Before I begin, I am aware of the alternative use of the product of this reaction, and I think it is very silly. I'm interested in dye chemistry, not
weight loss.
I don't have any of the reagent anyway, as it's seemingly impossible to get, and this is all purely a thought experiment. I'm very interested in the
reactions that produce sulfur black 1, for which I would need 2,4 DNP.
The reaction I am interested in is the alkaline hydrolysis of dinitrochlorobenzene. I have seen the Vogel synthesis and generally found it
underwhelming, and I am skeptical that it is particularly practical.
I have heard that the reaction with alkaline solution needs to have an excess of sodium hydroxide - in particular, I have seen 3-4x excess NaOH
concentrations suggested. Likewise, I have seen it said that one needs an autoclave - however, the reaction calls for a temperature of 100-105C. Won't
the autoclave raise the temperature higher than that? Every autoclave I've ever seen goes to about 130C. Why does this reaction form diphenyl ether at
all, and why doesn't it do that in an autoclave?
Secondly, I want to understand the purification process. According to the synthesis, the reagent is heated under reflux at about 100C for about 90
minutes or till it's clear. We then filter it hot, cool it, and then dump in some HCl until we get a yellow precipitate. We filter it again, wash with
cold water, and then dry the product. So, my question is this: Shouldn't the reaction produce 2,6 dinitrophenol? What confuses me here is this: My
reading shows me that 2,4 DNP is very insoluble in water and 2,6 DNP is highly soluble...but why? That's my reasoning for why this purification works,
but is this really all that's required?
Thanks for reading. I hope somebody can sort out this very simple reaction for me. Boffis - 15-6-2018 at 04:19
@DyeKinetics, welcome to SM. I thought I was the only person left in the world interested in dye intermediate and dye chemistry.
I have carried out this reaction and infact started a thread on SM about this and related reaction of 2,4-dinitrochlorobenzene at a time when you
could buy it off Ebay. Unfortunately, the thread attracted so much drivel and shit I eventually just abandoned it.
However, the hydrolysis does work but my experience is that the sodim carbonate route is very slow unless carried out in an autoclave at c 130-140 C.
With excess sodium or potassium hydroxide it can be carried out at atmospheric pressure but again I found the hydrolysis is slower than reported in
the literature, I found 4-5 hours to be necessary, but in the end it does work and if you have a little unconverted material it is easily recovered. A
large excess of NaOH helps prevent the formation of diphenyl ethers; a molar ratio of 2,4-dncb:NaOH of 1:3 seems to work fine. If the reaction mixture
contains undissolved material (usually residual 2,4-dncb and the equivalent diphenyl ether) they are easily removed by filtering the warm mixture
through sintered glass. The excess NaOH keeps the dinitrophenol in solution until it is acidified. Note that the K salt is less soluble so you need to
dilute the mixture before it cools too much or the dinitrophenol K salt crystallises out (the sodium salt does too but only at about 25 C and below
hence the reason you have to filter warm).
If you use a smaller excess of NaOH you get a larger amount of diphenyl ether, I have a paper about this somewhere but it isn't why some
nitrochlorobenzenes form ethers and some phenol preferentially. I suspect that since the 2,4-dncb is very sparingly soluble in water that the excess
alkali reacts with the available chlorocompound faster that it can react with the phenate ion and form an ether.
2,6-dinitrophenol only forms if the original material contains significant 2,6-dinitrochlorobenzene. My material claims to be less than 2% of the 2,6
isomer.
In the prepublication section of SM there is a preparation of 2,4-dinitrobromobenzene which should undergo hydrolysis more readily than the chloro
compound.Nicodem - 15-6-2018 at 09:17
Hydrolysis of 2,4-dinitrochlorobenzene most likely inevitably proceeds partially trough bis(2,4-dintrophenyl) ether, however at those reaction
conditions this ether also cleaves to give the same end product, 2,4-dinitrophenol (see DOI: 10.1021/ja01371a026). 2,4-Dinitrophenolate is actually a
pretty decent leaving group. That's why you need such relatively harsh conditions, as just the excess of hydroxide would not do much, particularly
not in a biphasic reaction mixture.
In the prepublication section of SM there is a preparation of 2,4-dinitrobromobenzene which should undergo hydrolysis more readily than the chloro
compound.
Actually, the reactivity in nucleophilic aromatic substitution goes like F>Cl>Br,I. The rate limiting step is the nucleophilic addition rather
than the elimination step. For this reason, F, which has the strongest inductive effect, is also the most easy halide substituent to substitute.
I knew that 2,4-dinitrofluorobenzene was in some reaction much more reactive than the chloro compound and for that reason is used as a derivatizing
agent for various organic compounds but I had it in my head that bromo and iodo compounds were also more reactive to this type of substituion. Never
mind Dyekinetic, chlorobenzene is also fairly readily nitrated with sodium nitrate and sulphuric acid so its still reasonably accessible if not
exactly OTC.
Just a thought, I found a synthesis for 2,4-dinitrofluorobenzene from the chloro compound by halogen exchange with KF. Can it be prepared by direct
nitration of fluorobenzene though? Can you dinitrate iodobenzene without oxidizing it?Pumukli - 21-6-2018 at 11:48
Boffis:
I think you'd encounter problems on the direct nitration route: the ortho positions in fluorobenzene (relative to F) are too strongly deactivated if I
remember well. DyeKinetics - 28-6-2018 at 00:57
Boffis: By no means are you the only one interested! I'm actually largely interested in DNCB's dye reactions, and the reaction of DNCB -> DNP is
useful for sulfur black 1, but that's only one dye of many it can make, it's a very versatile compound - as I am sure you know already.
To avoid aforementioned clogging with drivel/shit, do you think it might be best to discuss this via PM of some kind?
Thanks for taking the time to reply to this sensitive topic.
