[swintarka]

Hello!
Any idea what is the mechanism of the last step? How diazonium group is replaced by hydrogen in acidic conditions?

http://www.orgsyn.org/demo.aspx?prep=CV2P0592

[discodermolide]

There was a very recent discussion in this forum which may help you further.

http://www.chemicalforums.com/index.php?topic=80719.msg293854#msg293854

[swintarka]

Thanks, it is an interesting discussion, but the essence of my problem is not related. In case of hydride sources, the mechanism is quite clear. But here there's none. Only sulfuric acid and ethanol, which I can't imagine to produce hydride source. Neither radical mechanism, like in Cu(I) case, seems possible here.

[discodermolide]

Try loss of N₂ to produce a carbonium ion and protonation. You do have a strong acid.

[orgopete]

I don't know the mechanism though I am very doubtful that it is an aryl carbocation. Although an aryl carbocation can be a useful model to predict the reaction products, other reactions suggest this is not how the decomposition of diazonium salts occurs. Of course I'm willing to change my mind if someone can provide evidence to the contrary.

The manner in which the reaction has been written in the OrgSyn prep is the problem. I knew diazotization reactions needed to be kept cold to avoid a further reaction with nitrite. This example shows what that "competing reaction" is. The OrgSyn prep is actually a reduction of the diazonium salt with nitrous acid. The OrgSyn prep also references another reaction in which phosphorous acid is used as the reductant.

I hope this may give some help, though I cannot give a mechanism.

[clarkstill]

In the absence of any nucleophilic sources of hydride, it looks like it must be a radical reaction, something like the reaction described here:

http://pubs.acs.org/doi/pdf/10.1021/jo00111a032

Nitrous acid or nitrite reduces ArN2+ to ArN2·, which decomposes to N2 and Ar·, then hydrogen abstraction from ethanol?

PS I enjoyed the phrase "sodium nitrite [was added] as rapidly as the violence of the reaction will permit"

[pgk]

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 mechanism of Fe(III)/Fe(II) oxidation with the one of H2SO4 oxidation that work in different ways.
Hot H2SO4 is an oxidant mineral acid that can easily oxidize ethanol to acetaldehyde. To detail:
Simple mixing of H2SO4 with EtOH produces the corresponding mineral monoester that upon heating is transformed to the corresponding diester:
EtOH  +  H2SO4   →    EtOSO3H   +   H2O  →   (EtO)2SO2  +   H2SO4  + H2O
Monoestrification reaction is reversible and thus, by further heating and without removing the so formed diethyl sulfate and water, the monoester is hydrolyzed and gives back EtOH and H2SO4:
EtOSO3H   +   H2O   ←→     EtOH  +  H2SO4
The so re-formed hot H2SO4 oxidizes EtOH to acetaldehyde through hydride transfer and sulfurous acid is so formed (Remember that H2SO4 fully ionizes in ETOH):
EtOH  +  HSO4(-)   →   CH3CH=O   +   H2SO3 +   H2O 
(Please see, the detailed mechanism in the attached file.)
In presence of the aryl carbocation that is formed during heating of the diazonium salt, During Sandmeyer coupling hydride anion prefers to react with the aryl carbocation that is formed during heating of the diazonium salt, instead of the sulfur oxide and thus, sulfurous acid is not formed.
ArNΞΝ(+)  →  Ar(+)  + N2 
Ar(+)   +   H(-)  →  ArH
The reasons of hydride’s reactive preferences can be explained by the HSAB theory (please, see a recent discussion on the issue). Please note that the aryl carbocation is a borderline acid, H(+) is a hard acid, -S=O  is a borderline base and H(-) is a soft base.

[orgopete]

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 mechanism of Fe(III)/Fe(II) oxidation with the one of H2SO4 oxidation that work in different ways.
Hot H2SO4 is an oxidant mineral acid that can easily oxidize ethanol to acetaldehyde. To detail:
Simple mixing of H2SO4 with EtOH produces the corresponding mineral monoester that upon heating is transformed to the corresponding diester:
EtOH  +  H2SO4   →    EtOSO3H   +   H2O  →   (EtO)2SO2  +   H2SO4  + H2O
Monoestrification reaction is reversible and thus, by further heating and without removing the so formed diethyl sulfate and water, the monoester is hydrolyzed and gives back EtOH and H2SO4:
EtOSO3H   +   H2O   ←→     EtOH  +  H2SO4
The so re-formed hot H2SO4 oxidizes EtOH to acetaldehyde through hydride transfer and sulfurous acid is so formed (Remember that H2SO4 fully ionizes in ETOH):
EtOH  +  HSO4(-)   →   CH3CH=O   +   H2SO3 +   H2O 
(Please see, the detailed mechanism in the attached file.)

A reduction has occurred here.

In presence of the aryl carbocation that is formed during heating of the diazonium salt, During Sandmeyer coupling hydride anion prefers to react with the aryl carbocation that is formed during heating of the diazonium salt, instead of the sulfur oxide and thus, sulfurous acid is not formed.
ArNΞΝ(+)  →  Ar(+)  + N2 
Ar(+)   +   H(-)  →  ArH
The reasons of hydride’s reactive preferences can be explained by the HSAB theory (please, see a recent discussion on the issue). Please note that the aryl carbocation is a borderline acid, H(+) is a hard acid, -S=O  is a borderline base and H(-) is a soft base.

One would think that a hydride donor would react with acid, for example H(+) + H(-)  H2.
This occurs with hydride donors like borohydride, aluminum tetrahydride, sodium hydride, etc. I do not think either nitrous acid or hypophosphorous acid act as hydride donor. They may act as reductants or electron donors, but not as hydride donors. (I don't know the mechanism of the reductions.)

Although I could only read the abstract of the ACS paper referred to by clarkstill (kudos), this paper seems correct. Not only do I find HSAB theory weakly predictable, but I fail to see how it should have been applied in this reaction. Perhaps you could explain what is it about HSAB theory that applies here that these authors overlooked and perhaps why they did not invoke an aryl carbocation.

Try searching for "aryl carbocation". This is what Robert Grossman said, "Common error alert: If your mechanism has an alkenyl, alkynyl, or aryl carbocation as an intermediate, it is almost certainly incorrect."

[swintarka]

Thank you all for the replies!

Try searching for "aryl carbocation". This is what Robert Grossman said, "Common error alert: If your mechanism has an alkenyl, alkynyl, or aryl carbocation as an intermediate, it is almost certainly incorrect."

But N₂ is there mentioned as an exemption from the rule

As for hydride sources, I think that that aldehyde in Cannizzaro reaction can be counted as one, yet no hydrogen evolution is observed (as far as I know, based on my limited experience with this reaction). Similarly, biochemical NADH reductions also do not involve H₂.

[orgopete]

Re: Grossman
Okay, caught me on this one. It was like the 16th hit and the actual quote looked like this:

The Art of Writing Reasonable Organic Reaction Mechanisms
https://books.google.com/books?isbn=0387954686
Robert B. Grossman - 2003 - ‎Medical
Common error alert: If your mechanism has an alkenyl, alkynyl, or aryl carbocation as an intermediate, it is almost certainly incorrect. no stabilization of empty ...

So, I didn't actually read it. Sorry.

Re hydride source or "can I write a hydride reduction without hydride?"
See attached. I'm not saying this IS the mechanism, just that it could react like this. Given the pH, this could easily be written differently with protonation of the diazonium salt. The intermediate is then an imino analog of the Weisenheimer complex of nucleophilic substitution reactions. The NO2(+) would react with water to give nitrate or nitric acid.

[swintarka]

That mechanism seems quite reasonable, though unusual. Thanks!
I guess it could be verified quite easily by preparing appropriate nitrite complexes in lower temperature, so maybe some time in the future I will try it

[pgk]

From where do I start and where do I finish?
1). Ethanol is not a simple solvent in the given reaction. Contrary, ethanol participates in the reaction and is finally transformed to acetaldehyde. Transformation of EtOH to acetaldehyde is an oxidation reaction. Oxidation reactions are always accompanied by the corresponding reduction ones. In the given reaction, in question, transformation of aryl diazonium salt to the deazotated aryl compound, is the accompanying reduction reaction. In oxidation reaction of EtOH alone by H2SO4, transformation of H2SO4 to H2SO3, is the accompanying reduction reaction.
2). HSAB theory is just a useful tool that helps to understand how some reaction mechanisms occur. But on the other hand, HSAB theory is not the only and unique holly text. Other significant principles, such as thermodynamic laws and mass and charge equilibria must also be respected. Thus, it is obvious that if adding a mineral acid to sodium hydride, neutralization and a vigorous formation of H2 will occur. But if in acidic medium, the soft base H(-) meets a soft or a borderline acid, it will prefer to react with them rather than with the hard acid H(+).
3). In the given reaction, in question, the H(-) donor is ethanol and not the nitrous acid of the initial step.
4). I haven’t seen the said paper. So, I cannot have an opinion about this.
5). Dr. Robert Grossman is right. Alkenyl carbocations are difficult to exist. However, this is somehow different in conjugated systems, e.g. butadiene, aromatics, etc. Therefore, this great scientist and educational who perfectly knows how to use the words, declares that “it is almost certainly incorrect” and not that “it is certainly incorrect”.
6). Not only Canizzaro but also Favorskii reaction, HCO2H reductions, KMnO4, chromic and similar oxidations work by hydride transfer.
7). Please do not confuse the recently proposed mechanism (protonated nitronium group as an hydride donor) with the one of the given reaction, in question (ethanol as an hydride donor). Both are Sandmeyer, but they are different reactions and with different nitrous acid stoichiometry.

[pgk]

1). Error Correction: Favorskii reaction does not work by hydride transfer, as wrongly mentioned above but in a somehow similar way, by means of generation of a stronger base by a weaker one, under specific conditions. Sorry for any potential confusions.
2). In order to avoid further confusions, the stoichiometry of the reaction, in question is cited hereby:
ArNΞN(+), HSO4(-)   +   CH3CH2OH   →    ArH   +  CH3CH=O  +  H2SO4  + N2↑
and the corresponding mechanism is described in the attached file.

[clarkstill]

Do you have a reference for this mechanism? It seems unlikely that the best nucleophile in this system is hydride, not merely the lone pairs on the alcohol. Also, bisulfate seems like a woefully inadequate base to do this deprotonation....

[pgk]

1). No, I don’t.
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.
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).