Sciencemadness Discussion Board

Wanted: prep for Tosyl acid

Paddywhacker - 1-3-2009 at 13:25

Toluene sulphonic acid is cheaper to make than to buy, I think, and I could make it by SOP, but does anybody have an actual optimised procedure and workup?

SOP would be to reflux toluene and add H2SO4 dropwise. Workup would be to evaporate excess toluene and recrystalise from EtOH.

bfesser - 1-3-2009 at 14:41

You'd need <em>fuming</em> concentrated sulfuric acid, wouldn't you? Try searching for the synthesis in the usual places before asking (Org. Syn. might be a good start). Even Wikipedia yields some info, such as conc. HCl being used for the recrystallization.

[Edited on 3/1/09 by bfesser]

Sauron - 1-3-2009 at 17:13

The forum library contains Vogel's third edition of PRACTICAL ORGANIC CHEMISTRY (as well as a couple of other books with same name so check the author.)

This contains a prep for p-toluenesulfonic acid, aka tosylic acid, not tosyl acid.

A much better discussion is in FUNDAMENTAL PROCESSES OF DYE CHEMISTRY also in forum library.

Sometimes sulfonations can be done with conc H2SO4 rather than fuming H2SO4, as in the case of aniline, but this is always much slower. If you can get the toluene to dissolve in the conc H2SO4 however long it takes, it has now been sulfonated.

It is an equilibrium, remember.

benzylchloride1 - 1-3-2009 at 18:49

Toluene can be very easily sulfonated with concentrated sulfuric acid due to the activating effect from the methyl group; fuming sulfuric acid shortens the reaction time drastically.Benzene on the other hand takes forever to sulfonate with concentrated sulfuric acid and is best sulfonated with oleum. I have prepared the sodium salt of p-toluene sulfonic acid several times along with the acid. This procedure can be conducted with the 92% sulfuric acid available at hardware stores. After the toluene is sulfonated, the mixture is allowed to cool and poured into concentrated hydrochloric acid. The acid solution is heated to boiling and allowed to cool slowly, crystals of the p-sulfonic acid then form. You will need a sintered glass funnel anda vacuum filtration set up to recover the product. Strong acids instantly destroy filter paper. Here is the procedure for the preparation of this compound form Experimental Organic Chemistry by James F. Norris, available from the Sciencemadness Library.

Preparation of p-Toluenesulphonic Acid (SECTION 476).—
To 25 grams of toluene in a 200-cc. flask add 25 cc. of pure concentrated
sulphuric acid, and place on the steam-bath. Allow
the mixture to stand until the toluene has dissolved. This will
require about 2 hours if the flask is shaken once in a while. Let
the solution cool; before it solidifies pour it, with stirring, into
100 cc, of pure concentrated hydrochloric acid. Heat the mixture on the steam-bath until the solid dissolves. Set aside to
crystallize. Filter by suction, and wash the crystals with concentrated
hydrochloric acid. Press the crystals to remove as
much of the mother-liquor as possible and dry them on a porous
plate. Weigh the toluenesulphonic acid and calculate the
percentage yield from the toluene used.
p-Toluenesulphonic acid crystallizes from water, in which it is
very soluble, in long colorless needles. It is less soluble in concentrated
hydrochloric acid. It melts at 104°-105°. The yield
should be about 25 grams. The amide can be prepared by the
method described in experiment 176; it melts at 136°-137°.

[Edited on 1-3-2009 by benzylchloride1]

Sauron - 1-3-2009 at 20:01

The reason why oleum (fuming sulfuric, anhydrous H2SO4 + SO3) speeds up aromatic sulfonations is because as I said above this is an equilibrium.

Water is formed in the reaction and if conc H2SO4 is used, can build up, pushing reaction to the left

The free SO3 in oleum reacts with water. thereby driving reaction to the right.

That is why oleum is always faster than conc H2SO4 even for activated substrates and is mandatory for deactivating ones.

Aniline, toluene, halobenzenes can be sulfonated with conc H2SO4 if you have the patience.

Removing a sulfonyl group is always done with the concentrated acid because it's the same reaction in reverse and you now want the reaction driven to the left.

Sedit - 17-5-2011 at 17:03

I have attempted the above process twice now and have been unable to recrystalize this from HCl. 25grams of Toluene and 25ml of 97% H2SO4 was used. I have tried to recrystalize from EtOH with no success. I have tried to recrystalize from H2O with no success. It smells of SO2 when its done.

The only difference between my work and the above text is I use a much weaker heat source over a much longer period of time. However when the Toluene is all gone it crystalizes into a solid mass on cooling. First I tried the recommended 100ml of concentrated HCl but on cooling received nothing. I attempted to reduce the HCl by 1/2 using only 50ml but still it did not crystalize.

Any suggestions?


It appears I spoke to soon and chucked the first batch to soon. This reaction seems prone to super cooling. After moving the 50ml HCl / TsOH batch it solidified. I added the other 50ml of HCl and am heating it to dissolve it all and will see if I can get it to form filterable needle crystals like in the prep.

[Edited on 18-5-2011 by Sedit]

smuv - 17-5-2011 at 21:04

See this:

If you just follow my sodium tosylate prep, and after the reaction instead of diluting with brine and neutralizing, simply add an excess of HCl, you will get toluene sulfonic acid. The yields should be very good. In my experience, after cooling a little even the crude toluenesulfonic acid is in nicely formed xtals.

Sedit - 17-5-2011 at 21:16

I have ready it a couple times Smuv, very nice work. Every post dedicated to "Tosyl" has been read in the past week or so. :)

I considered attempting this if it did not go well but it appears to be working fine now. I just checked it and I have about 1 inch long crystals forming in round clumps all thru the liquid. Im going to leave it overnight and see where it stops but I really think this substance may just take a long time to crystalize and when I shock it before it just broke up a clump I didn't see into a bunch of pieces lending itself to seeds.

I really want TsCl but for now I will just play with TsOH since I can not find a TsCl prep that does not involve SO2Cl2. TsOH seems like it could lend itself well to Haloalkane synthesis from alcohols and halide salts. Its non oxidising so it could take the place of H3PO4.

Lol, well spoke to soon. From where I messed with the flask before it once again turned into a solid mass of crystals. Anyone attempting this do yourself a favor. Don't touch it for allloooonnggg time and you should be fine. Im just going to filter this and press all the fluids out, which should be fun considering this is fuming like mad in the wet weather from SO2 and HCl fumes, and then recrystalize it again. This should yeild the pretty white needles im looking for.

[Edited on 18-5-2011 by Sedit]

smuv - 17-5-2011 at 22:03

Well, I am glad you got things working out(ish). But a solid mass of crystals isn't bad, filter it, wash with HCl, and I think you should be OKl. If you really want finely divided needles, to avoid masses, you probably need to use more solvent. But I think what you have is probably pure, after all, you are only really removing toluene and sulfuric acid.

But one thing I have noticed is, watching a crystallization is like watching paint dry. I usually try to do my crystallizations overnight, so I don't fuss over them (and usually disturb them, or put them in the fridge too soon, etc).

Sedit - 18-5-2011 at 05:23

Yeh it should work out fine, it just looks like crap right now. I wouldn't have moved it had I seen any crystals growing but the green dye that was in my Sulfuric acid has turned a dark, dark purple making it impossible to see into the reaction. When I tilted it to look inside I seen a patch of crystals stuck to the bottom and it appears to have solidified shortly there after.

I managed to shake the mass into a slush so it should filter well and then a simple wash with cold HCl and another recrystalization should get me the purity im looking for. Its not a hard synthesis so I think I will try it again in the future and Im going to bleach my H2SO4 with H2O2 before going into it so I can get a better idea on what colors are normal for the reaction.

Any suggestions on what I can do with it when its done? I was thinking of trying to make anhydrous HBr with it from NaBr. It also seems like a good way to HI for those who cant get concentrated H3PO4.

Is there any way to chlorinate it to TsCl?

Am I wrong to assume that it behaves simular to carboxylic acids?

Nicodem - 18-5-2011 at 06:40

Quote: Originally posted by Sedit  
Am I wrong to assume that it behaves simular to carboxylic acids?

Yes, very wrong. It behaves very differently. It is not even isoelectronic with carboxylic acids and the S=O bond is not similar to the C=O double bond. It is nearly unprotonable and thus can not get further activated for the nucleophilic additions (thus esterification via the mechanism operating in carboxylic acids is not feasible). Esters of sulfonic acids can form from the acids via nucleophilic substitution reactions even though the sulfonate anions are terribly non-nucleophilic, but as far as I know, it is not possible to use this reaction in a preparative manner (but check the literature yourself). The formation of sulfonate esters has been reviewed in one of my posts a couple of years ago.
Sulfonyl chlorides can be formed from the sulfonic acids using PCl5 or SOCl2, probably other such chlorides as well. Alternatively they can be prepared by the electrophilic chlorination of sulfinic acids, or thiols, thiosulfates, isothioureas, thiocyanates, disulfides and sulfoxides in the presence of water. These last methods were reviewed in other threads (by Klute, I think) and experiences were posted (like the preparation of ethylsulfonyl chloride and its methyl ester by Ullmann).

I forgot to mention, but you don't need any concentrated H2SO4 for the sulfonation of toluene. With the use of the Dean-Stark trap you can start with a diluted acid. You just use an excess of toluene over H2SO4 instead of the opposite. Once the mixture becomes homogeneous, you can consider the reaction complete. This might be more suited for those who can get the diluted H2SO4 for electrolyte use, but have troubles acquiring the concentrated one. It worked fine for me and I started with the 35% acid.

In regard to the synthesis of alkyl tosylates from tosylic acid, it can be done by reacting it with various esters like phosphites, phosphates, sulfites, carbonates and carboxylates (the most known is the synthesis of methyl tosylate from trimethyl ortoformate). There is an interesting study and review of these reactions, for those who want to prepare simple alkyl tosylates directly from tosylic acid:

Reaction of carboxylic acid esters with p-toluenesulfonic acid.
Nitta, Yoshihiro; Arakawa, Yasushi.
Chemical & Pharmaceutical Bulletin, 33 (1985) 1380-1386.

Relatively good yields were achieved even by using simple carboxylates like ethyl benzoate (30% yield after 24 h reflux in toluene). The reaction might be possible to be combined in a one-pot procedure with the above mentioned TsOH synthesis using toluene reflux with the Dean-Stark trap. The authors assume a mechanism based on the nucleophilic substitution (O-alkylation of the sulfonate anion by the O-protonated esters). Ethyl acetate unfortunately has a boiling point too low for be of use for the preparation of TsOEt, though with a longer reaction time it might give some useful yields (dimethyl carbonate gave a 37% yield even though its bp is 91 °C).

[Edited on 18/5/2011 by Nicodem]

DJF90 - 18-5-2011 at 09:35

I'd like to mention that TsOH crystallises as a monohydrate, so dehydration will be required if you intend to make anhydrous HBr or TsCl.

Sedit - 18-5-2011 at 16:58

Nicodem I have some of the threads you mentioned and a few other references I will post when I get on my other computer. I also have what appears to be a good thesis on the photo sulfination of alkanes but it may be to large to upload here.

But first of all I have a big problem. Someone here has had to have tried this simple prep before. Heres my problem, I filtered the fine crystals and pressed most of the fluid from them. I then placed a large pile of them onto an unglazed ceramic plate. When I returned.... They where GONE!!!

This stuff don't sublime right? Since its in the form of the Monohydrate I doubt it to be hygroscopic.... Where did it go? Im left with only a few mg to perhaps a gram and I don't expect that to hang around much longer either.

I dont know what to do now... Rerun the prep, longer crystalization time..... And a time lapse camera?


I also want to address the possible insitu generation of SO2Cl2 by feeding SO2 and Cl2 into Toluene and activated charcol to generate TsCl but first I really would like to know where the hell all my Tosylic acid went.

S.C. Wack - 18-5-2011 at 17:28

Are there little droplets where the solid used to be? Of course it could be hygroscopic until all the o-product is gone.

Sedit - 18-5-2011 at 17:33

Its damp but not excessively so. It was kind of wet when placed there to dry that's why I was using the porous plate like I thought I was suppose to be using as per the above preparation in the start of this thread that I was following.

I was under the impression that little to no Ortho product formed since the Methyl group was activating the ring at the Para position.

S.C. Wack - 18-5-2011 at 17:59

"Little" is enough of the o-cpd.

smaerd - 18-5-2011 at 18:22

I've made TsOH by mixing toluene and conc. hard-ware store sulfuric acid in the amounts in the prep benzyl chloride posted. Then refluxing(with stirring) until the two phases ''homogenized''. Quickly transferring that to an erlenmeyer flask then adding HCl(be careful! HCl gas flies out). I added I think 5mL of 31.45% HCl and all of the water turned to steam and I was left with a flask full of yellow crystals. Surely this isn't pure, but it worked just fine. Sorry for a cook-book explanation...

[Edited on 19-5-2011 by smaerd]

Sedit - 18-5-2011 at 18:30

Did you manage to isolate those crystals and dry them?

5ml of HCl is well below what the prep calls for and about 20x lower then the amount I used. I also allowed mine to cool before the addition of HCl which prevented the HCl from boiling out right away. Still the fluids left fume horribly from the SO2 and HCl so a fume hood or good ventilation is a must.

I think I should dry them on a porous plate before adding them to HCl for a reflux as this will remove any Toluene and most of the H2SO4 before crystalization.

Paddywhacker - 18-5-2011 at 19:45

This is a general lab tip which is relevant here. During workup, the first time you see any solid grab a small sample so that later on you can use it as a seed to promote crystallization. A little crust on a stirring rod is sufficient, if it is not hygroscopic.

smaerd - 19-5-2011 at 07:57

Well I failed at recrystallization of the final product as this was my first organic reaction some time ago and I wasn't happy with the yellow appearance of the product.

Yes I made the ignorant mistake of adding conc. HCl to the solution while it was still very hot which lead to a very vigorous evolution of gases. I shouldn't have said that as if I was recommending it. This was done outside after a few drops scared me off with quite a large cloud of gases that make sweat sting(thankfully goggles and gloves were on).

I have no qualitative data on the final product except that 100g of NaOH and 100+g of sodium bicarbonate still had it at pH 1(so sayeth the pH papers, poor information I know). Edit - although this is after I tried to recrystallize from too much HCl(20mL's I think wish I didn't lose my old chem journal :().

My prep isn't ideal, I just wanted to share that a dean-stark is not necessary for the preparation(don't have one) as it is used in many of the other procedures, and implied in several others.

Nice tip paddywacker, seeding for crystallization is very handy.

[Edited on 19-5-2011 by smaerd]

maxidastier - 13-8-2011 at 04:36

What is the better way to precipitate the p-TsOH? Add conc. HCl aq. or passing in gaseous HCl? Do the methods differ in consumption of HCl or precipitating amount of p-TsOH ?