Sciencemadness Discussion Board - preparation of p-phenylendiamine

preparation of p-phenylendiamine

Aqua-regia1 - 10-2-2007 at 03:09

Have somebody ever synthetised pure (not dark coloured) p-phenylendiamine? How it can be prepare pure this one for synthesis? Thanks

Ozone - 10-2-2007 at 06:41

"Pure" p-phenylenediamine will become colored on storage with exposure to light, O2 and/or heat. These are oxidation products and will be found in most aromatic amines that have been stored for any length of time (particularly, aniline).

Either you distill the colored material (which is a pain w/high boilers) or, preferably stir with decolorizing carbon (Darco, Norit, etc.) and filter though celite in a buchner funnel.

One synthetic route could start with aniline. First, the aniline is solubllized into water as the hydrochloride salt. To this is added norit with stirring. This is filtered through celite and rinsed with water. To the filtrate is added acetic anhydride and sodium acetate with stirring. Cool the mixture and collect the acetanilide (recrystallize if you wish). Second, the material is nitrated with H2SO4/HNO3 slowly and with cooling so as to not exceed 5°C at any time. The reaction is quenched with ice, carefully neutralized and the product, p-nitroacetanilide is recovered. Third, to this is added dilute HCl (aq) and this is refluxed for 1-2hr to hydrolize the amide. This yields p-nitroaniline. Fourth, to this is added tin granules and HCl. The mixture is shaken and cooled (ice) until the reaction is complete. This is neutralized w/NaOH, rendered basic, and extracted into say, DCM. The Solvent is removed under vacuum to yield 4-aminoaniline, free base.

All of these reactions are fundamental and taught at the sophomore level in University courses.

Cheers,

O3

unionised - 10-2-2007 at 07:25

You can clean the stuff up quite well by dissolving it in dilute HCl, washing with ether (which takes out some of the black stuff) and reprecipitating the base with NaOH soln. Not as good as distillation, but much easier.

Aqua-regia1 - 10-2-2007 at 13:54

Thanks Dear Sirs

Nicodem - 11-2-2007 at 02:51

The black stuff (polyanilinic dyes, quinones and the like) can be removed by recrystallization if you discolor with active charcoal. However, vacuum distillation is a considerably more rational and practical solution.

Sergei_Eisenstein - 11-2-2007 at 05:36

Is it more practical? Personally, I'm not inclined to leave a distillation setup unattended. I'm usually doing something nearby to keep an eye on the process. As such, it requires some time. Discoloration/recrystallization does not require the same amount of time, and drying the substance usually doesn't require special attention either. So, from my personal point of view, I'll choose a recrystallization over a distillation as much as possible.

Nicodem - 12-2-2007 at 10:12

I had more like a kughlrohr in mind, not necessarily an actual distillation setup which would be more difficult to use since the distillate solidifies and easily clogs the vapors path. In my experience with compounds of that type, recrystallization with charcoal generally works if there is little polianilinic crap inside. But if there is a lot one recrystallization is not enough and lots of charcoal needs to be used. The recovery also sucks in comparison with distillation.

unionised - 12-2-2007 at 11:05

So, if you happen to have a kughlrohr handy it's earier to use it. I daresay it is, but for those of us struggling by with more moderate kit I think Sergei is probably right.
BTW, IIRC recrystalising PPD as the HCl salt from water/alcohol mixture works well and the acid conditions retard oxidation. For some purposes the salt is just as useful as the free base.

Nicodem - 12-2-2007 at 12:42

Quote:

Originally posted by unionised
So, if you happen to have a kughlrohr handy it's earier to use it. I daresay it is, but for those of us struggling by with more moderate kit I think Sergei is probably right.

But it was just because I thought Aqua-regia1 had no access to vacuum distillation equipment that I suggested him to recrystallize. Sergei's reply was about my claim that vacuum distillation is a "considerably more rational and practical solution". It is more rational because you get a purer product with an excellent recovery and practical because it takes less work (which is important for lazy people like me) and no solvents. The most unpractical part is in taking out the large chunks of crystals that form from the distillate.

Quote:

BTW, IIRC recrystalising PPD as the HCl salt from water/alcohol mixture works well and the acid conditions retard oxidation. For some purposes the salt is just as useful as the free base.

That indeed slows down the oxidation but recrystallizing the hydrochloride does not remove the polyanilinic crap that is already present. I do not know if this is true for p-aminoaniline, but in my experience it is true for certain other anilines that tend to colorize by prolonged air exposure. Those annoying polyanilines just tend to stick together with the original anilines, whether as free bases or hydrochlorides (these annoying dyes also dissolve in acidic media and can not be washed away). Recrystallization with active charcoal in non polar solvents is in my experience most efficient if distillation is not an option. If the recrystallization is done in water or water/alcohols the active charcoal is not equally efficient, but it is still better than nothing.