Alright guys so as you can see on pages before I synthesized some benzamide, the test tube with reaction mix was basically sitting 2 years. So I tried
to wash the contents with water they were insoluble in room temp water. So I added alcohol and dissolved the stuff, there were still some crystals
left which could be urea degradation products. So I evaporated alcohol extract and got some white crystals. Mixed them with about 10-15% more of 5%
NaClO. It was colder but as I added NaClO mixture started foaming and some gas started evolving. Benzamide almost completely dissolved. There was
still some on top but I guess there wasn't enough hypochlorite. So the gas was evolving for quite some time. Now i have almost colorless solution here
and how can I even know if I obtained aniline? I have no nitrites.Crowfjord - 30-10-2013 at 16:03
Have you smelled it? Aniline is some stinky stuff (typical fishy amine odor). That would be a good indicator. Perhaps saturate the solution with salt
and see if a separate layer forms?Random - 30-10-2013 at 22:25
I have tried to take a whiff but I didn't notice anything. Maybe I didn't smell it enough. Shouldn't aniline saparate as layer even before NaCl?
I mean the reaction was apparent so something definitelly happened but I am not sure what.
[Edited on 31-10-2013 by Random]Crowfjord - 31-10-2013 at 09:47
Aniline is water soluble according to Wikipedia (3.6g/mL). Maybe it is too dilute to have a noticeable odor. What amounts of benzamide and solvents
did you use?Random - 1-11-2013 at 01:25
Actually, 3.6/100mL. Still somewhat significant. Ratio of benzamide to solvent is like 1:15, test tube amounts. I didn't heat the test tube to 60-80 C
like it's often suggested but I found that hoffman degradations occur even at room temps. Isocyanate intermediate should be already hydrolysed by now
anyway? There is still very little white powder floating on solution but most of it dissolved I guess.
Now when I think about it there might be some aniline dissolved as 1/15 is 0.06 and 3,6/100 is 0.036. Given that not all benzamide reacted, all of
aniline could be dissolved.
I just hope that I can isolate even a little bit of it. Maybe I should filter the solution from unreacted benzamide and then add HCl to make aniline
hcl and evaporate. Then dissolve and add xylene to basified mix and again hcl. But I think that mechanical losses would be too big for such small
amounts.
Do you have any details?testimento - 7-2-2014 at 09:09
Can sodium benzoate be converted into benzoic acid with weak acids(acetic, citric or oxalic) since primary acids aren't available to me? Would sodium
bisulfate work?Ascaridole - 7-2-2014 at 13:59
Weaker acid favored, plus benzoic acid has limited solubility in water.
Acetic, citric and oxalic are not likely as they are all carboxylic acids like benzoic acid however benzoic acids limited solubility may drive the
reaction sufficiently though I think the bisulfate would work best.DJF90 - 8-2-2014 at 03:31
Oxalic acid would probably work just as well as it has a relatively low pKa also (1.24 jumps to mind but I'd double check that first) gdflp - 13-7-2015 at 12:38
I have recently been experimenting with this procedure and I'm having mixed success. I scaled up smuv's preparation of benzamide 2x, and ran the
procedure three times in parallel, yielding ~110-130g of water recrystallized product each run. Temperature control was not as precise as it should
have been, this is likely the cause of the 70% yields. I have found that keeping the temperature below 185°C is important, if the temperature
exceeds that for extended periods of time, the yield decreases greatly and lots of water insoluble polymerized byproducts are observed.
As for the Hofmann rearrangement, I have had less success. I prepared a solution of basic sodium hypochlorite by chilling 713ml(1 eq) of a commercial
bleach titrated at 1.04M in a salt-ice bath and slowly adding 29.7g of sodium hydroxide to minimize temperature increases. This solution was placed
in a 1L round bottom flask and 90g of benzamide was quickly added with stirring. The mixture quickly took on what appeared to be a black coloration.
The color was actually a deep red, as evidenced by swirling the flask, leaving a thin red layer on the walls. This solution, with strong stirring,
was heated to 60°C for 20mins with a reflux condenser, then setup for a simple distillation. At this point it had the strong smell of aniline.
250ml of distillate was collected, at which point the receiver was switched since the distillate appeared to be running clear. Another 100ml of
distillate was collected which contained no aniline upon saturating with sodium chloride. The initial distillate was saturated with sodium chloride
and the upper aniline layer was removed and measured, yielding a miserable 16ml of dark red aniline, or a 23.6% yield.
Upon cooling, a compound in the reaction mixture crystallized, I'm thinking of filtering it off and saturating the filtrate with sodium chloride to
see if I can recover any more aniline. The Hofmann rearrangement with TCCA seemed to be successful for smuv, but it would take
almost a kilo of NaOH for the amount of benzamide I synthesized. I'm rather confused as to why my yield was so low, perhaps steam distilling longer
would increase the yield, even though the last 100ml fraction didn't seem to contain appreciable amounts of aniline.Boffis - 13-7-2015 at 15:10
@gdflp
While I am no expert on the Hofmann degradation, using ideal stoichiometry I estmate that for 90g of benzamide you need 57.5g (0.33 Mequ) of TCCA and
and 60g of NaOH (2 Mequ). I think smuv in his post above meant 6 Molar equivalence of NaOH relative to the TCCA used not the benzamide. Allowing for
some excess TCCA and a corresponding amount of NaOH I think probably 60-70g of TCCA and say 80g of NaOH for 90g of benzamide. This is based on the
following equation:
So I think you would probably need about 300g of NaOH assuming 3x 110-130g = c 350g.
And you can also recover about 200g of cyanuric acid too
One last point since aniline is sensitive to oxidation would it not be better to add the powdered TCCA to the cod benzamide +NaOH. I can see that
hydrolysis may be a competing reaction but its probably pretty slow if the mixture is kept cold.AvBaeyer - 13-7-2015 at 18:57
Attached is a paper on the use of TCCA in the Hoffmann rearrangement. Note that the intermediate isocyanate is captured as the methyl carbamate. This
precludes further oxidation of free aniline during the reaction (very dark reaction mixtures and low yields) while at the same time providing a
derivative that can be purified by recrystallization. Unfortunately, the preferred base for this procedure appears to be DBU but this may be a result
of the particular substrate that needed to be rearranged. The authors do state that sodium methoxide in methanol also works as the base to give the
desired carbamates. Once the carbamate is in hand and purified,it can be converted to the aniline hydrochloride by aqueous HCl.
Very interesting, thank you both Boffis and AvBaeyer for those papers. Unfortunately, I have no TCCA, and none of
the pool stores in my area stock it in small quantities(25lb min). I do however have sodium dichloroisocyanurate dihydrate, which I will try
utilizing water as a solvent to directly yield the amine. This also may have the advantage that it is significantly more soluble than TCCA, thus
allowing for reduced solvent volumes and (hopefully) less tar formation. If this is unsuccessful, I will try synthesizing the carbamate, then
hydrolyzing it.
I cleaned the flask today from the first Hofmann rearrangement, it contained a ton of carbonized matter and tar. This is likely responsible for the
terrible yield.Boffis - 14-7-2015 at 15:28
@gdflp; given the availability and water soluble nature of sodium dichloroisocyanurate it would be great if this compound could be used. You could
then mix all of the other reactants before adding the chlorocyanurate in stoichiometric amounts slowly, ensuring that the amide is always in excess
right up to the end.
Another way to limit oxidation of sensetive amines like aniline might be to add a collector solvent that will dissolve the aniline but not mix with
the aqueous phase. A similar process has been developed for the preparation of furfural from pentosic materials where much of the product is usually
converted to tar before it can be removed from the reaction mixture, the addition of methyl isobutyl ketone has been investigated to extract the
furfural from the reaction mixture before this happens.
Let us know how you get on if you try out the sodium salt.AvBaeyer - 14-7-2015 at 20:26
@Boffis: I did mention that sodium methoxide also works as the base as stated by the JOC authors. Unfortunately, I overlooked mentioning ref 21 and I
thank you for pointing out that reference and providing a copy of the paper which I did not have in my files.
@gdflp: Running the Hoffman rearrangement (and related reactions) in water is not a preferred method. The problems are significant. One is the
overoxidation of the liberated amine leading to tar which has already been discussed. Another is the reaction of the liberated amine with the
isocyanate to form a urea. The urea can then go on and consume additional oxidant leading to further yield problems. It is almost always best to run
these rearrangements in an alcoholic solvent to form the carbamate. My advice, based in significant experience with these reactions, is to avoid
water. Perhaps you could work out a method to use your sodium dichloroisocyanurate in methanol. You would still need to find a suitable base (eg
methoxide).
Have you read the appropriate chapters in Organic Reactions, V3? This is in the Forum library. This will give you a good background.Mush - 23-8-2015 at 09:50
