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Chemistry Forums for Students => Organic Chemistry Forum => Organic Chemistry Forum for Graduate Students and Professionals => Topic started by: azmanam on August 31, 2011, 01:57:12 PM

Title: pKa of Sulfuric Acid
Post by: azmanam on August 31, 2011, 01:57:12 PM

I've got two organic chemistry texts in front of me.  One Appendix of pK_a values lists the pK_a of H₂SO₄ as -9.  The other lists it at -3.

What gives?  Is it pK_a in DMSO vs water or gas phase or something?  Neither appendix gives solvent or other conditions...  Evans (http://www2.lsdiv.harvard.edu/labs/evans/pdf/evans_pKa_table.pdf) lists it at -3 (2nd deprotonation at 1.99), and I can't seem to find it on Bordwell (http://www.chem.wisc.edu/areas/reich/pkatable/index.htm)'s table.

When you think sulfuric acid, what pK_a value pops into your head?

Title: Re: pKa of Sulfuric Acid
Post by: opsomath on August 31, 2011, 02:18:39 PM

Well I tend to think of it as having a pKa of "Yes."

I do think of it as actually being a stronger acid than HCl, which is not correct according to pKa tables; however, you use it in conditions where you need a very strong acid for forcing conditions, like in the double diazotization of a phenylenediamine.

Anyway, the consistent internet answer including Evans is -3, but if it were -9 that would be consistent with my experience of it having more punch than HCl (-8) in difficult reactions.

Title: Re: pKa of Sulfuric Acid
Post by: Borek on August 31, 2011, 03:32:40 PM

Never seen -9 before.