Meltonium
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2-Chloropropane Strange Distillation
Today, I attempted to make 2-chloropropane from isopropanol and concentrated hydrochloric acid with a zinc chloride catalyst. I refluxed the mixture
for about 2 hours and then set up for simple distillation. The boiling point of 2-chloropropane is ~35C and the boiling point of isopropanol is ~84C.
Throughout the distillation, I tried to keep the temperature down around 40C, but, I expected the temperature to stabilize at the bp of
2-chloropropane. It didn't. Instead, it would just keep rising if I increased the knob on my heating mantle. At one point, the temperature got to 80C
and a lot of distillate came over at this point. I assumed it was isopropanol and kept distilling; however, it became aware to me that my thermometer
probe should read 35C if the bp of the thing I'm distilling is 35C. I quickly cut the reaction short assuming I had failed. Yet, when I measured the
density of the liquid I collected, it came out to be .84g/mL when the density of 2-chloropropane is .86g/mL and the density for isopropanol .74g/mL. I
thought that maybe the density of the isopropanol came from the pure form. I was using the 91% so I attributed water content to the higher density.
But then as a final test, I poured some of the distillate into distilled water. 2-chloropropane is only sparingly soluble in water while isopropanol
is soluble. I expected everything to dissolve, but to my surprise, a layer formed on top of the water. My question is, why did my distillation not
take place at the temperature the distillate boils at and what temperature could I have raised it to before receiving significant impurities in my
product?
[Edited on 1-11-2016 by Meltonium]
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Magpie
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You are not giving us any quantities or a description of your work-up. Did you follow an established procedure?
The single most important condition for a successful synthesis is good mixing - Nicodem
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Meltonium
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The procedure I followed was my own using an excess of isopropanol, 165mL of 12M HCl, and 20mL of a concentrated Zinc Chloride solution. Because I cut
the reaction short, I only received 82.3mL of distillate.
I don't believe a work up was needed for this reaction.
[Edited on 1-11-2016 by Meltonium]
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gdflp
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If you're using an excess of isopropanol, then you are surely going to need a workup to get a clean product. The method I presented here should be suitable. (Apologies for the picture quality)
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Dr.Bob
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You are distilling a mixture of the 2 compounds. You might be able to redistill the smaller amount to a better purity, but not easy. Easier to use
an excess of HCl, drive the reaction to near completion, wash way the water soluables, and then distill some already mostly pure product.
If the desired product is only 10% or 20% of the mixture (I don't know the exact reaction conditions), you are going to have a tough time separating
the small amount of product from the large excess of isopropanol. The closer the reaction is to complete at first, the better.
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DDTea
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I had a similar experience with 2-chloropropane in an old job. It was forming through and undesired (and problematic) side reaction during
distillation. The column feed contained TiCl4 and heptanes and other nasties. During a bench scale experiment, we measured a full 25 wt%
2-chloropropane in the pot *after* distilling off the heptanes. Our speculation was that 2-Chloropropane was coordinating to TiCl4, something like
(CH3)2CH-(Cl+)-TiCl4(-). The complex would then have to be heated to dissociate before the 2-chloropropane would distill. It was a curiosity that
we never followed up on.
"In the end the proud scientist or philosopher who cannot be bothered to make his thought accessible has no choice but to retire to the heights in
which dwell the Great Misunderstood and the Great Ignored, there to rail in Olympic superiority at the folly of mankind." - Reginald Kapp.
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