations serine is converted into O-acetylserine, and glutamate is converted into gamma-glutamyl phosphate, respectively.