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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: lukas.stib on November 08, 2020, 04:49:58 AM

Title: How to remove 1-pentanol from 1-bromopentane?
Post by: lukas.stib on November 08, 2020, 04:49:58 AM
Hello chemists,

can you advise me how to remove 1-pentanol from 1-bromopentane? I made about 100 ml of 1-bromopentane, and I'm sure it also contains 1-pentanol.

Thank you, Lukáš S.
Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: Traumatic Acid on November 10, 2020, 11:35:50 PM
Aparently 1-bromopentane is practically insoluble in water while 1-pentanol is reasonably soluble in water. You could give your product a few water washes. Put your product into a separatory flask and add about 150ml clean (preferably distilled if you have it) water and give it a good mix. Don't shake it or you could form an emulsion which will be a pain to separate, just invert the flask repetativly for a few minutes.
Then drain off the lower layer (your 1-bromopentane). You can do that as many or as few times as you feel necessary.


I was suprised at the low solubility of 1-bromopentane, so I'll leave this here:

https://www.chemicalbook.com/ChemicalProductProperty_EN_CB6336015.htm
https://pubchem.ncbi.nlm.nih.gov/compound/1-bromopentane#section=Solubility

Good luck!
Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: lukas.stib on November 11, 2020, 09:37:30 AM
OK, thank you very much!  ;)
Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: Babcock_Hall on November 11, 2020, 11:44:38 AM
Do you think a distillation would be successful?  Why or why not?
Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: Guitarmaniac86 on November 11, 2020, 11:51:07 AM
If you think it still contains the alcohol, why not just resubmit it to the reaction and fully convert it? Would be easier than trying to separate them.
Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: lukas.stib on November 11, 2020, 12:59:10 PM
Because both substances have a close boiling point, so they cannot be easily separated by distillation.
Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: Babcock_Hall on November 11, 2020, 01:34:15 PM
Because both substances have a close boiling point, so they cannot be easily separated by distillation.

I agree.  Although a careful fractional distillation might give some separation, I would not be very optimistic.
Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: AWK on November 11, 2020, 04:55:38 PM
I'm guessing lucac.stib was doing a synthesis with HBr (KBr + H2SO4) and probably has a mixture containing 20-30% pentanol. He did not provide any data on the synthesis, processing of the reaction mixture, and the distillation product. We can only guess. It knows nothing about the possible formation of azeotropic mixtures. We do not know about its distillation capabilities (vacuum - although from a water pump, distillation column, etc.). Before lukas.stib starts any synthesis, he should prepare it well (I already pointed it out to him).
It is probably possible to save this synthesis and obtain quite pure 1-bromopentane, but we need to know all its previous procedures and possible his technical and analytical possibilities.
Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: lukas.stib on November 12, 2020, 10:24:27 AM
Hello chemists,

I performed the synthesis using concentrated HBr, H2SO4 and pentanol. Weighed everything according to the instructions and refluxed for 3,5 hours, then distilled the mixture of water and bromopentane, stored the lower layer, rinsed with water, HCl, water, NaHCO3 solution, water and distilled. My thermometer showed a distillation temperature of 128°C, so it held on to the end of distillation. I guess it's a thermometer fault that's from China.

And as I mentioned, I think 1-bromopentane contains the alcohol, because I smell the "musty" smell of pentanol +  smell of bromopentane.


Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: Babcock_Hall on November 12, 2020, 11:24:39 AM
Did you use a fractional or simple distillation?  Have you checked the product by NMR?
Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: AWK on November 12, 2020, 12:07:37 PM
The quality of the thermometer is easy to check - distill some distilled water (this will allow you to check not only the thermometer but the correct position of the mercury bulb also).
Also, check the boiling point of your alcohol (add 2 cm3 of water to 10 ml of alcohol) - this way you will check the boiling point of the azeotrope and alcohol at the same time and compare it with any correction measured during the distillation of water (watch the lower end of the cooler - the azeotrope will stratify (drops of water) and alcohol), pure alcohol will be a homogeneous liquid).

Where does the procedure for your synthesis come from?
What can you learn about the quality of your amyl alcohol (eg: 98%, pure, extra pure, lab-grade, for synthesis, for analysis)?
Is your distillate colorless or yellowish? Does washing with a little water still acidify the water?
What is the solubility of n-pentanol in the water?
What is the boiling point of the n-pentanol-water azeotrope and what is its composition? What is the boiling point of 2-bromopentane? What were your temperature constant points during the distillation? Do you have the possibility of vacuum distillation with a water pump?

I suppose 1-Bromopentane is likely to form an azeotrope with n-pentanol (although a simple internet search gives nothing, I expect a boiling point of ~ 125°C).

If you do all of this with understanding, you will probably understand the idea of ​​splitting your mixture (but ask beforehand).
Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: lukas.stib on November 12, 2020, 01:01:59 PM
I used simple distillation because high-boiling substances do poorly distillation.
1-Pentanol is 99% for analysis.
My distillate is colourless, and i wash H2O.
No, vacuum not possibility.
Distillation comes from 90°C in little drops, then everything distilled at about 118 - 120°C.
Thank you for your tips of quality my thermometer.
After that distillation, I rinsed everything H2O, dried it, and next time I will try to perform a simple distillation again.
NMR haven´t.
Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: AWK on November 12, 2020, 01:30:46 PM
Binary azeotrope 1-pentanol-water boils at 95.8°C and contains 45.6 wt. % of n-amyl alcohol.  Add 15-20 ml of water to your mixture/ The first fraction below 100°C will contain n-pentanol, next fraction - water, at about 115°C maybe a few drops of 2-bromopentane (if it occurs), and above 125 - pure 1-bromopentane.

In this volume of water, the pentanol solubility will be less than 0.5 g, so you can separate the alcohol and measure its volume to estimate the alcohol content of your product.
Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: lukas.stib on November 12, 2020, 02:22:34 PM
OK, thank you very much for your informations. I add water to my bromopentane and distill.
Should I use mixing? I do not use mixing, just a large number of small and large pieces of porcelain for good boiling.
Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: lukas.stib on November 14, 2020, 05:18:09 AM
OK, I tested the accuracy of the thermometer when I was boiling pure water, and the thermometer showed 100°C, so it's probably accurate.
Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: lukas.stib on November 15, 2020, 02:39:59 AM
Hello chemists,

I distilled bromopentane with water. The first drops dripped at 65°C, and so it dripped to 115°C. Odor I can't tell what it is, something between bromopentane, pentanol etc. The product dripped at about 115 - 123°C. I think it is pure.
Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: AWK on November 15, 2020, 04:14:37 AM
At the beginning of the distillation, did you observe the pentanol drops floating on the surface of the distillate around 100C? No smell but floating drops of pentanol would indicate its presence in the sample. The boiling point of 1-bromopentane is about 129°C (12-bromopentane is about 118°C). What were you distilling?

locant corrected
Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: lukas.stib on November 15, 2020, 04:21:56 AM
No, no drops floated on the surface, everything seemed like one mixture.

Based on the fact that from 115°C and higher, everything was dripping quickly and to the end, so I think it is already a pure product. It smells different from pentanol, strangely, sweeter.
Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: AWK on November 15, 2020, 04:27:45 AM
Check the boiling point of your pentanol.
Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: lukas.stib on November 15, 2020, 04:40:19 AM
They write 136 - 139°C on the bottle cover.
Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: AWK on November 15, 2020, 04:47:51 AM
Still, check it experimentally.
Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: lukas.stib on November 15, 2020, 04:54:15 AM
OK, thank you.
Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: rolnor on November 15, 2020, 05:15:57 AM
I think you should avoid sniffing on the mixture, this is a alkylating agent, not healthy for you.
Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: lukas.stib on November 15, 2020, 05:27:13 AM
Yes, I know.
Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: lukas.stib on November 17, 2020, 02:32:00 AM
I tried distilling pentanol. Slowly it started dripping at 132°C, then a lot of dripping from 135°C to 137°C. All it smell as pentanol.
Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: AWK on November 17, 2020, 03:08:01 AM
When using simple distillation without a rectifying column, a sharp boiling point is rarely achieved.
The result obtained by you does not allow for unambiguous interpretation.
You can also try to carefully wrap the fragment of the distillation set shown in the drawing with aluminum foil and repeat the distillation.
Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: lukas.stib on November 17, 2020, 04:06:13 AM
But I had the distillation set whole wrapped in aluminum foil; this is how I perform each distillation with a boiling point higher than water to "get" the fumes to the condenser.

Interestingly, when the water boiled, it showed 100°C.

Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: AWK on November 17, 2020, 04:19:24 AM
For now, you only have a good chance that you have received the correct chemical, but there is no absolute certainty. One could still check the thermometer by distilling some known compound with a boiling point in the range of 130-150°C.
The NMR science lab would give a definite answer very quickly.
Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: lukas.stib on November 17, 2020, 05:07:14 AM
OK, thank you very much. I appreciate it very much.
Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: lukas.stib on November 28, 2020, 06:53:54 AM
Hello chemists,

I distill again bromopentane, to which I added water, about 20 ml. Now the drops drip at 88 - 90°C. Drip style: in this temperature range, nothing drips for a while and suddenly a large amount of liquid flows, and for a while the drops drip in quick succession. According to the refractive index, however, it looks like bromopentane (high refractive index of the liquid), I don't understand it when the boiling point is much higher (possible azeotrope with water?). . Everything takes place between 88 - 90°C. After about 2-3 hours, the temperature stagnated. Slowly I started to increase the performance, and started to drip the substance at 124°C, here I started to collect, and it looks like bromopentane. At the beginning it was cloudy with water, + drops of water from the cooler, I dry it with CaCl2.
Now the problem: it's about half as much as I had before, I don't understand. It seems that a lot of bromopentane dripped at 88 - 90°C because: the first distillate with two layers I separated, and the upper one is actually water with pentanol (according to the smell), but the lower, larger layer looks like bromopentane: it is a mobile, shiny liquid . Can you please explain to me why such an amount (about 50 ml) dripped from the beginning at a much lower temperature?

Can I try to mix my pure distilled bromopentane with this and try to distill without added of water?

I will add that I do not have a stirrer, so I do not mix the mixture; I have pieces of porcelain inserted there like cooking stones.

One more thing. I dried the first heavier layer from the first distillation with CaCl2, and an interesting thing: while I dried the distilled product well, bromopentane is already transparent, so 50 ml of liquid is not: CaCl2 is still at the bottom and the solution is white, see photo. Could it be that even though there is bromopentane, there may be a lot of pentanol in it, which is probably soluble in it, and CaCl2 has formed this white turbidity with it?


Thank you very much. Lukáš S.
Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: AWK on November 28, 2020, 06:55:24 PM
If you get the condensate below 100°C and it separates into two layers with the organic phase floating on the water surface - this is pentanol. Bromopentanol has a density greater than that of water to keep it below the surface of the water. Without the rectification column, the condensate collected even below 100°C will contain traces of bromopentane which will be dissolved in the pentanol. I am rather sure that pentanol and bromopentane form an azeotrope and I expect boiling points of the order of 120-125°C.
Drying solutions containing alcohols with CaCl2 is pointless because alcohols form solvates with CaCl2 (probably with pentanol as well) and there will always be some water left in the dried preparation.
If you have a refractometer, distill a small amount of your preparation with the addition of water and check whether the refractive index of the last fraction increases.
Title: Re: How to remove 1-pentanol from 1-bromopentane?
Post by: lukas.stib on November 29, 2020, 02:44:13 AM
I don't have a refractometer. OK should I perform fractional distillation? That I would mix the product and the second distillate? I don't like fractional distillation for high-boiling substances, because it's more difficult, I'll have to wrap the column with foil or rags to keep warm.
Fractional distillation is much more efficient than simple I think.
.