Reaction mechanism of TCCA with HCl

Quote: Originally posted by byko3y

N-Cl bond is always shifted to nitrogen side. pKa of HCl is -7, which means that it is completely dissociated. For chloramine pka=14 and pkb=15, which is close to that of water. I'm sure the chloramide has similar dissociation constants. Thus, the chloroamine is mostly protonated by the strong acid. Attack of chlorammonium by nucleophile leads either to displacement of one chlorine atom with another (products and reactants are the same, reaction is unfavorable because the nitrogen is shielded with H and Cl atoms from 4 sides) or to displacement of ammonium with chlorine by attack on chlorammonium's chlorine [Cl]- -> [Cl-NH3]+ Cl-Cl NH3 Chlorocyanuric acids are slowly hydrolyzed to HOCl by water. Sodium dichloroisocyanurate For cyanuric acid the dissociation constants are pKa1 = 6.88; pKa2 = 11.40; pKa3 = 13.5 (for comparision pKa of water is 15.7; pka of ammonia is 9.25). [Edited on 15-10-2015 by byko3y]

First of all, thanks for your reply

Two parts:
First in regards to the reaction mechanism:
I was thinking of the N-Cl bond: Is it actually shifted to the N side? I mean: Shouldn't the chlorine pull the electrons stronger than the nitrogen, thus shifting the bond to the chlorine? (Then again: Nitrogen does have a lone pair, which could be vulnerable to protonotion).

I can see the chloroamine's nitrogen being protonated. But what follows? I mean:
Option 1: Something clearly has to leave that nitrogen, and it could either be Cl+ or H+. My way of seeing it is the H+ will leave, given that Cl+ is not a good leaving group.

Option 2: If we look at the protonated chloramine as a chlorammonium: Cl- has a full electron shell, as does the chlorine in the chloramine. How would it be able to attack it? Even if the chloramine chlorine donates electron density to the nitrogen: It would have to actually donate an electron pair to be vulnerable to attack by the Cl-, wouldn't it?

Second: In regards to the sodium dichloroisocyanurate:
I've read references to a sodium TRIchloroisocyanurate, but can't seem to find it. Any ideas on this?

How are the ions of TCCA formed anyhow? What has to happen for an ion to be formed?

Third: Would you happen to have a source for the hydrolysis of TCCA to HOCl in the presence of water? It'd really help! D:
[Edited on 15-10-2015 by Sniffity]

[Edited on 15-10-2015 by Sniffity]

Quote: Originally posted by Sniffity

Other things I've read:Isocyanurate ions are actually hydrolized to form HOCl, is this true? Can't seem to find any basis for this?

You're on the right track. And the dechlorinated compound you're referring to is cyanuric acid. It exists as a pair of tautomers, one where the nitrogens are protonated and one where the oxygens are protonated. Which tautomeric form dominates generally depends on pH.

First of all, I highly doubt TCCA's nitrogens are what get protonated under acidic conditions. If anything's going to get protonated, it's going to be the carbonyl oxygens. The reason is due to resonance stabilization. If an oxygen gets protonated, the charge can be stabilized through electron delocalization. If a nitrogen gets protonated, there's no way to stabilize the charge, making protonation at a nitrogen a lot less favorable. I made the following to show you what I mean:



As you can see, with the top, the charge is more or less completely localized on the one nitrogen atom. With the bottom, however, the charge gets delocalized and spread out across four different atoms.

Secondly, I doubt that a chloride ion directly attacks the protonated TCCA to form Cl₂. According to the literature I've read in the past, the general consensus seems to be that, under acidic conditions, TCCA is first hydrolysized to HOCl, even in the presence of excess chloride ions. (I may attach a reference later to back this up. I'll have to get it off my phone) It seems that it's actually the hypochlorous acid that is the source of the Cl₂, not the protonated TCCA. Thus the chlorine gas is a result of the usual reaction between hypochlorous acid and HCl:

HClO + HCl ⇌ Cl2 + H2O

And as far as what byko3y said about TCCA hydrolyzing slowly in water, keep in mind that the hydrolysis reaction is reversible. This means TCCA is in equilibrium with HOCl. So while the hydrolysis may be slow under normal conditions, under highly acidic conditions with excess chloride ions the hydrolysis becomes extremely rapid due to Cl₂ production. (can also be backed up with a reference if necessary) The reaction with HCl that produces chlorine gas continuously lowers the concentration of HOCl, which, in turn, shifts the equilibrium over to the right to favor more HOCl production via hydrolysis:

TCCA + 3 H₂O ⇌ Cyanuric acid + 3 HOCl
HClO + HCl ⇌ Cl2 + H2O

Anyway, here's the general idea behind my proposed mechanism. Like most reactions involving chlorine and its various oxidative species, the exact mechanism for the reaction between HOCl and HCl isn't known, either. But I imagine that it proceeds through either an activated hypochloronium ion via nucleophilic attack by chloride, or though some kind of autocatalytic mechanism involving chloride, hypochloronium and water.



Here's a much higher quality (and resolution) version of the mechanism in .png format if you're interested:

Magnificent; exactly what I was looking for!!!

Yeah, something did seem off about a chloride attack on TCCA itself; but by forming HOCl it all makes sense.

Thanks again!!!

Ps: I U2U'd you.. If you've got time give it a read