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  • Professor Dave here, let's do some distilling.

  • We've learned a few different techniques for separating the components of a mixture.

  • Extraction utilizes differences in solubility.

  • Chromatography utilizes differences in polarity.

  • There is another technique that utilizes differences in boiling points, and it's called distillation.

  • Let's go ahead and check out a distillation apparatus now, along with some instructions

  • and tips for how to do this properly.

  • The idea behind distillation is quite simple to understand.

  • Given a mixture of two miscible liquids, if they have very disparate boiling points, we

  • should be able to heat up the mixture to a temperature that is above the boiling point

  • of one compound, but below the boiling of the other.

  • This will cause the one with the lower boiling point to go into the gas phase, such that

  • we can collect the vapor and condense it elsewhere, while the one with the higher boiling will

  • just stay right where it is.

  • And in fact, the technique is pretty much as simple as that.

  • But there are a lot of things to discuss regarding the setup, so let's see what this looks like.

  • Here we can see a distilling flask, usually a round bottomed flask, and our mixture will

  • go in here, along with a couple of boiling chips.

  • This will sit above the heat source, be it a hot plate, bunsen burner, or whatever we

  • are using.

  • We bring the mixture to a gentle boil, and vapor is produced.

  • This vapor will rise into the side arm, and a thermometer will sit here, measuring the

  • temperature of vapor right as it approaches this horizontal section.

  • The vapor continues into the condenser.

  • This has a central hollow section where the vapor will pass, surrounded by another section

  • where cold water will enter on one side and then exit from the other.

  • This will be constantly running, and against the direction of the vapor.

  • This cold water will cause the temperature inside the condenser to drop, so when the

  • vapor enters this section, it will condense back into a liquid.

  • This liquid is called the distillate, and we will collect it in a receiving flask.

  • And just like that, separation is achieved.

  • Now let's mention a few quick tips to make sure your distillation goes smoothly.

  • First, make sure your distillation flask is not more than half full.

  • If it is too full, some of the unwanted substance might make it into the distillate.

  • Second, make sure your thermometer is in the right position, up here, as we want to measure

  • the temperature of the vapor, not the liquid.

  • Third, make sure your heat source is easy to remove very quickly.

  • If it's a bunsen burner, make sure the setup is such that you can just pull the burner

  • away instantly.

  • If a hot plate, make sure it's easy to loosen one clamp and pull things apart easily.

  • It's possible for the mixture to suddenly begin boiling violently, and we need to remove

  • the heat right away if that happens.

  • Fourth, make sure every piece of glassware is clamped properly, especially the condensor.

  • Clips should connect everything.

  • And lastly, it's sometimes a good idea to just collect distillate at the desired temperature

  • range that corresponds with the boiling point of what you want.

  • This will minimize contamination.

  • So we can see how this technique is useful for separations.

  • It can even be used in the context of a reaction.

  • Say you're doing something like this, the dehydration of cyclohexanol by phosphoric

  • acid, to produce cyclohexene.

  • The starting material has a boiling point of around 162 because of its size and hydroxyl

  • group, while the product boils at around 83.

  • Since we want to heat this reaction up for it to proceed anyway, we can just perform

  • the reaction in a distillation flask, and the distillate will be our product.

  • Reaction and isolation of product all at once.

  • There are other ways to do distillation too, like fractional distillation.

  • This utilizes a fractionating column to separate mixtures with many components, like the atmosphere,

  • or certain mixtures with industrial applications.

  • Things also get tricky if the mixture being distilled is an azeotrope.

  • This is a mixture of two or more liquids that when boiled, the vapor will contain those

  • constituents in the same proportion as the liquid, and it will boil at a temperature

  • lower than any of their individual boiling points.

  • Ethanol and water is an example of such an azeotrope, and we will need special techniques

  • for such mixtures.

  • But we will have to look at these techniques another day.

  • For now, that wraps up our introduction to basic separation techniques for the organic

  • chemistry laboratory.

Professor Dave here, let's do some distilling.

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B1 中級

Separating Liquids by Distillation

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    ウィリアム オ ルウェル に公開 2021 年 06 月 25 日
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