Reactivity of ions

[FranciumFluoride]

Could somebody please tell me how to work out the reactivity of ions like OH-, CN-, SO₄ 2-, CO₃ 2- and so on.

Is there a reactivity series for ions like these, or some rule to their reactivity?

Can more reactive ions displace less reactive ones- say, if the CN- ion was more reactive than the OH- ion then it could displace the OH- ion in methanol or ethanol?

Any help would be much appreciated.

[peter]

I think that there are no general reactivity series of ions. But you of course can order them in according to reactivity in some specific reaction. For example you can compare how these ions behave in water -> compare their Kb.

[FranciumFluoride]

Thanks for replying to my post.

Say, could the SiO₃ 2- ion displace the CO₃ 2- ion in carbonic acid making sililic acid?

Does an ion with a greater charge have a greater reactivity, or does one with a smaller charge have a greater reactivity?

[Dr Fotios]

"Reactivity" is a very relative term. Reactivity is relative to the specific type of reaction, the counter ions, the solvent and the reactant. In regards to your question about whether a cyanide anion could "displace" a hydroxide from something like methanol in general without specifying any counter ions, solvent, pH or temperature the answer would be no. Just compare the pKa of HCN to the pKa of water. HCN -> H+ -CN pKa: 9.2 vs. H₂O -> H+ -OH pKa: 15.7 . If instead of cyanide anion you use an anion with a pKa greater than 15 and reacted it with methanol you still would not get displacement. This is because of the pKa of methanol MeOH -> MeO- H+ pKa: 15.2 . So the anion rather than acting as a nucleophile could (and would) act as a base deprotonate the methanol. Using the pKa in this way can be helpful in thinking about how things may react but it is not the whole story in and of itself. Again solvent, pH of reaction mixture, temperature etc. play major roles and warrant necessary consideration when trying to predict the type of reaction that may occur and the products that may result. Note pKa values are based on an equilibrium.
!!!!!!!NOTE THE FOLLOWING IS ONLY AN EXAMPLE, CYANIDE IN THE PRESENCE OF ACID WILL RESULT IN HCN GAS, VERY DANGEROUS AND DEADLY DO NOT ATTEMPT UNDER ANY CIRCUMSTANCES!!!!!!!
Hypothetically if aqueous cyanide anion and methanol were reacted in the presence of an acid the hydroxyl group on the methanol could be protonated forming a hydronium ion MeOH₂+ (pKa ~-2.0) but remember pKa values are based on an equilibrium so even though HCN pKa is 9.2 in aqueous solution some -CN will still be present and some MeOH₂+ will be present as well. The hydronium ion results in a better leaving group H₂O compared to hydroxide. So while the cyanide anion can and will deprotonate the hydronium ion the result is just HCN and methanol, what we basicaly started with and this methanol can again be protonated and react with another cyanide anion. at some point some will react to displace H₂O yielding CH₃CN (acetonitrile) and when thi occurs this reaction is not reversible so eventualy a significant amount of methanol will be converted to acetonitrile. This is rather simplified, there are specific details/actions that would facilitate this type of a reaction but these are beyond the scope of this explanation and example. Also there are other/better ways to synthesize acetonitrile, its also commercially available. Hope this clears things up a little.