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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: lukas.stib on July 28, 2020, 04:55:11 AM

Title: How to remove of pentan-2-ol from 2-bromopentane?
Post by: lukas.stib on July 28, 2020, 04:55:11 AM

Hello chemists,

I syntetized about 50 ml of 2-bromopentane from pentan-2-ol and hydrobromic acid. Given almost the same boiling point of pentan-2-ol and 2-bromopentane I think my 2-bromopentane still contains alcohol (I think I feel very little alcohol from 2-bromopentane) and I want to ask you for advice on how to remove the alcohol from it?

Thank you very much. Lukáš S.

Title: Re: How to remove of pentan-2-ol from 2-bromopentane?
Post by: AWK on July 28, 2020, 05:20:36 AM

The problem is known and therefore manufacturers sell 90-95% 2-bromopentane.
Other method uses expensive PBr₃.

Title: Re: How to remove of pentan-2-ol from 2-bromopentane?
Post by: lukas.stib on July 28, 2020, 05:37:53 AM

PBr3 is very ugly compound I don´t like this. Is there no other more available method? What sodium hydroxide, does not form any pentoxide with alcohol, and 2-bromopentane remains unreacted?

Title: Re: How to remove of pentan-2-ol from 2-bromopentane?
Post by: rolnor on July 28, 2020, 04:13:38 PM

You can use thionyl bromide, dont know if its expensive. There is no easy chemical way to separate the bromide and alcohol, chromatography would work but I guess you have large-scale. Also you would need to separate the mobile-phase from your bromide by distillation. Ether and pentane would be best.

https://en.wikipedia.org/wiki/Thionyl_bromide

Title: Re: How to remove of pentan-2-ol from 2-bromopentane?
Post by: lukas.stib on July 28, 2020, 04:25:20 PM

Ok, thank you very much. And water? 2-Pentanol is soluble in water (little), but 2-bromopentane practicaly none.

Title: Re: How to remove of pentan-2-ol from 2-bromopentane?
Post by: wildfyr on July 29, 2020, 12:01:05 AM

Repeated aqueous washes aren't a bad idea. It might be the cheapest.

Title: Re: How to remove of pentan-2-ol from 2-bromopentane?
Post by: hollytara on July 29, 2020, 01:00:07 AM

supposedly azeotropes with toluene

Title: Re: How to remove of pentan-2-ol from 2-bromopentane?
Post by: AWK on July 29, 2020, 01:38:21 AM

As I wrote previously, if the purification of 2-bromopentane was easy, it would not be sold with a purity of 90-95% (
some manufacturers honestly report that the product contains several percents of 3-bromopentane). You have to consider whether it is worth doing it, and it depends on the quality of the 2-pentanol used. Besides, what is the point of purification, if there is no way to check if it is effective (GC, NMR).
Several decades ago, when very pure 2-bromopentane was needed to study the reaction mechanisms, it was obtained from synthetic 2-pentanol by reacting its tosylate with LiBr or NaBr.

Title: Re: How to remove of pentan-2-ol from 2-bromopentane?
Post by: lukas.stib on July 29, 2020, 03:03:17 AM

OK, thank you very, very much for all.

Title: Re: How to remove of pentan-2-ol from 2-bromopentane?
Post by: lukas.stib on July 29, 2020, 03:04:54 AM

Next time a will used synthezise 2-penten with HBr.

Title: Re: How to remove of pentan-2-ol from 2-bromopentane?
Post by: AWK on July 29, 2020, 03:13:49 AM

Are you sure that 2-pentene will give you pure 2-bromopentane?

Title: Re: How to remove of pentan-2-ol from 2-bromopentane?
Post by: lukas.stib on July 29, 2020, 03:18:16 AM

I'm not sure, but I'll try. I've had enough 2-pentene, and HBr too, just for fun.

Title: Re: How to remove of pentan-2-ol from 2-bromopentane?
Post by: AWK on July 29, 2020, 03:51:26 AM

How do you check if you have received a mixture of 3:1 of 2-bromopentane/3-bromopentane?

Title: Re: How to remove of pentan-2-ol from 2-bromopentane?
Post by: lukas.stib on July 29, 2020, 04:33:34 AM

I will believe that there will be of 3-bromopentane little ;D

Title: Re: How to remove of pentan-2-ol from 2-bromopentane?
Post by: Enthalpy on July 29, 2020, 05:01:07 AM

Melting points:

 -73°C  2-Pentanol
 -95°C  2-Bromopentane
-126°C  3-Bromopentane

would that be usable?

Title: Re: How to remove of pentan-2-ol from 2-bromopentane?
Post by: lukas.stib on July 29, 2020, 05:09:49 AM

Hm, very low melting points ;D