Transcription
[Music] Hello, my dearest science nerds, and welcome to my channel, Let's Chemistry! It's Christmastime in the city, and we are advancing to the next step in our synthesis. Today, we are going to brominate the product we got in the previous video, a little alcohol. The product of this bromination reaction is allyl bromide, a very useful reagent and building block in organic synthesis, especially for green chemistry reactions. Allyl alcohol bromination is a primary alcohol bromination reaction, which is called the SN2, or the substitution reaction, in organic synthesis when a bromine atom substitutes a hydroxyl ion in the alcohol chain and forms the corresponding halide.
The reaction mechanism is rather simple in normal conditions. The hydroxyl ion, OH-, is a poor leaving group. However, treating alcohol with acid leads to an interesting personality adjustment. The alcohol, R-OH, is converted to its conjugate acid, R-OH2+, which now possesses a decent leaving group. Treating alcohol with hydrogen chloride, hydrogen bromide, or hydrogen iodide results in the formation of corresponding alkyl halides. As you can see, it is a two-step process: first, the alcohol is protonated to give its conjugate acid; secondly, a substitution occurs. You can see how this reaction works for different alcohols. The same applies to our product. Allyl alcohol is brominated in the strongly acidic condition of a mixture of sulfuric and hydrobromic acid. We don't have hydrobromic acid, and we'll use the potassium salt to prepare it. In our case, it is potassium bromide; sodium bromide could also be used, but in less quantity.
From the name of this post, you probably have noticed that we had screwed up the first attempt of this reaction, so I am going to tell you the story of how to do and how not to do the alcohol bromination. I decided to follow the procedure I had found in Rhodium's ZKK Emma story archives. According to the procedure, in a 1-liter round-bottom flask, we placed 100 milliliters of water. The flask itself was placed in an ice bath, and 120.4 grams of concentrated sulfuric acid was added in small portions. It is always mandatory to add acid to water and not vice versa, because during addition, massive heat is generated, and boiling acid might be spilt on your skin. To the diluted sulfuric acid mixture, 140.4 grams of potassium bromide was added. The mixture was stirred for 10 minutes, and all solids formed were filtered with a vacuum.
On the first run, I had been following the procedure meticulously, but I failed. As you will see, it is not because of this step, but I found out that it is much better to, at first, dissolve the potassium bromide in the water, then chill it down, and only after that add sulfuric acid. On this stage, hydrobromic acid and less soluble potassium bisulfate are formed. Filtering off the solids is preferable. When I changed the addition sequence for the second time, more solids were formed, and the funnel was full of bisulfate salt. After having filtration done, we placed our freshly prepared hydrobromic acid mixture into the flat-bottomed 500-milliliter, 3-neck flask. To the center, we placed an addition funnel charged with 60 grams of sulfuric acid and added 64 grams of allyl alcohol to the side neck. We had connected a simple distillation setup. We are going to distill the azeotropic mixture of water and allyl bromide. The removal of water also drives the reaction forward in favor of the formation of our desired product. The reaction itself is exothermic, which means the heat is generated in the process, but external heating is also necessary for distillation to start. We had the stirrer turned on, and after we started the slow addition of sulfuric acid, after a while, we noticed that self-heating was not enough to start the distillation and applied heat, 150 degrees on the hot plate.
Here is when the problems began. The liquid was changed in coloration from red to deep brown—in fact, almost black—the clear signs of bromine formation. Instead of allyl bromide, we were getting bromine in the flask. In the reaction flask, we got an almost unremovable black organic tar. To clean the flask and the beaker—we managed to involve the beaker as well—we used up almost two liters of isopropyl alcohol, and we had to start everything from the very beginning. A small portion of distillate came over, and I decided to purify it by shaking with a strong calcium carbonate solution and, after drying on anhydrous calcium carbonate, in total 20 milliliters of crude allyl bromide was collected. This reaction failed. The reasons for this failure were the incorrect sequence in the first step of hydrobromic acid formation and, secondly, our impatience. We messed up with temperature and overheated the brominated mixture. I had assembled a different setup, which we placed in a water bath to have better control of the heating temperature. The same amount of substances was used this time. The reddish color of sulfuric acid in the funnel is due to bromine contamination from the dirty beaker—my fault—but it is not an obstacle for this reaction to go on. That time, we were patient and had not heated the flasks until we added all the sulfuric acid. 70-80 degrees was enough for the reflux to start in the flask, and the distillate began to come over.
[Music] [Music] [Music] Season [Music] [Music] This time, we did everything correctly and are expecting yields up to 92%. [Music] You will see in the next part of this video how we dry and fractionally distill the crude allyl bromide and what our total yield is. [Music]