Talk:Pyrithione

A fact from Pyrithione appeared on Wikipedia's Main Page in the Did you know column on 18 January 2017 (check views). The text of the entry was as follows:

Did you know... that pyrithione is used to prepare medicated shampoos to treat dandruff and seborrhoeic dermatitis?

A record of the entry may be seen at Wikipedia:Recent additions/2017/January. The nomination discussion and review may be seen at Template:Did you know nominations/Pyrithione.

Wikipedia

Acid/base properties

Tautomerisation of the sodium salt of pyrithione

"Pyrithione is a weak acid with pK_a values of −1.95 and +4.6 (thiol proton)...but is a markedly stronger acid than either of its parent compounds, both of which have pK_a ~ 8.

There is something wrong here. An acid with a pK_a<2 is effectively a strong acid. Most probably there is confusion between pK_a and pK_b values: see Acid dissociation constant#Bases and basicity. The OH group is weakly acidic and the pyridinic N is basic. pK_a of -8 is certainly wrong; it implies that the compound is a strong acid.

I don't like the diagram which appears to show ion-pairs. the Na⁺ ion is very unlikely to associate strongly with -O^- and even less likely to associate with -S^-. It may be normal practice for organic chemists to show N oxides in a canonical form with "charges" on N and O, but this is misleading in regard to acid-base properties. Petergans (talk) 11:05, 18 January 2017 (UTC)[reply]

@Petergans: It does not say a pK_a of -8, it says a pK_a of the order of (with a tilde, ~) 8. This could be changed to "both of which have pK_a values of around 8" or similar. The source for the pK_a data is here. There are two values as the thione form is a stronger acid than the thiol form. As for the salt tautomers, the illustrations are commonly used in organic chemistry. EdChem (talk) 13:44, 18 January 2017 (UTC)[reply]

OK, I misread the tilde (~). A better symbol for approx. is ≈. I could not check your source as there is mo preview available on the web, However, I insist that something is wrong with the pKa value of -1.95. Such a value implies that the compound is effectively a strong acid, much stronger than acetic acid pKa ≈ 4.7 but not quite as strong as hydrochloric acid pKa ≈ -6.3.

What may be acceptable to organic chemists is not necessarily right for the general reader of Wikipedia. At the very least I would prefer the notation 0^-Na⁺ but it would be even better to show the acid forms, -OH or -SH as it is they that are involved in the tautomerism.. Petergans (talk) 16:38, 18 January 2017 (UTC)[reply]

I have changed the image as suggested, and noted it can be taken as an example of resonance in just the anion. The image of the tautomerism in the neutral species is already in the infobox. I have removed the tilde as well as the source specifies both are above 8. The google book image of that is available in Australia, you could access it from an Australian IP, or perhaps from the .com link here. I agree, from a chemical perspective, the pK_a of −1.95 is strange, but I was reluctant to ignore it from a reliable source. I'm willing to post at the Chemistry WikiProject for input, consensus may well be to just leave the 4.6 value that makes sense, or to add a note that a reference indicates a second value. Also, thanks for the feedback, extra perspectives often help to strengthen an article. :) EdChem (talk) 22:41, 18 January 2017 (UTC)[reply]