[Anthasci]

I'm having trouble understanding the meaning of pKaH. We mentioned it casually during lectures but I don't remember receiving any specific definition. From what I gather, pKa is derived from the equilibrium of a deprotonated acid and it's non-ionic form (A-/HA) but I utterly fail to understand what pKaH is. I have in my textbook some pKaH values for different LGs under the carboxylic derivates, pertaining to reactivity.

It says there that pKaH of Cl^- is 7 and for example, RO^- (ether) has 16, and R^- (ketone) has 50. It seems to me that groups with lower pKaH are better LGs. Now if someone could just help me define pKaH, that'd be grand

[Rutherford]

pKaH is the pKa of the conjugated acid. For the chloride ion it is HCl, so the pKa of HCl is -7.

"It says there that pKaH of Cl- is 7" It should be -7.

[Anthasci]

My mistake, it says -7, yeah. So, pKaH of Cl- is just another way of saying pKa of HCl?

[Rutherford]

Yes.