Toluenesulfonic acid - tips and tricks.

[Antoncho]

Hi everyone!

This post was inspired by Rhodium who asked me to translate a Russian article on making p-toluenesulfonic (tosic) acid, a very useful reagent in Organic Synthesis. Which i will do yet more gladly since it just so happens that we at HyperLab have accumulated quite a bit of know-how on making it.

So. First - to the article, (see the original at

http://hyperlab.bravepages.com/tolueneacid.djvu

) It goes as follows (i think this will bee nothing new, but still):

In a 200 ml RBF eqiupped w/reflux condenser, there are placed 32 ml toluene and 19 ml H2SO4 (sp. gravity = 1,84 g/ml) and the mixture is heated to reflux. Gentle boiling of toluene is maintained for 1 hour as the mixture is constantly being homogenized - for this purpose, the flask is vigorously shaken every 2-3 mins (if you don't have a stirrer hotplate, that is). After an hour the layer of toluene almost completely disappears. The warm mixture is poured into 100 ml water; the flask is also rinsed w/some water. 1 g of activated charcoal is added to this solution and the mixture is boiled under draught until it decolorizes. Then charcoal is filtered off, and the solution is evaporated under draught to 50 ml volume, chilled to 5-7 C and saturated w/HCl gas. When using muriatic acid instead, the yields drop sharply.
The precipitated crystals of p-toluenesulfonic acid are filtered and as much as possible moisture is pressed out of them. The crystals are then placed into an excicator over conc. H2SO4, simultaneously inside is placed a beaker w/solid NaOH to absorb residual HCl. Drying is continued for several days.
The yield is 35-40 g (65% of theory), mp. 104-105 C; below that temp p-TSA forms colorless very hygroscopic monohydrated crystals. Unhydrous acid boils at 140 C/ 20 torr.
Ref: S.S. Gitis; A.V. Ivanov "Practicum in Organic Chemistry", Moscow, Vysschaya Shkola, 1991

See my practical experimentations with this method :

Post 199573

(Antoncho: "p-Toluenesulfonic acid (Tosic acid) synthesis", Chemistry Discourse)

Then there is the simplified version of this method (no gassing) researched chiefly by Chromic:

Post 261000

(Chromic: "p-toluenesulfonic acid", Chemistry Discourse). Unfortunately, thus pptated acid is difficult to filter.

Now - here's a REALLY simple way brought to you by Zaratystra of HyperLab:

"I did it like this: refluxed w/stirring sulfuric and toluene for 15-20 mins, then tenderly decanted the upper toluene layer into a saucer. On cooling, there happened crystallisation of pretty white xtals. The trick works beecause tosic acid (monohydrate) is very well soluble in hot toluene, and very little soluble in cold."

As you can see, this is the perfect way to make some tosic acid real quick and easy. Moreover, the product is already quite clean!
Needless to say, toluene can bee returned into the flask and the cycle repeated as many times as desired.

Naturally, if you want to make a big amt of tosic acid, the above route doesn't seem very attractive. For such purpose one is recommended to use the variant perfected by Fallen_Angel of HyperLab, which goes as follows:

To a 1 liter flask there's added 200mls conc. H2SO4, 500-600mls toluene and the mixtr is refluxed w/water separation on a Dean-Stark trap. In the beginning the mass consists of two layers which gradually disappear. This is accompanied by precipitation of TSA xtals - in my case there was half of the flask full. If you don't care for the purity, just filter it and use as is (the chief impurity is sulfuric acid). The only reason of concern is not to remove too much water: unhydrous TSA, unlike the monohydrate, is well soluble in toluene.

Or you can do it like this: remove all the water until full dissolution, then pour 75-80% of the distilled water (taking into account the water initially contained in the sulfuric acid) back into the flask. The pptated crystals are washed a couple of times with toluene and then - with cold conc. HCl. Drying over KOH.

Thus obtained tosyc acid is significantly more pure and can bee used straight for any practical purpose.

<knowing Angel's fondness of cyclidines, i assume that 'practical purpose' means synthing PCP or some such >


That's all i have to say for now - enjoy your chemical experience!


Avec l'amour,

Antoncho