8. The published procedure called for the solution to be heated at reflux for 90

Question

8. The published procedure called for the solution to be heated at reflux for 90 minutes, which would take up half of a 3-hour lab period. If this were shortened, conversion of starting material might not be complete, and all of the compounds shown below may be present in the crude product. OH 0 OH 0 acetylsalicylic acid salicylic acid methyl acetylsalicylate a) (15 points) Assume that you have dissolved your crude product in dichloromethane. Propose a series of extractions that would successfully separate the desired methyl salicylate from these other three components. Be specific about what aqueous solutions you would use, the order that you perform the extractions in, which component is dissolved in each layer, and how the pure i.e. no remaining solvent or water) methyl salicylate is finally isolated.

Explanation / Answer

You have in these mixture, two strong acids (the salicylic acid and the acetylsalicylic acid), and a neutral esther component, which you wan to separate from these mixture. In order to do extractions of these components you need to use the acid-base extractions. As esther is a neutral component, it will (in theory) not react in a large quantity with a base or acid, however, the esther may become carboxilic acids if they suffer hydrolisis in acid or base medium (if the acid or base is strong).

The NaOH is one of the strongest base, and it's used in many reactions including extractions, however, it's only recommended to use the NaOH in the extraction, when you have a weak acid (like a phenol) and you have not another compound in the mixture that can react with it.

In this case, if we use NaOH in these mixture, both acid components will be extracted by the base, however, it will react with the esther too because esther in basic medium can hydrolize, and form a different compound, in this case a carboxilic acid, so, if we use NaOH we will get a different compound and not the desired methyl acetylsalicylate. So what can we do? in this case, as we have two carboxilic acids, there are no way to separate one and then the other, so the best thing we can do here, is separate both of the acids with one aqueous solution and leave in the organic layer the esther. Then, with the aqueous layer, we can separate both acids by distillation (maybe a simple distillation is enough). (You may use this paragraph to answer part b).

Now, the best way to separate the carboxilic acid is using a weak base like NaHCO3 which is strong enough to extract both acids, but weak enough to not react with the esther and do a hydrolisis. So, we use aqueous NaHCO3 to separate both acids from the organic layer. We do at least 3 or 4 extractions with NaHCO3 to assure a better separation and better yield (and a better partition coefficient). After we do the extractions, we use some of NaCl to dissolve the emulsion formed by the mixing of the layers, (if it's neccesary of course), and then, let evaporate the organic layer, to get the methyl salycilate. Once that is separated, you may use a solvent or solvent mix (like water-ethanol) to recristallize the esther obtained, by heating the mix, until the solid dissolve, and then, cooling in an ice bath to promove the cristals to appear. Once they appear you may use the vaccum filter to dry the compound, and then, measure the melting point and the IR spectrum.

In the case of the other two acids, they are in an aqueous layer, so you can separate both of them by distillation. As their melting point are distant (140 and 211 °C), you may use simple distillation. The first distillate will be the acetylsalicylic acid, and to regenerate the compound, just use the strong acid like HCl or H2SO4.

Hope this helps

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