Transcription
So today, we're going to prepare some benzaldehyde through the oxidation of benzyl alcohol. The oxidant in this case is going to be nitric acid, concentrated, a diluted nitric acid, and a catalyst amount of sodium nitrate.
First of all, we have to make a dilution of our concentrated nitric acid to reach a concentration of about 1.59 molar. Then, inside the hood, we prepare a 500-milliliter round bottom flask inside a crystallizer dish with some water so we can start. After the addition of the nitric acid, we can start heating the solution. So, first of all, we're going to add the concentrated nitric acid solution into our water. Then, with strong stirring, we're going to add the catalytic amount of sodium nitrate. [Music] As we can see, some vapors of nitrogen dioxide are being liberated, so it's important to carry out this reaction in a field hood or a well-ventilated area.
Next, we're going to add the 79 milliliters of benzyl alcohol. [Music] So, for the reaction to take place, we have to heat the reaction to around 90°C for at least four hours. [Music] As we approach the desired temperature of 90 degrees centigrade, we can start to see the formation of bubbles of nitrous oxide and also the changing in color from whitish to yellow or orange color at the end of the reaction.
So let's talk about the mechanism of reaction. In the first step, we have here our sodium nitrate and nitric acid. So sodium nitrate is going to take a proton from nitric acid, then creating sodium nitrate and nitrous acid. Then nitrous acid, in an acidic media, is going to take a proton; it's going to protonate and it's generating this uh positive charged form. So water leaves, and then we generate the nitrozonium ion that it has; it has these two forms. Then nitrozonium—we have nitrozonium here. In the second step of the reaction, we have nitrous sodium and benzyl alcohol. Then benzyl alcohol; its oxygen is going to attack the nitrogen from the nitrous ion. So we have here uh the the following structure that is charge positive. So in this case, water is going to take a proton from here, and it returns its charge generating the acidic media. And then we have this molecule that is called benzyl nitrate. Benzyl nitrate is unstable, and it's more unstable in an acidic media, so it becomes protonated here in this intermediate. So this part of the molecule leaves, it turns its charge, and we have our final product that is benzaldehyde and H and O. This form is going to further react with nitric acid to form another mole—another two molecules of nit of nitrous acid—and thus regenerating the nitrozonium, and the cycle repeats itself.
After four hours of reaction, we can now observe the separation of phases. The upper phase contains the spent nitric acid, and the bottom layer contains our desired product. Before we can separate the phases, we need to cool down the reaction flask. And then, using a separatory funnel, we can now obtain our crude benzaldehyde.
Now that we have our desired product, our crude benzaldehyde separated, we can start adding saturated sodium bicarbonate solution in order to remove any spent acid. Always remember that it is a good idea to shake and vent to prevent the accumulation of gas. So after we finished washing the product, the crude benzaldehyde twice with saturated sodium bicarbonate solution and one time with saturated sodium chloride solution, we can now proceed to set up a simple distillation or a fractionated distillation using a short path apparatus—it is the short path apparatus—so we can effectively purify, purified, or benzaldehyde.