PrimoPyro
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Mechanics of Aromatic Diazotization (with diagrams!)
LOL.
Um hi......how is everyone? This was meant to be part of a much longer post, that I had intended to post several days ago, and am still not finished
with. I have decided not to post the original mega-long thread, but instead gradually post each individual part by itself as a stand-alone.
This part happened to be the middle reaction of the pathway I was working with. Sorry for the delay Polverone, I have had very little time to further
the paper since our last correspondance, and it is still unfinished. Here is one of the more relevant parts, the mechanism of aromatic diazotization
reactions:
Aromatic Diazotization
Aromatic diazotization is the reaction of anilines with nitrous acid to form diazonium salts, which are ionic compounds that contain the Ph-N=N+ ion,
which is stabilized through electron resonance to Ph-N(+)#N, where # is a triple bond. The cation can also be partially distributed throughout the
aromatic ring for added stability, although the compound still reacts as though the nitrogen carries the charge, not the ring.
It is common to not use nitrous acid as the diazotizing agent, but rather to use a nitrite salt to provide the nitrite ion upon addition of a stronger
acid such as HCl. HCl serves a dual purpose, first to facilitate HO-N=O formation, and then to convert the nitrous acid into a better electrophile,
O=N-Cl + H2O. The reaction mechanism of diazonium salt formation is like this:
Alright, first we see sodium nitrite and hydrogen chloride in solution, dissociated to their ions. HCl being in excess drives the equilibrium to the
right so that sodium chloride and nitrous acid predominates. Next, another proton from another molecule of HCl protonates the hydroxide of the very
weak acid, nitrous acid, giving the oxonium ion of nitrous acid. This acts as a leaving group in a standard nucleophilic substitution reaction where
the leaving group is H2O and the nucleophile is the chloride ion, producing notroxyl chloride and water. This reaction is an equilibrium reaction, but
is easy to use since the nitroxyl chloride entity is highly reactive toward primary amines, and the reaction continues to progress easily because of
reaction of the nitroxyl chloride intermediate with the aniline, which decreases the concentration of nitroxyl chloride in solution, keeping the
equilibrium in favor of nitroxyl chloride production rather than hydrolysis.
Next, our nitroxyl chloride, which is highly electrophilic, bonds to the lone pair in aniline’s amine nitrogen, followed by extrusion of the
chloride ion to give the intermediate pictured in the very center of the diagram. (note: In the picture of aniline, I messed up and drew
dimethylaniline. Imagine each of those groups off the nitrogen being a hydrogen, not a methyl, please.) One of the two protons on the amine then
leaves from attraction to chloride ion, and the nitrogen cation is neutralized to the middle molecule of the left column.
The next step is tautomerization in the acidic environment, where the nitroso group rearranges to the oxime by electron resonance followed by
migration of a proton. This happens by the nitroso-oxygen withdrawing an electron from its nitrogen, forming an oxide ion, and the nitrogen shuttles
the positive charge to the other nitrogen, which then retrieves and electron from the proton bound to it, and the proton, now ejected from the amine,
migrates to the oxide ion to become a hydroxyl group for the oxime. This step is reversible, but since the oxime decomposes, again the equilibrium is
forced to the right because the concentration of oxime never gets very high due to decomposition.
The decomposition which I speak of is resultant of the next step, where another proton from another molecule of HCl interacts with the oxime, forming
another oxonium leaving ion, facilitating another nucleophilic substitution where water is the leaving group and chloride ion is the nucleophile. The
formed nitrogen cation is very prone to resonance, which I show in the last step. Both resonance forms react exactly the same, and both are present in
all diazonium solutions.
PrimoPyro
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Lowry
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I bet your Organic-chemistry teacher would weep if he/she saw the way you draw electron arrows.
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PrimoPyro
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1.I dont have an O-chem teacher. I am self taught and have yet to go to professional school.
2.I know they are wrong. I found this out recently. My mistake was assuming that the arrows represented any kind of movements, be they of electrons or
of particles, such as protons. When viewed in this light, the arrows are correct. When viewdd technically, they are far from correct.
Simply reverse the direction of the proton-movement arrows and you will have a correct schematic.
PrimoPyro
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