Sciencemadness Discussion Board

Ways to mix toluene and sulfuric acid

Hexavalent - 10-7-2012 at 12:52

I recently tried the sulfonation of toluene and conducted the experiment at a high temperature to favour formation of the p-isomer. However, I got no product at all and I attribute this to bad mixing in the reaction vessel which was under reflux. I used magnetic stirring, but this clearly wasn't enough to mix them properly. The original guide I'm following suggests inserting some glass tubing down the condenser, into the reactants and bubbling air through it to mix them as they boil. However, I do not have any glass tubing and my rubber and plastic hoses would probably be damaged by the acid or the toluene. I have Claisen adapters which would mean the need for less tubing, i.e. only from the top of the flask to the liquid would be needed as opposed to all the length of the condenser. I do not have any mechanical stirrers at this time, BTW.

I need therefore a way to help mix my toluene and sulfuric acid better. My first idea was some sort of emulsifier....any suggestions? I also looked into the possibility of finding a solvent or solvent pair that could give me increased solubility of the non-polar toluene in the polar sulfuric acid, but this idea seems bleak. Perhaps a phase boundary catalyst? More toluene or acid to be used? I want to keep the toluene in excess, though, to help eliminate the possibility of forming the disulphonate product.

Any ideas?
Thanks in advance.

DJF90 - 10-7-2012 at 13:32

You don't say what procedure you're following. I've done this twice before myself and apart from an impurity in the toluene that darkened in contact with the conc. sulfuric acid I obtained crystals of the monohydrate both times. I used a dean stark apparatus to remove water formed in the reaction, and an excess of toluene (as solvent). No stirring is needed (reflux was sufficient!), and once the reaction is complete (theoretical amount of water collected, approx 4 hrs using 20ml H2SO4 and 100ml toluene) the calculated amount of water is added in and a rather exothermic crystallization occurs (I suggest transfer to a beaker or conical flask prior to adding the water). I cant help you any more than that without further details.

[Edited on 10-7-2012 by DJF90]

Sublimatus - 10-7-2012 at 14:47

I did this three times recently.

I used magnetic stirring and it worked fine. I used equal volumes of toluene and sulfuric acid, per the procedure in Experimental Organic Chemistry.

When the toluene and sulfuric acid come in contact, the nonpolar phase goes sort of milky. Then as heat is applied, the polar phase starts turning yellow, eventually going amber. Through out the reaction the polar phase grows, consuming the nonpolar phase until they're no longer separate. I heat it to reflux.

Once the reaction is over, if I let it cool down, the whole thing goes solid with needle shaped crystals. At that point I reheat, move it over to a beaker, and recrystallize from concentrated HCl. The yield has been between 25-29% of theoretical.

I know in Experimental Organic Chemistry they mention periodically shaking it by hand. You might try that. Are you sure of the identity of your starting materials?

barley81 - 10-7-2012 at 15:04

This thread is useful:
http://www.sciencemadness.org/talk/viewthread.php?tid=15522

Hexavalent - 11-7-2012 at 08:55

Thanks a lot guys - barley, that thread was extremely useful, thanks.

My original synthesis from the book 'Organic Experiments, Vol. 2' said to reflux for 15 minutes using - gasp - a Bunsen burner. That's probably what's wrong, as at the end almost all of the toluene I originally added could still be seen floating on top of the acid.

DJF90 - 11-7-2012 at 12:06

Whats wrong with using a bunsen burner? I expect the issue is the length of reaction, but what do I know.

Pyro - 11-7-2012 at 12:19

he means the fire hazard. but if you stay on it with an extinguisher and a good reflux condenser you should be fine

Hexavalent - 11-7-2012 at 12:45

Yes, that's what I meant, Pyro.

I would still rather use a heating mantle or hotplate, though, when using flammable solvents such as the toluene here.

I will re-try but with a greater length of time for the reaction. Do you think 6 hours would be sufficient?

DJF90 - 11-7-2012 at 13:08

No need to gasp about it. I've used a bunsen burner in organic labs before, and I also recall other members here and elsewhere using a burner. Its just a matter of being able to decide what is safe, and ensuring the flask is heated evenly. A hotplate is not without dangers, although I agree its preferable in many cases, and its my first choice when it comes to heating a reaction mixture.

Sublimatus - 11-7-2012 at 13:54

From what I remember, it only took about 2 hours of reflux and magnetic stirring before both layers had disappeared into a single amber phase (this was with 100 mL of both reactants). I can only assume the reaction is done at the point, but I went for 3 hours every time just to be safe.

I think in the future I might run a few small reactions, varying the reaction time to see how long it takes to get the optimal yield.

Just for future reference, since I don't see it in the other thread that was linked:

From Experimental Organic Chemistry (you can find it in the Sciencemadness library):
Quote:

177. 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 7/11/2012 by Sublimatus]

Hexavalent - 12-7-2012 at 07:22

Thanks for that.

I should point out that I only have hardware store sulfuric acid, which has a striking pink/red colour to it to help people avoid mistaking it for water. It's unlikely, but are there any problems associated with this?

Sublimatus - 12-7-2012 at 11:36

The first time I tried it, I used sulfuric acid used for unclogging drains. That stuff has something dark brown suspended in it, and isn't terribly strong, despite the MSDS claiming something like 97% H<sub>2</sub>SO<sub>4</sub>.

Even so, I ran the reaction and ended up getting the same spiky clusters of clear crystals (reminiscent of sea anemones) when I recrystallized that I get now using pure 98% sulfuric. The only problem was for the life of me I couldn't get the brown color out, even after a few times recrystallizing. I should've used some other decolorizing techniques, but I just chucked them and went and ordered some good clean acid instead.

I can only guess, but I imagine the only problem will be that you end up with pink/red colored crystals.