[orgopete]

The proposed mechanism is mainly based on the generally accepted, mechanistic schemes of HxMOy oxidations. Besides, the proposed mechanism fully respects the reaction’s stoichiometry.

When I have written oxidation mechanisms, I have found they follow a H-C-O-X pattern in which the X-group is a good electron withdrawer or acceptor, e.g. Cl, Br, Cr, Mn, etc. I am not aware of any instances in which X is hydrogen (to give hydride). Please provide an example of this oxidation.

Hydride is one of the strongest nucleophiles but please, note that in Cannizzaro and Favorskii reactions, as well as in HCO2H/NET3 reductions, the corresponding bases also seem woefully inadequate to generate strong nucleophiles, though their generally accepted, mechanistic schemes are similar with the proposed one, hereby.
2). Or course, the JOC reference that you provided, is correct but as mentioned before: "Oxidation/reduction reactions may occur by a variety of mechanisms, e.g. radical, electron transfer, hydride transfer, etc. Please do not compare and confuse the electron transfer mechanism of Fe(III)/Fe(II) oxidation (and the corresponding reduction) with the one of H2SO4 oxidation (and the corresponding reduction) that works by hydride transfer".
Sorry for the misunderstanding but it is a JOC and not a ACS paper (but it’s not your fault).

Just writing stuff isn't persuasive. "Substitution reactions can occur via SN1 or SN2 mechanisms."

...Fe(III)/Fe(II) oxidation...

Huh? There wasn't any iron in the OrgSyn prep or did I miss something?

In the post discussing the Sandmeyer mechanism, it was argued that an aryl carbocation formed and was captured by a halogen. I fail to see how the aryl carbocation should change affinity in this reaction. If we were to assume the subatomic charges also carry a field associated with them, then I would expect the carbocation to have a positive field which would be attractive to the negative field of electrons. Similarly, if I were to predict the fields associated with ethanol, I would expect the non-bonded electrons of oxygen to have a greater negative field than the electrons shared between a proton and oxygen. (I would further argue the OH-proton would have the greatest positive field of all of the protons of ethanol. This greater positive field would be consistent with it being the most acidic, proton donor.) All of this not withstanding, the electrons remain with the hydrogen and not the oxygen. Is this something I don't understand about HSAB theory?

These questions only challenge how the mechanism was written. I also saw in quarterly review of chemistry (or something like that) in 1952 on the title page the oxidation of ethanol to acetaldehyde with a diazonium salt. I can no longer find where I saw this nor verify the reaction. (Google is my only reaction searching tool.) I would expect a CH to be the source of the reduction and isopropanol to be a better reductant, for example, Oppenhauer oxidation (I'm not suggesting this could be).

I still think nitrous acid is the reductant and would be consistent with two equivalents of nitrite. I think this is in analogy to hypophosphorous to reduce diazonium salts.

[pgk]

Dear Orgopete,
1). The chemically correct mechanistic model of HxMOy oxidations assumes the formation of the corresponding mineral monoesters as intermediates. However, the chemically correct mechanistic model, among others, fails to explain the following:
1a). Why the corresponding mineral monoesters and diesters are not isolable, even spectromectrically, even at ultra low temperatures and even under strong dehydration conditions (contrary, molecular sieves accelerate chromic oxidations).
A few exceptions such as ruthenate and osmate vic-diesters at -70oC, as well as di(t-butyl)-chromate can easily be explained by alternative mechanisms such as (4n+2)π pericyclic and direct attack of the chromate anion to the t-butyl carbanion that is formed by E1 elimination mechanism, respectively.
1b). Why the corresponding carbonyl compounds are not formed during alkylation by sulfate and carbonate esters, respectively.
1c). Why oxopropanedial and pentaerythrital are not formed during the explosion of nitroglycerine and pentaerythrite nitrate, respectively.
1d). Why arylsulfonic acid is not formed during the said Sandmeyer reaction, in question.
And so on..
Contrary all above can easily be explained by hydride transfer during the said oxidation reactions.
2). Apart the SN1/SN2 Substitution reactions, nucleophile attack also occurs in many other and among them,  Cannizzaro autoxidation, Favorskii rearrangement and HCO2H reductions that are cited above.
3). No, there wasn't any iron in the Organic Syntheses preparation, in question. Just EtOH and additional H2SO4.
4). Not at all. Contrary, in the similar post discussing the Sandmeyer mechanism, it was argued that apart the soft base I(-), the borderline aryl carbocation acid fails to be captured by the rest of halogenides that are hard bases. Therefore, Cu(II) salts or NaBF4 are used, depending on the specific case.
Sorry for my English, if this is the source of any unclear and obscured description, during discussions.
5). Agreed. –OH protons are more electropositive, therefore HSO4(-)  attack would stop the reaction up to a proton exchange equilibrium. Contrary, HSO4(-) attack to C-H, favors the redox reaction, according to the corresponding potentials. In this case HSAB theory can explain why the hydride soft base prefers to react with the borderline aryl carbocation, instead of reducing H2SO4 to H2SO3.
7). Pease see the links, bellow
http://mystudyexpress.com/12%20state%20science/12th%20chem%20state/compound%20containing%20nitrogen/pdf%20file/7.pdf
and page 742 in:
https://books.google.gr/books?id=8wIQwCmWz9EC&pg=PA742&lpg=PA742&dq=ethanol+acetaldehyde+diazonium&source=bl&ots=DmP
. The confusion start by the nitrous acid as being the reductant and the hydride transfer in another example given in a previous reply. But this reaction occurs via a preliminary nuclophile attack on the bromobenzene ring that could also happen in the given reaction in question, too. However and by that mechanistic model, there is no accordance with the equilibrium of the reaction, in question. Besides, the reaction in question, also occurs in abscence of bromide substitution of the aromatic ring.
Regards

[orgopete]

@pgk
Yada, yada, yada, all irrelevant, but if it works for you, fine.

Since some hydrogen abstractions have been reported to accompany diazotization reactions, ethanol can be the source of the hydrogen, no question. What seems puzzling is the reversal of polarity. If an aryl carbocation should prefer to abstract a hydride from ethanol, why did the oxygen not react?

What happens if nitrous acid is omitted?
Methanolysis of 4-bromobenzenediazonium ions. Effects of acidity, [MeOH] and temperature on the formation and decomposition of diazo ethers that initiate homolytic dediazoniation
Alejandra Fernández-Alonsoa and Carlos Bravo-Díaza
Org. Biomol. Chem., 2008,6, 4004-4011

I didn't read the paper (no access), but let me summarize what I could learn from the abstract. If the reaction is less acidic, more hydrogen abstraction occurs. They theorize methanol adds to the diazo nitrogen similar to the formation of diazo dyes. The Ar-N=N-OMe decomposes to give an aryl radical. It abstracts a hydrogen from methanol. As the solution is made more acidic, then 4-bromophenol and 4-bromoanisole become the major products. They are present if the solution is less acidic as well.

Re posters original question, I still argue there is a stoichiometric significance for two equivalents of nitrite being used in the reductive decomposition of the diazonium salt. I further think that nitrous acid may act similarly to the often reported use of hypophosphorous for this reaction. I do concede that at least some of the reaction may occur via a radical abstraction from ethanol.

[pgk]

Yada, yada, yada and unfortunately more yada, yada, yada because my replies do not seem to be carefully  read. It was already mentioned above, as well as in a previous similar discussion that:
1). Ethanol is ionizable (slightly but ionizable) and as a consequence, HSO4(-)  attack to hydroxyl hydrogen would stop the reaction up to a proton exchange equilibrium and being localized to the right side and therefore,  no further redox reaction would occur.
2). Alcoholysis of diazonium salts, followed by formation of aryl radicals and initially proposed by Langley in 1957, is based on the on the Gomberg–Bachmann reaction. However, Gomberg–Bachmann reaction occurs in presence of copper powder or in an alkaline medium, contrary to the conditions of the said Sandmeyer reaction that are highly acidic and free of any form of copper.
http://arizona.openrepository.com/arizona/bitstream/10150/319333/1/AZU_TD_BOX2_E9791_1957_32.pdf
https://en.wikipedia.org/wiki/Gomberg%E2%80%93Bachmann_reaction
https://en.wikipedia.org/wiki/Diazonium_compound
3). The stoichiometric necessity for two equivalents of nitrite being used in the reductive decomposition of the diazonium salt, demands a preliminary nucleophile attack on the bromobenzene ring. However, hydrogen replacement of the aryl diazonium bisulfate by ethanol, can also occur in absence of bromide substitution of the aromatic ring.
4). The question is not “if it works for me” but rather “if it fits with the experimental and spectral data.” Furthermore, experimental data that do not fit, must be fully explained and not be considered as irrelevant and be hidden under the carpet.