Acid halides from amides

Quote: Originally posted by Frodo_Baggins

Comparing the two, I came to the realization that perhaps the amide could be instead reacted with HX/CuX like a simple amine would to form an acid halide, perhaps in a mechanism similar to the below. RCONH2 + HNO2 + H+ -> RCON2 + 2H2O RCON2 + HX (CuX)-> RCOX + N2 This is obviously a modified sandmeyer reaction.

Your equations are incorect since they do not balance by charge and RCON2 is probably not what you wanted to write since it would be a radical (you obviously meant to write RCON₂⁺.
Of course this can not be a Sandmeyer reaction since besides the diazonium intermediate it has nothing in common - this is not about a nucleophilic aromatic substitution as you are talking about carbonyl compounds.
First of all you should understand what the Sandmeyer reaction is. There are essentially two types of Sandmeyer reaction different in the practical approach, conditions, catalysis and solvent(lessness). One is the SN1 aromatic substitution and due to the terrible instability of the Ar⁺ it only works when the nucleophile is water or fluoride (actually BF₄^-. The second, more commonly taught in the pedagogic institutions, is the S_RN1 reaction of aromatic diazonium salts which therefore requires a Cu catalyst.
Now, obviously RCON₂⁺ can not react via a radical species since the RCO⁺ resulting from normal decomposition is so very stabilized. Therefore copper catalyst have nothing to do with what you propose. The RCON₂⁺ is already very unstable as it is, it decomposes instantaneously, and therefore needs no catalysis to react with the nucleophile.
However, if you would want to obtain an acid chloride from the amide you must avoid the presence of any other nucleophile besides Cl^-. This means the nitrosation must be done with a reagent that will form no water or any other nucleophilic species during the reaction. If you consider the nitrosation of an amide with NOCl you will see that one equivalent of water forms. Similarly with an alkyl nitrite/HCl you would also get one equivalent of water and one of the alcohol. You would have to use NCl₃ which is however unlikely to react with amines without troubles and since it is a hazardous explosive it would be nonsense to even bother.
Of course you could also use almost anhydrous conditions at low enough temperature where acyl chlorides hydrolyse only slowly and absorb water as it forms. This way you could use NOCl and obtain some acyl chloride in mixture with the carboxylic acid...
I did not bother checking the literature, but you are welcome to do so as you might find something interesting on the topic. I hope this helped.

Sauron: Another common method of preparing acyl azides is also by the nitrosation of hydrazides, RCONHNH₂ + HNO₂ => RCON₃ + 2H₂O.

[Edited on 8/4/2009 by Nicodem]

Appreciate the answers. Yeah I know I need to go back to school and re-learn all the technical stuffs

Anyways it was an idea that I had one night and I didn't see anything wrong with it personally (other than it requiring anhydrous conditions which would be super hard in such a reaction), but I knew there was a reason it wouldn't work as listed. That's why I put it in the beginner section Thanks for explaining that to me.

From what I've taught myself I usually understand that most oxidized groups react quite differently than their reduced counterparts (ex, acids vs alcohols, amides vs amines and acyl halides versus haloalkanes.)

Well, the idea was not stupid in itself. A method of "activation" of a -CONH₂ can be synthetically useful. The only problem is in that activation into an acid chloride is not desirable unless it can be used in a one-pot methodology. Otherwise, it is just much simpler to hydrolyse the amide to an acid and use a common coupling reagent (including cheap ones like CDI or DCC). Amides, and not just of the -CONH₂ type but also -CONR₂, can also be hydrazinolyzed to hydrazides which upon nitrosation give acyl azides (as mentioned in the previous post) which can be considered as activated carboxylic acids (they are pretty electrophilic and react with amines just as acid chlorides just much less rapidly so).
It might be possible that by using a combination of NaNO2/AlCl3 in diethyl ether at 0°C you could get acyl chlorides from RCONH₂ type of amides since the water formed during the nitrosation would be immediately consumed by the acid used (AlCl3).

About the reactivity of the functional groups... the ones you mentioned are all very different and it is not about their oxidation state per se, it is about electronic densities, resonance structure and reaction mechanisms. You must be at the learning stage where you look up for symbolisms in the chemical structures, but there is no such things. Electrons just do not care about symbolisms, they only follow the rules of molecular orbitals. Don't worry, you will grow over this stage once you read a few dozens of books, a few hundred of reviews and several thousands of papers. Enjoy your learning process!

Quote: Originally posted by Nicodem

It might be possible that by using a combination of NaNO2/AlCl3 in diethyl ether at 0°C you could get acyl chlorides from RCONH<sub>2</sub> type of amides since the water formed during the nitrosation would be immediately consumed by the acid used (AlCl3).

No, it was not from a reference. Actually, I didn't even bother checking SciFinder or Beilstein. I just gave that as an option where the acid used is not protic, reacts with the water formed, while at the same time it provides a source of HCl.
I don't think aluminium nitrite could exist as a characterizable salt. I think the Al(III) cation is too acidic to keep the nitrite in the ionic form and prevent the decomposition to N₂O₃. But I mentioned no Al(NO₂₃ and don't really see why you call for it. I just proposed the use of AlCl₃ ethereal solution as an acid for nitrosation with NaNO₂ in the sense of:

R-CONH₂ + NaNO₂ + AlCl₃ => R-COCl + N₂ + NaCl + "Al(OH)₂Cl"

... where "Al(OH)₂Cl" is obviously not a compound but rather an undefined hydrated aluminium oxychloride, kind of what one gets by quenching AlCl₃ with water and evaporating to dryness.

Quote: Originally posted by Nicodem

But I mentioned no Al(NO<sub>2</sub><sub>3</sub> and don't really see why you call for it. I just proposed the use of AlCl<sub>3</sub> ethereal solution as an acid for nitrosation with NaNO<sub>2</sub> in the sense of: R-CONH<sub>2</sub> + NaNO<sub>2</sub> + AlCl<sub>3</sub> => R-COCl + N<sub>2</sub> + NaCl + "Al(OH)<sub>2</sub>Cl"

I fully understood your point !
I was just trying to imagine what could happen...assuming the electropositivity of Na(+) vs Al(3+) and the electronegativity of Cl(-) vs NO2(-), one can imagine that Na(+) will be more attracted to Cl(-) than to the NO2(-)...also in etheral maybe precipitate NaCl and favourise the equilibrium to produce Al(NO2)3.
Also the decomposition of the Al(NO2)3 might be a competitive reaction...so everything depends on the kinetic and nothing is better than a trial experiment...but with the due cautions and microscale of course!
It would be, if it succeded, a very interesting alternative route to Acyl halide from cheap material or easily available reactants...

Quote: Originally posted by PHILOU Zrealone

I was just trying to imagine what could happen...assuming the electropositivity of Na(+) vs Al(3+) and the electronegativity of Cl(-) vs NO2(-), one can imagine that Na(+) will be more attracted to Cl(-) than to the NO2(-)...also in etheral maybe precipitate NaCl and favourise the equilibrium to produce Al(NO2)3.