Make pToluenesulfonic Acid

Warning: Toluene is flammable. Sulfuric acid is corrosive. Wear gloves when handling them and fire safety protocols must be in place. Greetings fellow nerds. In this video I'm going to make p-Toluenesulfonic acid. So first an introduction, what is it and why do we want it. P-toluenesulfonic acid is basically a toluene molecule where we've attached on a sulfonate group. Sulfonates are pretty much a piece of sulfuric acid and in fact behave similarly. They are acidic and will form salts upon reaction with bases. Although a single sulfonate group will only have one acidic proton unlike sulfuric acid's two protons. So why use p-toluenesulfonic acid as opposed to sulfuric acid. The answer is that most of the time you won't actually. If you just need an acid then sulfuric acid and hydrochloric acid are preferred. I've been working on my youtube channel for ten years and never bothered with p-toluenesulfonic acid. That being said it does have some useful properties. First the toluene attached to it makes this acid somewhat more soluble in organic solvents. It dissolves very well in not just water, but alcohols, ketones, ethers, esters and so on. You'll get much less phase separation or even avoid it entirely. The resulting salts are also more soluble and that can aide separation if you plan your synthesis correctly. P-toluenesulfonic acid is also much less oxidizing than sulfuric acid. You saw in one of my videos of making hydrobromic acid that sulfuric acid is slightly oxidizing and will react to create undesirable side products like bromine. If you have sensitive chemistry and can't use something like hydrochloric acid then p-toluenesulfonic acid might be a good choice. Now i'll be honest though, for the amateur, there is not much need for it. Most reactions can still be done with more common and cheaper acids. But if you just want to have some it's pretty straightforward to make with the right equipment. It's one of the classic demonstration experiments to use with a dean stark apparatus. And all you really need to do it in terms of chemicals is sulfuric acid and toluene. So let's give it a try. First we get a 500mL flask and start with 200mL of toluene. I got mine from the hardware store as paint thinner. Then we add in 54mL of concentrated sulfuric acid, which corresponds to about 1 mole. I'm using low quality sulfuric acid from drain cleaner. Now on top of the flask we affix a dean stark apparatus and on top of that we install a condenser. Technically i'm using a clevenger apparatus but since i'm using it in light return mode it's the same as a dean stark apparatus. Now we turn on heating and stirring and raise the temperature until the mixture begins refluxing. What's happening is the sulfuric acid is reacting with the toluene to form p-toluenesulfonic acid. There is actually an explanation of why we get mostly the para-substituted product and not the ortho and meta versions. But it's pretty complicated so i'll skip over that. If you want to know why anyway then i've added it to the end of the video. Moving on, this reaction produces water which forms an azeotrope with the excess toluene we added earlier. This azeotrope boils out and phase separates in the dean-stark apparatus. Water on the bottom and toluene on the top. The lighter toluene then flows back and the reaction continues. Periodically we remove water from the trap so it doesn't also flow back into the reaction. Now, by removing the water from the right side of this reaction we drive it forward. You can actually convert p-toluenesulfonic acid back into sulfuric acid and toluene by refluxing it with excess water and an acid like hydrochloric acid. But that's an experiment for another time. Right now, just keep refluxing and collecting water until it stops accumulating. This took me about 12 hours. Then turn off the heating and let it cool. Now empty the trap of water, and then of toluene. Add the toluene back in to the reaction mixture so we can get more volume to filter. Now the p-toluenesulfonic acid is actually dissolved in the toluene. While anhydrous it's soluble but if we convert to the hydrate form it'll be insoluble. So to separate it out we actually add in 20mL of water to form the monohydrate. Give it a stir and... whoaů That was a lot more dramatic than i was expecting. Looks like it's formed a suspension with the toluene. I tried manually breaking it up but it was too thick. So instead i added even more toluene, 200mL worth and finally it was stirrable and the particles of p-toluenesulfonic acid hydrate were visible. Now we filter it to recover the product. And there is our impure p-toluenesulfonic acid. For some amateur purposes this is good enough but it contains large amounts of toluene still soaked into the particles and whatever impurities from the drain cleaner and paint thinner sources as well as any side products like ortho-toluenesulfonic acid. To purify further i'm going to try to recrystalize it. First to get rid off the insoluble impurities. P-toluenesulfonic acid is highly soluble in water so we first add in 350mL of water to dissolve it with stirring. Whatever doesn't dissolve cannot be our product. Filtering it out we can see we have quite a lot of this insoluble white material. I'm not exactly sure what it is. Some of it is toluene and impurities from the drain cleaner, but there is far too much to be just that. Anyway no matter, what we really want is the filtrate containing our product. And here it is, still impure but at least it doesn't have the insoluble impurities from earlier. Now we boil it down to try and recover our p-toluenesulfonic acid product. This is a little complicated by the fact that there are two common forms of p-toluenesulfonic acid. An anhydrous form that is just the product. And a monohydrate form where it crystalizes with a equal stoichiometry of water. The anhydrous form has a lower melting point than the monohydrate and is much more hygroscopic. So it's actually the monohydrate form that's preferred. Now boiling down to the monohydrate form is a bit of a hit or miss as you can overshoot if you boil for too long. So we'll have to go through an iterative drying process. We start by boiling down to where we expect the monohydrate volume to be, about 150mL, and let it cool. Unfortunately it does not seem to be crystalling so we'll have to boil off more water. I wish I had some easy method of knowing how much to boil off but without sophisticated apparatus like a karl-fischer titrator we'll just have to eyeball it. I boiled off another 30% of the volume or about 50mL before stopping and allowing it to cool. And it looks like we have our p-toluenesulfonic acid monohydrate. Okay it's probably not exactly the monohydrate so you'll need to store it a dessicator to fully dry it. For the most part this is pure enough for amateur purposes. But I want to purify it further by recrystallization from ethanol. So i took my entire stock and added it to 50mL of ethanol. Looking back I should have dried the excess water as much possible in a desiccator before doing this recrystallization step. Anyway, I heated the ethanol until it all dissolved and then cooled the mixture to hopefully crystalize out the product. Unfortunately nothing happened so I boiled off some ethanol, about 20mL, and then let it cool again. Finally i had crystals coming out. When it was completely cooled i broke up the mass and then filtered it to obtain purified product. Okay it's not quite pure but still much better than earlier. I recommend storing it a desiccator until completely dry before transferring to air tight containers. Anyway, total yield is 57g or about 30%. I think most of the loss was because i used low-quality drain cleaner acid which has a lot of water and impurities. The rest is probably in the ethanol filtrate. It can probably be recovered for better yield but i'm not going to bother. The fortunate thing about p-toluenesulfonic acid is that it's really cheap in terms of both labor and chemicals. Anyway i have no immediate use for it but I wanted to have some in my library of chemicals. Maybe to make aromatic esters where the sulfuric acid normally used would react with the aromatic groups to create undesirable side products. We'll see what happens. Thanks for watching. Special thank you to all of my supporters on patreon for making these science videos possible with their donations and their direction. If you are not currently a patron, but like to support the continued production of science videos like this one, then check out my patreon page here or in the video description. I really appreciate any and all support. Now a quick chemistry lesson for those that are wondering about why our predominant product is para-toluenesulfonic acid and not the other possible products like ortho or meta-toluenesulfonic acid. It has to do with how electrons move around the aromatic ring. Let's take a look at benzene for a second. Benzene has three double bonds but without moving the atoms we can move the double bonds over by hoping the electrons. This structure is essentially equivalent but with the double bonds in a new place. This is known as a resonance structure. Because there is no real energy change or barrier to this movement, it happens all the time and happens very quickly. In fact if you measured the bond distance between the carbon atoms on benzene, you'd find they were all the same. The hopping happens so fast, and the structures shift between resonance forms so quickly that we observe what is essentially an average of all structures. The electrons floating around the ring in a cloud. This makes the aromatic ring have special properties in chemistry. Let's look at phenol and i'm going to abstract the acidic proton to get a phenoxide ion. Now the electron doesn't always need to sit on the oxygen. It's connected directly to a sp2 hybridized carbon which means we can move the electron into the ring by moving the double bonds in the opposite direction. So now we have a sort of ketone benzene molecule and the electron is inside the ring. The double bonds can still move so we can draw more resonance forms moving the electron around as well. This happens extremely quickly so overall we get an average of all three forms. Now i want you to pay attention to where the electrons are. The extra electron is only on every other carbon, it cannot move to adjacent carbons. So overall we get higher negative charge at only these positions. These are known as the ortho and para positions. The meta positions are avoided. Now when we perform an electrophilic aromatic substitution, only the most negatively charged positions perform the attack. So that's why we get predominantly ortho and para products. Now you might be wondering, wait a minute, we're not reacting phenol, we're reacting toluene. How can this directing effect still occur? You're absolutely right, it seems odd that a methyl group, without a free negative charge, can still direct where the substitution occurs. It can do this because the methyl group is still somewhat electron rich compared to just hydrogen so it contributes more electron density. At the quantum mechanical level you don't need a formal electron to have an effect, just the cloud of electrons in the methyl group can push a bit of electron density into the cloud of the aromatic ring and thus make the aromatic ortho and para positions more attractive for a reaction. One way of visualizing is through the model of hyperconjugation. We know the methyl donates electron density so we visualize it as actually deprotonating itself and then drawing the consequent resonance structures with this restriction. At this point i want to make clear this is just a way of visualizing and rationalizing the results. This is not formally happening. We are not making zwitterionic toluene and your toluene is not going to spontaneously isomerize into methylidenecyclohexadiene. It's just not happening, as awesome as that would be. But we are pushing electron density from the methyl group and influencing where the electron density ends up. In the end, we still get the same ortho and para directing effect just like phenol. Granted the overall effect is indeed weaker for methyl as opposed to oxygen so you will get a different distribution of products with a bit more meta substitution. Okay so that covers why we get ortho and para products. But that doesn't explain why in our experiment we're getting predominantly para substitution. If all these sites are reactive, just by random chance we should be getting 2/3rds ortho products and 1/3rd para products. Indeed this has happened before. You can recall way, way, WAY, back during my pyrimethamine synthesis i was halogenating my toluene with chlorine. And in that case I actually had predominantly ortho products, it was actually a massive problem and I had to devise a complicated separation step to remove them. Why is this experiment different? Well it's because we're substituting with sulfuric acid. The sulfonate molecule is big, much bigger than chlorine. It actually bumps up against the methyl group. This physical effect is what we call a steric effect and substituents will try and stay away from each other to minimize such effects. One way to think of it is two guys going into a washroom with a row of urinals. All else being equal they will not use urinals next to each other if it can be avoided. So on this aromatic ring the sulfonate ortho-product is the minor product due to how large sulfonate is and how close it would be to the methyl. Its formation is unfavourable. So our majority product will be p-toluenesulfonic acid. I hope that explains it.

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