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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: LogicX on September 10, 2012, 05:45:38 PM

Title: If pH is below pKa, chemical will be protonated: corresponding rule for pKb?
Post by: LogicX on September 10, 2012, 05:45:38 PM

The rule that if the pH of solution is below the pKa of a molecule then that molecule will be protonated is a handy rule. However, I'm finding it difficult to find the pKa of the chemicals I'm working with. This mostly applies to looking at nitrogens in organic compounds.

For example, it is easy to find the pKa of an amine. However, this is usually the pKa for the uncharged species. I don't really care about the hydrogen in NH3 that has a pKa of 35. I want to know the pKa of NH4+ so I can predict whether it will be protonated at physiological pH. The same goes with many compounds like aniline, pyridine, etc. I need to know the pKa of protonated, ammonium group and it is hard to find.

Who cares about the pKa of the uncharged amine in lysine? We care more about the fact that R group with R-NH3+ has a pKa of 10 so it is pretty much always protonated at physiological pH.

Is there a way to use the pKb of a neutral group to predict at what pH that group will grab a proton? This is the only way I can think of to possibly skirt this problem of not knowing pKa values.

Title: Re: If pH is below pKa, chemical will be protonated: corresponding rule for pKb?
Post by: Borek on September 10, 2012, 06:07:37 PM

Quote from: LogicX on September 10, 2012, 05:45:38 PM

I don't really care about the hydrogen in NH3 that has a pKa of 35. I want to know the pKa of NH4+ so I can predict whether it will be protonated at physiological pH.

Strange, in my experience finding pKa (or pKb) values for amines in water (which are the ones you are looking for) is much easier than finding values like 35. Sometime sit is easier to find pKb, but then you can easily calculate pKa from

pKb + pKa = pKw