Well i think i have a couple of thoughts on all this.
First, although it wasn't my suggestion that H2SO4 is used to make nitrites i still think that it should work, possibly better. The bottom line is that HCl is just as suitable for producing nitrites as is sulfuric, it's just not used because HCl vapors would contaminate the nitrite.
I'm quite certain that formation of alkylhalogenides has nothing to do with this rxn at all, because HCl turns into NaCl the very instant it hits the rxn surface. You can't make ethyl chloride w/NaCl
Now to the possible rxn mechanism. This is an exert from
https://www.thevespiary.org/rhodium/Rhodium/chemistry/nitrite2nitro.txt
:
The theory of isomeric conversion not being adequate to explain the formation of the entire quantity of the nitro-compound actually produced, the formation of unstable intermediate compounds becomes the only alternative explanation of the formation of the aliphatic nitro-compounds. This additive hypothesis is
summarised thus: Nitrous acid, which forms aliphatic nitrites, and not nitro-compounds with alcohols, evidently possesses the oxylic constitution HO-N=O, and the inorganic nitrites, including silver and mercurous nitrites, being obtained simply by the neutralisation of nitrous acid with inorganic oxides, would also possess the same constitution. When the alkyl iodides react with silver or mercurous nitrite, or when the alkyl sulphates react with alkali nitrites, two reactions take place, namely, (1) the aliphatic nitrites are obtained by direct interchange, and (2) unstable intermediate compounds are formed, which decompose with the formation of nitro-compounds thus:
AgO-N=O + C2H5I => AgO-N(C2H5)-OI => C2H5-NO2 + AgI
KO-N=O + C2H5KSO4 => KO-N(C2H5)-OKSO4 => C2H5-NO2 + K2SO4
I vaguely recall seeing another paper on the web that explained the mechanism (speaking of NaNO2/DMF) some other way, but i can't remember it for my life's sake.
OK, anyway. From what i have recently learned
NO2- ion has two nucleophilic centers, N and O
which obviously compete w/each other in rxns like ours. In this case it would mean that we get nitro as soon as nitrogen prevales and forms the menioned above adduct. Now as you easily can see in NO2- ion one of the oxygenes has an extra electrone which makes it a better nucleophil than the N atom. Now, in
undissociated HNO2 it doesn't have that advantage and makes nitration possible. It's entirely consistent w/the fact that in all nitrations (AgNO2; EtBr/DMSO; NaHSO3) the nitro carrier is not dissociated.
Which brings us to suggest that we need some undissociated HNO2, which means - unhydrous conds!
Now, the role of acid. NaNO2 doesn't react w/glacial AcOH at all. In fact, glacial AcOH doesn't even react w/carbonates. So, just like Antibody2 suggested in the old thread, it is to create some initial amt of HNO2 that we need acid for. As soon as some water appears in the rxn, AcOH itself begins to dissociate and liberate HNO2 - and indeed, when you add some acid,
all the nitrite immediately changes its appearance but - note that - no NO2 is formed! Which means that we have a HNO2/AcOH soln. As the alcohol is processed, though, more water appears into the rxn which causes more HNO2 to dissociate and form nitrite, which formes yet more water - and faster than w/nitroalkane... The snowball effect. It does indeed start very gradually - first a couple of tiny bubbles, in ten minutes a fucken volcano.
Again, the purpose of adding CH2Cl2 seems to be its forming an emulsion with water/AcOH. Since all alcohols in the original article were water-insoluble, they'd stay in the DCM reacting w/undissociated HNO2, while its dissociated portion was contained in the aqueous medium. I think that's why the yields were so high. In our case the alcohol is water soluble, and that's bad
.
So...Where does that leave us? Obviously, the water must be removed somehow. But i really see no way to do this... The point is to remove the extra water and not dehydrate HNO2 lest it should fly away - H2SO4's no good...
Another idea that comes to my mind is rxn between alc. and NO2/NO mixture at, say, 130 C. I suggest that catalytic trace amt of water is needed here - see nitrite2nitro.txt, section "Influence of moisture" for details. - BTW, this fact also supports the HNO2 hypothesis
One might say that it's the same as isomerizing nitrites - well, no it's not the same, the yields should be much better, since the rate limiting step in that isomerization is the dissociation of nitrite, not the formation of nitrocompd. In practice, that could mean that instead of designing a 3 meter all-glass anakonda one could settle w/smth that fits into his usual oil bath...- BTW, with a 2 meter pipe filled w/asbesthos the claimed yield is around 25%...yuck.
Well, that's it for now. Please feel free to flame me if i fucked smth up. Any ideas are highly appreciated.
Antoncho