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Solubility/Purity Problem

I have a chemistry final tomorrow and this question on my sample exam through me for a loop. I'm not entirely confident I can work out problems B and C, but those questions rely on the answer to A.

If you could please especially explain how I'm supposed to read the solubility chart to understand which solvent is the best, that would be awesome.

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  • $\begingroup$ The best solvent would be the one with the greatest ratio of solubility for 3-phenylacrylaldehyde and a high solubility for benzaldehyde. You should be able to work out why from what I said. $\endgroup$ – LDC3 Mar 28 '14 at 13:20
  • $\begingroup$ Hi, thanks for answering, but I still dont quite understand. $\endgroup$ – TaliPHARAOH Mar 28 '14 at 16:40
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Expanding on LDC3's comment a little.

The information in the table (irrelevant of how good or bad the method proposed is) tells you what the saturation points are for solvent mixtures at given temperatures for each compound.

Fundamentally, you want to separate your desired product from your undesired product. In this case, you are trying to recrystallise the 3-phenylacrylaldehyde, leaving the benzaldehyde in solution. You know this because for all temperatures and solvent mixtures it has lower solubility (this cannot be assumed for a more complex solvent array).

You need the minimum of hot solvent, here 100 degrees, to dissolve all of your crude compound, and you want to maximise the return of pure compound on cooling. This explains why you want the biggest ratio of hot to cold solubility as LDC3 indicated.

You also want to minimise the amount of your starting material that crashes out with your product. So, calculate the same ratios for the starting material, but here you want to minimise the return on cooling, so you want the smallest ratio.

You now have a table of solubility ratios, columns being 'solvent mix', 'rP' (ratio of product solubilities) and 'rSM' (ratio of starting material solubilities). The largest value of rP/rSM gives you the best solvent mixture.

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Is the double bond cis ${Z}$ or trans ${E}$? (Terrible nomenclature.) What is the scent of the product? What is it commonly called? Don't taste it. A tiny bit on your tongue will excavate a deep ulcer. The ${E}$-isomer melts at -7.5 C. How did you get the putative ${Z}$-isomer, and how can it melt at least 40 C degrees higher than the ${E}$-isomer?

Two solvents are used, not five. Different mixtures are used. The technique is terrible! You would dissolve contaminated product in warm ethanol (below product mp) with stirring, then titrate with water until the cloud point. Add another drop of ethanol to clear. Very slowly cool without stirring (cheesecloth wrap) to obtain large clean crystals. Refrigerate for more crystals. Repeat a second time to maximize product yield. Do not combine to two crops! The second is not as pure.

As the product is most likely a liquid, you would fractionally distill it under reduced pressure (aspirator).

http://chemistry.st-andrews.ac.uk/labs/misc/nomo2.gif

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  • $\begingroup$ Although an interesting piece of information, this has hardly any connection with the question the OP is asking. OP wanted to know how to select the best solvent for the reaction from the solubility chart. $\endgroup$ – Michiel Mar 31 '14 at 5:35
  • $\begingroup$ The knowledge of trivial names can be helpful :) As for the isomerization, the answer lies in the 300 nm range ;) $\endgroup$ – Klaus-Dieter Warzecha Mar 31 '14 at 8:11
  • $\begingroup$ And yes, the options given in the question are terrible, the nomogram is helpful and distillation under reduced pressure an obvious choice. Storing the product under nitrogen or argon in the freezer in a flask wrapped with aluninium foil isn't a bad idea either. $\endgroup$ – Klaus-Dieter Warzecha Mar 31 '14 at 8:19

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