Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: swintarka on June 16, 2015, 07:14:43 PM
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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
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There was a very recent discussion in this forum which may help you further.
http://www.chemicalforums.com/index.php?topic=80719.msg293854#msg293854 (http://www.chemicalforums.com/index.php?topic=80719.msg293854#msg293854)
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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.
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Try loss of N2 to produce a carbonium ion and protonation. You do have a strong acid.
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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.
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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"
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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.
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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(-) :rarrow: 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 (https://books.google.com/books?id=FVXC2Ky792wC&pg=PA109&lpg=PA109&dq=aryl+carbocation&source=bl&ots=_lR2KP2wNs&sig=_1efsKaPp7h1ePB6TMeU0z3xbVk&hl=en&sa=X&ei=bdqBVZ-bJIqhNpCqgOAB&ved=0CKUBEOgBMBU) said, "Common error alert: If your mechanism has an alkenyl, alkynyl, or aryl carbocation as an intermediate, it is almost certainly incorrect."
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Thank you all for the replies!
Try searching for "aryl carbocation". This is what Robert Grossman (https://books.google.com/books?id=FVXC2Ky792wC&pg=PA109&lpg=PA109&dq=aryl+carbocation&source=bl&ots=_lR2KP2wNs&sig=_1efsKaPp7h1ePB6TMeU0z3xbVk&hl=en&sa=X&ei=bdqBVZ-bJIqhNpCqgOAB&ved=0CKUBEOgBMBU) 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 :D
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.
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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.
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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 :D
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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.
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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.
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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....
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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).
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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.
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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
8). 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
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@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 (http://pubs.rsc.org/en/content/articlelanding/2008/ob/b809521c)
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.
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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.