[Big-Daddy]

There are several conjugate bases which are amphiprotic, i.e. they can both accept a proton or lose one (or accept or lose a hydroxide). For example, H₂PO₄^- and HPO₄^2- are both amphiprotic. And there are salts of both of these, e.g. (NH4)2HPO4 or NH4H2PO4.

Are there any similar salts of conjugate acids which are amphiprotic? (Such that, if the salt were placed into solution, we would have to consider both the acidic and basic effects of the cationic conjugate acid.) Most don't seem to be (e.g. NH₄⁺ will only donate a proton; realistically I don't think we need to consider the chances of it accepting a proton; NH₃ does both but isn't really going to be a part of salts; NH₂^- will only accept a proton).

[Borek]

H₂PO₄^- is both an acid and the base at the same time, so it already fits your description.

But when it comes to cation - think harder. Plenty of compounds analogous to ammonia.

[Big-Daddy]

H₂PO₄^- is both an acid and the base at the same time, so it already fits your description.

But when it comes to cation - think harder. Plenty of compounds analogous to ammonia.

A quaternary amine? But then it can only donate H, not accept. Which cations are still in a state to accept more H?

A good case would be something like Ca(OH)⁺, but I doubt whether the Ca(OH)⁺ ion exists within any salt (e.g. Ca(OH)Cl such that dissolution gives Ca(OH)⁺ and Cl^-). So I'm not sure ...

[Dan]

So you want an amphoteric cation?

Think about compounds with two basic groups, such that if one is protonated you still have another one left in the conjugate acid.

e.g. Ethylenediamine

[Big-Daddy]

So you want an amphoteric cation?

Think about compounds with two basic groups, such that if one is protonated you still have another one left in the conjugate acid.

e.g. Ethylenediamine

Hmm, not just an amphoteric cation, but one that can exist as part of a salt. e.g. KH₂PO₄ has got an amphoteric anion.

1,2-diaminoethane just seems the same as a normal diprotic base to me; the middle form is amphiprotic but is it ever found as a cation in a salt?

[Dan]

Hmm, not just an amphoteric cation, but one that can exist as part of a salt. e.g. KH₂PO₄ has got an amphoteric anion.

Any ion could exist as a salt, all ions need counterions. You can't buy cations and anions anions in separate bottles. Maybe I have missed the point.

1,2-diaminoethane just seems the same as a normal diprotic base to me; the middle form is amphiprotic but is it ever found as a cation in a salt?

Ethylenediamine hydrochloride (2-aminoethylammonium chloride) for example. Not sure how commercially available it is but you could make it by mixing ethylenediamine and HCl in a 1:1 mole ratio. If this is a "normal" dibasic base, what is an abnormal one?

The fact that a base is dibasic doesn't mean you can't make a salt from monoprotonation - just as a polybasic base like potassium phosphate forms salts at different stages of protonation, so can ethylenediamine.

[Big-Daddy]

Hmm, not just an amphoteric cation, but one that can exist as part of a salt. e.g. KH₂PO₄ has got an amphoteric anion.

Any ion could exist as a salt, all ions need counterions. You can't buy cations and anions anions in separate bottles. Maybe I have missed the point.

1,2-diaminoethane just seems the same as a normal diprotic base to me; the middle form is amphiprotic but is it ever found as a cation in a salt?

Ethylenediamine hydrochloride (2-aminoethylammonium chloride) for example. Not sure how commercially available it is but you could make it by mixing ethylenediamine and HCl in a 1:1 mole ratio. If this is a "normal" dibasic base, what is an abnormal one?

The fact that a base is dibasic doesn't mean you can't make a salt from monoprotonation - just as a polybasic base like potassium phosphate forms salts at different stages of protonation, so can ethylenediamine.

But in the example compound of ethylenediamine hydrochloride, we've got a mixture of HCl and ethylenediamine itself; so is this essentially the same as a salt with Cl^-, and the once-protonated ethylenediamine?

[Dan]

But in the example compound of ethylenediamine hydrochloride, we've got a mixture of HCl and ethylenediamine itself; so is this essentially the same as a salt with Cl^-, and the once-protonated ethylenediamine?

Ethylenediamine hydrochloride is the same as 2-aminoethylammonium chloride, it is an ammonium salt. Nobody does, but you could call ammonium chloride "ammonia hydrochloride".

There are two representations: RNH₂·HCl (the amine hydrochloride) and RNH₃⁺Cl^- (the ammonium salt) - both mean the same thing.

[Corribus]

Bicarbonate and bisulfate are two examples that fit your need.

But just keep in mind that just about anything with a proton can act as both an acid and a base.  It's all a matter of degree.  You don't think of methane being either an acid or a base, but in the presence of a strong enough base, it will lose a proton, and in the presence of a strong enough acid, it will gain a proton.  When we speak of acidity/basicity, a given substance isn't one or the other.  This is why it's better to speak in precise terms like pKa values, which are less ambiguous than "acid" or "base".

So we usually define an amphoteric species as something that "readily" can behave as either an acid or base.  What what does "readily" mean?  It's a bit up to interpretation.  Just something to think about.

[Big-Daddy]

Bicarbonate and bisulfate are two examples that fit your need.

But just keep in mind that just about anything with a proton can act as both an acid and a base.  It's all a matter of degree.  You don't think of methane being either an acid or a base, but in the presence of a strong enough base, it will lose a proton, and in the presence of a strong enough acid, it will gain a proton.  When we speak of acidity/basicity, a given substance isn't one or the other.  This is why it's better to speak in precise terms like pKa values, which are less ambiguous than "acid" or "base".

So we usually define an amphoteric species as something that "readily" can behave as either an acid or base.  What what does "readily" mean?  It's a bit up to interpretation.  Just something to think about.

Thanks for the suggestions. But could you suggest some cations (HCO₃^- for instance is an anion, of which there are several examples already!)?

Yes, I was aware of this. I suppose NH₄⁺ has a K_b value of its own, rather than just a K_a, for the equilibrium NH₄⁺ + H₂O  NH₅²⁺ + OH^-. But the equilibrium constant will be incredibly small so unless the solution is extremely acidic (so [OH^-] is very small indeed) we won't get an appreciable yield of NH₅²⁺.

[Big-Daddy]

But in the example compound of ethylenediamine hydrochloride, we've got a mixture of HCl and ethylenediamine itself; so is this essentially the same as a salt with Cl^-, and the once-protonated ethylenediamine?

Ethylenediamine hydrochloride is the same as 2-aminoethylammonium chloride, it is an ammonium salt. Nobody does, but you could call ammonium chloride "ammonia hydrochloride".

There are two representations: RNH₂·HCl (the amine hydrochloride) and RNH₃⁺Cl^- (the ammonium salt) - both mean the same thing.

Thank you. This then is a strong example of what I'm looking for!

So they do exist then :p

[Borek]

NH₅²⁺

Never heard about something like that. As they say, paper (or computer screen) is patient and will survive everything you write/type - but just because something can be written or typed doesn't make it correct.

[Big-Daddy]

NH₅²⁺

Never heard about something like that. As they say, paper (or computer screen) is patient and will survive everything you write/type - but just because something can be written or typed doesn't make it correct.

Surely I justified this in my last post? I think the whole point of Corribus' post was that if things like this exist, they are almost always negligible. Maybe this association (of NH₄⁺) is significantly less appreciable than that of CH₄.

[Corribus]

@Borek

Just because something hasn't been observed in nature doesn't mean it can't exist.  We know CH₅⁺ exists because it's been observed.  CH₆²⁺ has never been observed, but theory predicts it is kinetically stable enough to be observed (J. Am. Chem. SOC. 1983, 105, 5258-5263).  The acidity of this complex would be incredibly high and decomposition would probably occur even faster, but the point remains that in principle it should have a K_a value.  Therefore both methane AND even CH₅⁺ are, strictly speaking, amphoteric, in that they can both accept and discard a proton.  (Although this isn't really a useful definition since virtually anything would qualify.)  Hence the key qualifier "readily", but this is rather subjective and any definition of "readily" would be arbitrary.

Similar unconventional compounds like diprotonated water (H₄O²⁺) [J. Am. Chem. SOC. 1986, 108, 1032-5] have been inferred from experiments as transient species. Likewise diprotonated ammonia, the molecule under discussion, has been predicted to be kinetically stable by theoretical measurements [J. Am. Chem. Soc. 1997, 119 (20), 4594–4598.], although exposure to superacids failed to produce it experimentally, so it must be very acidic indeed (not surprising).  The same work DID show experimental evidence of the similar (CH₃)₃NCH₄²⁺, so it stands to reason pentavalent protonated nitrogen could exist if a strong enough acid could be found to prepare it.  The same group even did theoretical calculations of NH₆³⁺ as well as the pentavalent version and penta- and hexa-valent versions of phosphorous, arsenic and so-forth analogues. [J. Am. Chem. Soc. 1997, 119, 12984-12985] They're all predicted to be incredibly unstable, but you never know whether some experiment will be designed to observe them experimentally.  Finally, gold analogues of tetra-valent oxygen and pentavalent nitrogen have been synthesized, so those are certainly possible. [Nature 377, 503 - 504 (12 October 2002 and Nature 1990, 345, 140]. Singly-positively charged pentavalent nitrogen, NH₅+, has also been experimentally observed. [Chem Phys Lett, 1988, 143, 1, 13-18.]

Point being: a lot of things haven't been observed but that doesn't mean they're not possible or that they're not "correct".  Many have been computationally studied and predicted to exist under certain conditions.  In the end the whole reason I brought it up is to say that "acidity" is a continuum concept.  If you have a strong enough acid you should be able to protonate just about anything.  How long the protonated form hangs around is a whole other issue.