Author Topic: Hofmann Rearrangement Yields  (Read 3632 times)

letters

  • Subordinate Wasp
  • ***
  • Posts: 138
Re: Hofmann Rearrangement Yields
« Reply #180 on: September 16, 2012, 11:09:25 PM »
cyclic/acyclic, water-miscible, ether solvents work satisfactory. sulfolane is also a good hofmann co-solvent.

fries

  • Larvae
  • *
  • Posts: 26
Re: Hofmann Rearrangement Yields
« Reply #181 on: October 14, 2012, 10:39:50 PM »
You're not getting a %45 yeild on the hoffman no matter what your solvent. Aint happening. I don't care about theory and I don't care about how much you got your unclean product to finally weigh in at, %45 isn't happening.

Wait, what? You must be doing it wrong cause i got a good 45% yield after a few runs where i got <10%

fries

  • Larvae
  • *
  • Posts: 26
Re: Hofmann Rearrangement Yields
« Reply #182 on: October 14, 2012, 10:48:43 PM »
Mixed amide into bleach first, and shook around for 20 minutes.  The magnetic stirrer did not work for such a mix.  This by itself took on a brownish color, before any base was added.  Strange?

If you're getting brown then the temperature is getting too high. For me the colour was an off white colour until i removed it from the fridge to warm up, then it turns red/brown and goes darker as it gets warmer. I didn't use any serious heating and i doubt the temperature got above 50c.

As you probably know, i dint use a magnetic stirrer, shaking the coke bottel did a better job and kept it cooler.

fries

  • Larvae
  • *
  • Posts: 26
Re: Hofmann Rearrangement Yields
« Reply #183 on: October 14, 2012, 10:56:13 PM »
The ferret appeared to have a decent amount of this dark orange oil and thought success! NO, fail. Upon extraction, extraction is black.  A/B lead to a very very small white/yellow oil, which pretty much gave nothing.  Whered it all go?!?!?

Extraction was always black or very dark brown for me. Sounds like your a/b might be the problem. You have to titrate very carefully with a good ph meter and good stirring. I shook the coke bottle after every few drops of hcl were added, separated off the polar part, then took a ph reading of the non polar solution before adding more hcl. I found that over titrating, even a little, would ruin the whole batch and turn it all into red tar.

LilDookie

  • Larvae
  • *
  • Posts: 16
Re: Hofmann Rearrangement Yields
« Reply #184 on: October 15, 2012, 08:37:52 PM »
You're not getting a %45 yeild on the hoffman no matter what your solvent. Aint happening. I don't care about theory and I don't care about how much you got your unclean product to finally weigh in at, %45 isn't happening.

Wait, what? You must be doing it wrong cause i got a good 45% yield after a few runs where i got <10%
I agree. I have gotten as high as 50%, once. I'll be doing my first hofmann in a year soon. We'll see how well I do.  I also agree with the poster who said brown/black means temp too high. I run mine so cold it'll have ice floating in it.
The cost of sanity in this society is a certain level of alienation.
-Terrence McKenna


dream0n

  • Subordinate Wasp
  • ***
  • Posts: 204
Re: Hofmann Rearrangement Yields
« Reply #186 on: October 16, 2012, 01:21:15 PM »
You do? Is this a post saving server issue, or a java...script issue...
off to bigger and better things - don't worry I will visit from time to time

fries

  • Larvae
  • *
  • Posts: 26
Re: Hofmann Rearrangement Yields
« Reply #187 on: October 16, 2012, 03:44:51 PM »
I agree. I have gotten as high as 50%, once. I'll be doing my first hofmann in a year soon. We'll see how well I do.  I also agree with the poster who said brown/black means temp too high. I run mine so cold it'll have ice floating in it.

I got black when adding no external heat, ice cold for 24 hours then let it warm up to rt for about 10 hours.

Sneak

  • Junglist
  • Subordinate Wasp
  • ***
  • Posts: 105
Re: Hofmann Rearrangement Yields
« Reply #188 on: October 17, 2012, 08:15:16 AM »
Those are some great images dopebee. Cheers
If you really want to enjoy a pure, clean product the only way... is to make it yourself...

You miss 100% of the shots you don't take...

uchiacon

  • Larvae
  • *
  • Posts: 46
Re: Hofmann Rearrangement Yields
« Reply #189 on: December 27, 2012, 10:42:35 PM »
I'm having a very peculiar problem with my hofmann. My amide had not been recrystalized bc I didnt need to when I first attempted and succeeded with the reaction several months ago. is from the same bottle, same goes for all other reagents. I have tried 2 brands of bleach with the same result.

Here are the notes summarized. my bleach is 4.2% with 9g/L NaOH

10.1g amide, 5.05g NaOH in solution and 127g 4.2% bleach (which has 9g/L naoh) all mixed together and temperature raised to 70C. Turns brown, dark grey then black. Above 25C the black colour in the liquor PPTs out as a fine black particulate. Black does not respond to more heating or base.

Run cold: 325ml bleach and 19.2gNaOH in 260ml water and 25.5g amide; bleach and hydroxide added and cooled to 1-2C in freezer, amide added and shaken. About an hour in reaction is black. Same response as last time when heated above 25C

Above reaction rerun making sure bleach and hydroxide cooled to 0C. No change.

Above reaction rerun in an icebox with magstir underneath. Same problem.

Above reaction repeated, but addition of reagents changed. Bleach cooled, amide then added. Turns black in 30 min w/o hydroxide.

Before heating, the black colour in the liquor is from dissolved solids and there is a fine suspension of translucent clear colourless crystals. I am unsure what causes the black color, as there are no black particles currently in liquor.

After heating and going up to 80C, the black falls out of solution as a fine PPT and will settle quite quickly, leaving the liquor as clear as water.

What do?? I'm going to recrystallize but I do not think it will help... when I recrystallized amide in boiling water I a slightly lighter amide, but not much change at all. Certainly not white...

(I have notes I could contribute in regards to other parts of reactions but I dont know if it would be appreciated or wanted.)
« Last Edit: December 29, 2012, 05:38:04 AM by Enkidu »

poisoninthestain

  • Dominant Queen
  • ****
  • Posts: 298
Re: Hofmann Rearrangement Yields
« Reply #190 on: December 29, 2012, 09:01:36 AM »
The black color is due to impurities from your amide that is not recrystallized. Dry IPA, MeOH, and EtOH work very well. This is one reason why your rxn mix was black regardless of rxn parameters. Garbage in, garbage out. The recrystallized looks remarkable compared to the crude amide. Check the mp on that amide.

uchiacon

  • Larvae
  • *
  • Posts: 46
Re: Hofmann Rearrangement Yields
« Reply #191 on: December 29, 2012, 09:05:59 AM »
Somehow what I ended up with wasn't an amide. Pretty basic test; heating with NaOH in a flask. Only a very faint ammonia smell... I never bothered to do it because the amide looked identical to what others reported and the pictures I saw along with the fact that it looked the same as the stuff I used months ago.


LilDookie

  • Larvae
  • *
  • Posts: 16
Re: Hofmann Rearrangement Yields
« Reply #192 on: December 29, 2012, 09:30:15 PM »
If you smelled ammonia then you had amide, no? Also, with the tiny amounts of amide you are using I wouldnt expect much from the Hofmann. Also I feel like you are heating too soon. I run the Hofmann at 0C for a full 24hrs then let it come up to room temp on its on then inch it up 10 degrees at a time till I hit 80C, it takes me almost 3 days to run but I always get 30+%

Edit: A better test rather than smell is to dangle a piece of wet litmus in the test tube.
« Last Edit: December 29, 2012, 09:32:04 PM by LilDookie »
The cost of sanity in this society is a certain level of alienation.
-Terrence McKenna

uchiacon

  • Larvae
  • *
  • Posts: 46
Re: Hofmann Rearrangement Yields
« Reply #193 on: January 02, 2013, 12:50:12 PM »
I put a good 50-100mg of amide in the flask, dumped in a tiny amount of NaOH and a few droplets of water to make a slurry and then heating. Surely the ammonia smell would be stronger, its really barely noticeable. There are other smells(possibly something nasty) so I havent whiffed it since.


Surely the ammonia smell would be there, are clearly irrirating, if I had amide? You try with a tiny amount of yours and tell me the intensity of the vapours. I'm sure I have some amide, but over 30% would be doubtful. Additionally, my amide does not melt upon contacting a metal hotplate. It smokes a little bit and goes darker brown.

The only conclusion I can come to is that the dark colour of the xylene is indicating something not amide is forming. Quantum dude suggested that the heat of refluxing xylene (138-140C) is too high and that other compounds will be forming. Someone isolated the nitrile and reported it was a dark red oil. This explains the colour of the xylene mix once the temp got up to reflux, it was a dark cola brown with tones of red. (the oxime is clean and pure, does not dissolve in water as 1-2 of my earlier oximations did)

The first time I ran this reaction I used toluene for the beckmann. This worked pretty well iirc(but rxn mix was enough to push me towards , the reason for using toluene being that it was purer than the xylene and I assumed the difference in temperature during reflux wouldnt effect the reaction.  Next time I plan to run it at 100 and 120C for 7-8hours, or until the amount of amide deposited on the bottom stops increasing.

LilDookie

  • Larvae
  • *
  • Posts: 16
Re: Hofmann Rearrangement Yields
« Reply #194 on: January 03, 2013, 01:35:09 AM »
You shouldn't see the amide come out of solution when you're forming it. I want to say you aren't using enough solvent if you are seeing it come out of solution. Also, I push the Beckmann hard as fuck and have never had a problem, excepts almost having hot xylene come out of my reflux condenser. Mine is usually done in 6-7 hours. Regarding the ammonia smell when testing for amide, it has typically been faint for me as well, which is why I say dangle a piece of litmus wet with distilled water in the test tube instead.

I've been reading more about the Hofmann lately just to try and understand the mechanism better, but it's notoriously inefficient. I usually have what I would consider good luck with it, but I have a feeling I'm about to beat all my records, including 50% once. I also plan to run the reaction on some acetamide I have, to give methylamine. If I ever crack the code and have some major happening I'm not sure if I'll tell the world or not lol. I would say 9 times outta 10 I dont even use heat in the Hofmann. Regardless of what the published data and everyone says, I have real world experience that has shown no increase in yields by using heat and trying several different heat profiles, including pushing to 80 as fast as possible. It seems to me that with chemistry our individual mileage can vary greatly, so, grain of salt, dude.
« Last Edit: January 09, 2013, 10:50:41 PM by java »
The cost of sanity in this society is a certain level of alienation.
-Terrence McKenna

Tsjanga

  • Larvae
  • *
  • Posts: 27
Re: Hofmann Rearrangement Yields
« Reply #195 on: January 09, 2013, 09:25:58 PM »
Does somebody knows a guy who had success with TCCA route and lowering the solvents amount?

Tsjanga

  • Larvae
  • *
  • Posts: 27
Re: Hofmann Rearrangement Yields
« Reply #196 on: January 15, 2013, 06:35:03 PM »
Just stumbled on an article about the formation of 2-chloro-4,5-methylenedioxyamphetamine when excess hypochlorite is used.
Somebody more info about this substance and is it possible to get it out of our desired product.

fractal

  • Subordinate Wasp
  • ***
  • Posts: 110
Re: Hofmann Rearrangement Yields
« Reply #197 on: January 16, 2013, 12:28:47 AM »
Having a large excess of sodium hydroxide present prevents this as free chlorine exists in equilibrium with sodium hypochlorite.

Cl2 + 2NaOH ? NaCl + NaClO + H2O

You could separate by distillation though, I'd think the boiling points should be pretty different. I'd check with TLC to make sure before wasting time trying to get rid of something that's not there.

fractal

  • Subordinate Wasp
  • ***
  • Posts: 110
Re: Hofmann Rearrangement Yields
« Reply #198 on: January 16, 2013, 12:30:18 AM »
The ? mark was an equilibrium sign. Doesn't work for some reason.

atara

  • Dominant Queen
  • ****
  • Posts: 256
Re: Hofmann Rearrangement Yields
« Reply #199 on: January 16, 2013, 12:36:11 AM »
Quote
is it possible to get it out of our desired product.

If you suspect your stuff is contaminated, reflux with NaOH should convert the 2-chloro to the 2-phenol, and the phenolate is then insoluble in most nonpolar solvents.