[azmanam]

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 lists it at -3 (2nd deprotonation at 1.99), and I can't seem to find it on Bordwell's table.

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

[opsomath]

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.

[Borek]

Never seen -9 before.