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To obtain phenylamine after it is formed from the reduction of nitrobenzene with tin, we carried out steam distillation.

This is what I learned in class:

"During the process,steam is injected into the flask (containing the reaction mixture) to create artificial vapour pressure on the surface of the liquid. When vapour pressure is equal to atmospheric pressure, the liquid starts to boil at 95 degrees"

This didn't make much sense to me. So I searched it up on the net and I got another explanation (source):

As the hot steam passes through the mixture it condenses, releasing heat. This will be enough to boil the mixture of water and phenylamine at 98°C provided the volume of the mixture isn't too great

Here, it is saying that the heat produced from the condensation of the steam will allow the mixture to boil. I am really confused, I mean I carried out steam distillation but how is it different from normal distillation and how is phenylamine collected at a lower temperature through this process?

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You boil the water and the product co-distills. There is an anomalous selective interaction between steam and many hydrophobes. If you heat the pot you deplete the water contained and may pyrolyze contents on the hot wall. Blowing in steam limits temp to near 100 C, moves a lot of energy into the flask and out (540 cal/g enthalpy of vaporization), and maintains the water content.

You could filter off the solids, acidify the aqueous raffinate, extract non-basics, then alkalinize and extract the aniline. Since aniline is air sensitive to oxidation and extremely toxic by skin contact, you can appreciate advantages of steam distillation.

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Water and any rather non-polar organic substance form a "positive" azeotrope, which lowers the boiling point, and makes a steam that contains both substances. You take a large excess of water, so you are quite far from the point where you have the actual azeotropic mixture. Thus the whole thing boils essentially like water, i.e. you don't go far from 100°C.

It doesn't matter if the two substances (water and diphenylamine in your case) mix in the condensed phase, or if one is even a solid. Except that a solid will tend to clog your condenser, unfortunately. ;-)

One well known example is extracting water from an esterification reaction by boiling it off with toluene: They separate again after condensation, and you can let the lighter toluene phase flow back into the reaction vessel. Same principle, only water is the minor compound in this case.

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  • $\begingroup$ Hm, perhaps we are in fact at (or very close to) the azotropic mixture. After all, it has the lowest boiling point. Comments? $\endgroup$ – Karl Aug 27 '16 at 20:53

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