[moderate]

Let the starting reagent be called RCON.

Some sources suggest that the elimination of Bromine occurs in the same step as the formation of the R-N bond.

Therefore, no nitrene intermediate is formed.

Links:

Source 1

(page 4)

Source 2

(page 302)

Source 3

(page 1)

But, some other sources suggest that bromine is first eliminated to form a nitrene, and only then the isocyanate is formed.

Source 1

(page 139)

Source 2

(scroll to 'Rearrangements of Acyl Carbenes ')

Can anyone please explain this to me? What is going on?

EDIT: is this a situation where there is a transition state that has a high "nitrene-like" character? Still, it should either be a 2 step or a 1 step reaction.

[SVXX]

Well I've studied that there is a definite nitrene intermediate through which the reaction proceeds. But it's a matter of debate...I read somewhere on the net that the nitrene doesn't form at all, just like you've found. It's sufficient for me within my (competitive exam) syllabus to think of it as a nitrene-intermediate reaction, but I think we'd better wait for more replies if you want to know what's actually going on.

[OC pro]

Elimination of bromine leads to formation of an intermediary isocyante which is highly reactive.

[SVXX]

Actually OC pro, one school of thought says that the alkyl isocyanate is obtained after the elimination of bromine yields a nitrene, which rearranges for that purpose.
The latest school of thought says that a nitrene is not formed at all.
Ideally it is something of a mix between the two. The nitrene formation and rearrangement is so fast it can be taken as a transition state.
(PS : I just noticed that the thread creator mentioned this in his last line too)

[OC pro]

One can draw the nitrene intermediate. But it´s not a must. More important is to draw the isocyanate as an intermediate.
Don´t forget: mechanisms are in most cases only proposals!!

[SVXX]

Well indeed so but it makes a marked difference to me as it can be mentioned in competitive engineering examinations!

[OC pro]

I see no difference. One can directly form the isocyanate when drawing a mechanism from RC=ONHBr.
For an examination this will be handled as correct since one can refer to a textbook!!

I have prepared exams for bachelors studs in organic chemistry years back. When it came up to mechanisms I was always fair with the correction.
Leaving a nitrene or not in the Hofmann rearrangement is such a tiny detail. Not even worth the discussion. Nice to know but I feel the isocyanate is more important to know.

[moderate]

I see no difference.

Well, one difference is the production of side products.

If a nitrene intermediate (RC=ON) does exist (nitrogen has 2 lone pairs), then there is probably more of an opportunity for other nucleophiles to react with the nitrogen.

On the other hand, if a nitrene does not exist, and the transition is made from (RC=ONBr)^- to R-N=C=O directly then other nucleophiles would not be able to attack the nitrogen.

One such nucleophile could be water, and its reaction would yield a hydroxamic acid.



So, I wanted to know what the accepted mechanism is today.

SVXX: you are saying that the newer school of thought does not have a nitrene intermediate?

[orgopete]

While the arguments for a hydroxamic acid seem reasonable, I don't consider them conclusive. Let us assume the reaction is concerted, no nitrene forms, and hydroxamic acid is a by-product. That would simply mean that another mechanism is competing, such as direct substitution. Similarly, let us assume the reaction is stepwise, a nitrene intermediate is present, and no hydroxamic acid is produced. Then the intramolecular rearrangement is simply faster than the intermolecular reaction.

I doubt that concerted versus stepwise can be determined in this forum. I however prefer to take the philosophic position that all reactions are stepwise. That is, if time can be broken into infinitely small units, then it becomes impossible for three balls to collide simultaneously. One will always collide before the other. (That was Jeremy Knowles' argument as I recall.) I find that philosophic position to be more useful than concerted options. I prefer to think of bromination reactions, hydroboration reactions, addition of HX, mercuration reactions, and others as stepwise. That allows an alkene to react as the nucleophile and polarizations are consistent with carbocation stabilities. It also avoids the awkwardness of suggesting 2 + 2 electrocyclic reactions. I have even read arguments that Diels-Alder reactions are actually not concerted. Indeed, I think of the Hammond Postulate about intermediates being reactant-like or product-like anticipating electron movements not being concerted. If they were concerted, there should be no difference. I only use concerted mechanisms if a stepwise mechanism provides no insight to how a reaction may take place or how its products might be controlled.

For the purpose of this question, I think it is doubtful that we could come to a categoric decision on whether a nitrene is involved or not. I suspect that we could find some examples that are more nitrene-like and others not. That is frequently the case with many reactions. The more closely we examine the steps and the strength of the proof, we may find that we really cannot determine one versus another.

The problem with this question is not about what the mechanism is, but rather what the questioner expects. This is common to this forum, posters have to try to guess what was discussed in a class they did not attend to give an answer. If a professor is going to favor one answer versus another, I would ask him which he preferred if I hoped to give that answer to his question. (I hope you see that answer as different from the philosophic question about what the mechanism or mechanisms actually is.)

[moderate]

While the arguments for a hydroxamic acid seem reasonable, I don't consider them conclusive. Let us assume the reaction is concerted, no nitrene forms, and hydroxamic acid is a by-product. That would simply mean that another mechanism is competing, such as direct substitution. Similarly, let us assume the reaction is stepwise, a nitrene intermediate is present, and no hydroxamic acid is produced. Then the intramolecular rearrangement is simply faster than the intermolecular reaction.

Hello,

Thank you for your answer!

You have given me some things to think over.

As a side note, I am not actually currently taking a class in organic chemistry. I was simply studying on my own, and found it very confusing that different sources seemed somewhat dogmatic in suggesting different mechanisms without even suggesting the possibility of another mechanism.

[SVXX]

The problem with this question is not about what the mechanism is, but rather what the questioner expects. This is common to this forum, posters have to try to guess what was discussed in a class they did not attend to give an answer. If a professor is going to favor one answer versus another, I would ask him which he preferred if I hoped to give that answer to his question. (I hope you see that answer as different from the philosophic question about what the mechanism or mechanisms actually is.)

You've put the nail in the coffin. This is exactly what I wanted to convey. In my case, the questioner expects a nitrene-intermediated mechanism.