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Topic: The approximate pKa of hydroxide ion  (Read 23905 times)

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Offline Babcock_Hall

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The approximate pKa of hydroxide ion
« on: September 22, 2012, 12:23:31 PM »
There are times when it would be helpful to have an estimate of the pKa for HO-  :lequil: O2- + 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 pKa of 42, I simply looked at the difference in the pKa values of ammonium ion (9) and ammonia (35). and added this difference to the pKa 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 pKa 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 pKa is a factor.  BTW, in these two transformations serine is converted into O-acetylserine, and glutamate is converted into gamma-glutamyl phosphate, respectively.
« Last Edit: September 22, 2012, 12:34:01 PM by Babcock_Hall »

Offline zolarpwr

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Re: The approximate pKa of hydroxide ion
« Reply #1 on: April 16, 2014, 02:40:29 AM »
That's funny... I was just searching the net for an answer to this question so I could use it in my own class I'm teaching.

I think your estimate is a good one. I found an alternative that gives an even higher pKa.

According to wikipedia, the pKb of O2- is -38, which is based on the equilibrium constant for

O2- + H2::equil:: 2 OH-

The basicity constant is related to the acidity constant of the conjugate acid (in this case OH-) by the equation:

pKa + pKb = 14

So if the pKb of oxide is -38, that would make the pKa of hydroxide ion equal to 52!

Of course this is a very rough approximation but it makes sense, since organolithium reagents, generally considered among the strongest bases, are unable to deprotonate hydroxide ion under normal conditions.

Offline Babcock_Hall

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Re: The approximate pKa of hydroxide ion
« Reply #2 on: April 16, 2014, 03:51:08 PM »
Just out of curiosity, were you planning to use this value in an organic class?

Offline Big-Daddy

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Re: The approximate pKa of hydroxide ion
« Reply #3 on: April 17, 2014, 10:06:20 AM »
According to wikipedia, the pKb of O2- is -38, which is based on the equilibrium constant for

O2- + H2::equil:: 2 OH-

Yes but below that they immediately wrote "equilibrium constant of aforesaid reactions is pKeq ~ −22" which seems contradictory. I would also wonder how they reached this value.

TBH though I don't understand why you would need the actual number for this pKb? Can't you be happy with just saying "goes to completion" - after all, if this equilibrium occurs with a pKb of 52, it is only going one direction in reality - the concentration of OH- needed for even one oxide ion to be present will be impracticable.

Offline Babcock_Hall

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Re: The approximate pKa of hydroxide ion
« Reply #4 on: April 18, 2014, 01:57:38 PM »
In biosynthetic reactions there is often a step which converts a bad leaving group into a good leaving group.  All else held equal stronger bases make poorer leaving groups than weaker bases.  Sometimes it is easy to find the pKa of the conjugate acid of a potential or actual leaving group, but in some reactions it is not.  If one imagined trying to synthesize asparagine directly from aspartate, the oxygen would leave as O2-, and I wanted to find the pKa of its conjugate acid to include in a table.  In the actual synthesis of asparagine, aspartate is converted into aspartyl adenylate, and the leaving group is AMP.

Offline zolarpwr

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Re: The approximate pKa of hydroxide ion
« Reply #5 on: April 25, 2014, 05:40:58 AM »
Just out of curiosity, were you planning to use this value in an organic class?

Yes.

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