Oxidation of a primary alcohol to an aldehyde usually gives so...

Question

Oxidation of a primary alcohol to an aldehyde usually gives some over-oxidation to the carboxylic acid. Assume you have used PCC to oxidize pentan-1-ol to pentanal.

(a) Show how you would use acid–base extraction to purify the pentanal.

(b) Which of the expected impurities cannot be removed from pentanal by acid–base extractions? How would you remove this impurity?

Accepted Answer

All right. Hi, everyone. So this question says that hexane one all is oxidized to hex. Now using PC C, however, some over oxidation occurred and some hexane, one oil was oxidized to hexose. Part one says to provide a step by step process of purifying Hexon. Now using acid base extraction and part two is asking us which substance in the reaction mixture cannot be separated from Hexon out using acid base extraction. How can the two be separated? So in this case, before we can start planning out our extraction process, let's get a sense for what molecules are present or what chemical species are present in the actual mixture. So in this case, we're going to have our starting material and our two products that are shown in the beginning. So we have one Hexon all, we also have Hexon now and Hexaco acid. However, the other chemical species present in the reaction mixture come from PC C recall that PC C is a complex of chromium trioxide that's cro three with pyridine and HC. So we have Pyridine HC and cro three in solution as well. So now let's go ahead and talk about separation. The first approach we can take is attempting to separate some of these species in the reaction mixture using their relative solubility in water. Because for example, HCL is a polar molecule which means it's water soluble. And cro three is ionic and therefore soluble in water as well. Pyridine is also soluble in both polar and nonpolar solvents. So the first thing we can do is shake the solution in a mixture of hexane and water. Because the three species in the reaction mixture that came from PC C can be found in the aqueous layer and the aqueous layer can therefore be discarded. However, some purine would still be present in the organic or nonpolar layer. So from here, we can go ahead and use acid base extraction depending on how the remaining species in the organic layer react. First heany being in alcohol is relatively neutral. Let me actually write that in a different color but heanor is relatively neutral. Hexon OIC acid is of course acidic thine is a neutral amine which is or which makes it basic. And Hexon now being an aldehyde is also neutral. So if we start with treating the organic layer using a base, that's going to go ahead and separate compounds with acidic properties. So if we react the organic layer with something like sodium hydroxide, sodium hydroxide being a base is going to react with ecan OIC acid and thereby separated from the organic layer. Because if we use aqueous sodium hydroxide, the conjugate base of hexaco acid would move to the aqueous layer. This leaves us now with one heany Hexon L and purity. Now because pyridine is basic, if we go ahead and treat what remains in the organic layer with some kind of acid, that's going to react with purine and move it to the aqueous layer. So now we can treat it with HCL for example. So once the conjugate acid of pine moves to the aqueous layer, after using aqueous acid, that aqueous layer can be discarded. So that leaves us now with our two neutral chemical species right from the organic layer. Now we have Hexon and He Xana now notably, right, because one heany and hexane now are both relatively neutral, they cannot be separated by acid base extraction. So the only way in which we can go ahead and separate these compounds is by evaporating hexane and then using distillation to separate them by their boiling point. Recall that Hexon all is capable of forming hydrogen bonds with itself due to the fact that it has a hydrogen attached to an electron element like oxygen hea now cannot do that. However, so that's going to go ahead and contribute to a difference in their boiling points that is significant enough for distillation because hexen now as a boiling point of 129 °C, whereas one hexagon has a boiling point of 157 °C So this difference of 28 degrees is significant enough for them to be separated via distillation. And with that, we have answered parts one and two. So I'm going to go ahead and pause the recording to rewrite our answer. All right. So here I've resumed the recording and I figured it would be easier to go ahead and type it out since it would be easier to see as well. So for part one, the separation procedure is as follows. Step one shake with hexane and water discard the aqueous layer. Use the organic layer for step two, step two shake the organic layer with an aqueous sodium hydroxide solution, discard the aqueous layer. Use the organic layer for step three, step three shake the organic layer with aqueous HCL solution discard the aqueous layer. Use the organic layer for step four, step four evaporate hexane, the solvent to obtain a mixture of only heany and heany in step five, separate hexaco and hexane via distillation. So from here, the answer to part two reads as follows Hexaco and hexane now cannot be separated using acid base extraction, we can separate the two by distillation and there you have it. So with that being said, if you watch this video all the way through to the end, thank you so very much. And I hope you found this helpful

Key Concepts

Oxidation of Alcohols

Acid-Base Extraction

Impurity Removal Techniques

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