Preparation of p-Toluidine

I have prepared p-toluidine with 12.2g of p-nitrotoluene using Vogel’s (3rd ed) “Method A” for aniline, at 0.44 scale. Adjustments were made for the differing molecular weights of the respective precursors, nitrobenzene and nitrotoluene. Method A utilizes tin/HCl as reducing agent.

The reaction and subsequent steam distillation were conducted in a 250mL RBF. I could tell that the p-nitrotoluene had all been reduced as I could no longer detect its pleasant almond-like smell. This was replaced with the out-house smell of p-toluidine. The melted p-toluidine is shown below as a pale yellow oil floating on top of the water:


p-toluidine in steam distillate

My pot residue consisted of stannous/stannic hydroxides that didn’t dissolve, as shown below.


steam distillation pot residue

The Vogel procedure says that this hydroxide should dissolve after enough NaOH is added. Testing 1mL of the gray slurry I found that it would not dissolve even after 4mL of aqueous NaOH was added. Therefore, I did not add any more NaOH. I’ve since seen other procedures that indicate that these hydroxides do not necessarily dissolve.

I substituted DCM for ether to extract the p-toluidine from the steam distillate. It is shown here being dried over potassium carbonate:


DCM extract of steam distillate, drying

The dried extract was distilled to remove the DCM. The p-toluidine pot residue was crystallized by air drying. The crystals were contaminated with a reddish-brown colored substance. The weight of these crude crystals was 8.5g, indicating a 90% yield at this point.

A recrystallization was done using 85% ethanol. Three successive crystal batches were harvested. This provided only limited purification.

The melting point for these crystals was ~ 35°C. The literature value is 43.8°C.

Noting that the procedure on the LambdaSyn (a German chemistry forum) website specified using petroleum ether as a solvent for recrystallization I decided to give this a try. The ACS pet ether that I used had a boiling point range of 30°-60°C.

Here I did three recrystallizations. The crystals form quickly as very thin plates, like fish scales. These recrystallizations were tedious and I lost considerable product due to all the handling. I did obtain two small purified batches. The first batch was 1.6g of apparently quite pure p-toluidine as it had a mp of 44-45°C, agreeing with literature values. Even this material, however, was gray and not the pure white I was expecting. See photo below.


p-toluidine after 3rd recrystallization in pet ether

The 2nd batch of crystals was 1.4g of less pure material, having a mp of 40.5°C.

Taking the total weight of product as 3.0g, my yield therefore dropped to 32%.

Discussion
p-Toluidine is a solid at room temperature. I therefore warmed it to melting before making transfers. It did tend to clog the condenser during steam distillation as indicated by Vogel. But this was easily cleared by cutting back on the condenser cooling water flow, as suggested by Vogel.

The p-toluidine was difficult to purify. The crystallization with 85% ethanol did not work well. I attribute this to a poor solubility vs temperature curve. I washed the crystals on the Buchner funnel using ice-cold water. The sticky flake crystals made this washing difficult.

Conclusions
I do like the use of tin/HCl for the reduction of nitro-aromatics. It works the best of several methods I have tried. I have not had good luck using powdered iron and it makes a real mess of my glassware/sink, and makes cleanup tedious.

I think the root problem in using crystallization is that the p-toluidine is very soluble in just about every common solvent. And making things worse the solubility vs temperature curve is relatively flat. In retrospect in might be better to purify p-toluidine by vacuum distillation, as recommended by Armarego & Chai in “Purification of Laboratory Chemicals.”

Questions/comments/suggestions are welcomed.

Quote: Originally posted by organicchemist25

I have a huge, new pressure cooker for generating the steam. I have the fitting I am going to tap the existing hole with to accept my 3/8" ID hose. Is that a good way to generate my steam? It is a 41.5 Qt size, so it should provide steam for a while. Do I need anything in-between for condensate removal? or just straight over to my inlet connection?

Yes, a pressure cooker makes a fine steam generator. I have a 4-qt size which is adequate when I fill it about 1/2 full of water. On occasion I have had to stop and refill it, but this is rare. I heat it on a hot-plate, setting the power to about 65% using a Variac.

Yes, a steam trap intermediate is very useful to collect condensate. I use a 500mL sep funnel as shown in the picture below. In this picture I am injecting steam and heating with a mantle simultaneously.





[Edited on 27-3-2014 by Magpie]

ok, nice. I see exactly what you did Ill use a sep funnel too. Ingenious. Thanks so much!

Oh, and you said you were heating simultaneously. So, do you keep the temp in the flask as close as possible to 100C to be consistent with the steam coming over? Or would it depend on the compound one is trying to separate? Higher temp for more viscous material? I know water/steam will only be 100C?, but could/would you pass it through the mixture in the flask with it being of higher temperature ? or keep everything 100C

Thanks

Quote: Originally posted by Magpie

p-Toluidine is a solid at room temperature. I therefore warmed it to melting before making transfers. It did tend to clog the condenser during steam distillation as indicated by Vogel. But this was easily cleared by cutting back on the condenser cooling water flow, as suggested by Vogel.

Hi, hope I'm not too late with my question I'm currently doing p-toluidyne in my class and I wanted to ask you about the extraction. As you said - the product was crystalizing in the condenser, so did it cause any traubles later on? I'll be performing extraction using Diethyl ether

Quote: Originally posted by AbbyS

Quote: Originally posted by Magpie   p-Toluidine is a solid at room temperature. I therefore warmed it to melting before making transfers. It did tend to clog the condenser during steam distillation as indicated by Vogel. But this was easily cleared by cutting back on the condenser cooling water flow, as suggested by Vogel. Hi, hope I'm not too late with my question I'm currently doing p-toluidyne in my class and I wanted to ask you about the extraction. As you said - the product was crystalizing in the condenser, so did it cause any traubles later on? I'll be performing extraction using Diethyl ether

I'm just worried if it starts crystalyzing during extraction or evaporation, both have temperature below 40 degrees. Sometimes I have the worst luck