[bubaKazouba]

in short, my question is: what happens when concentraed HCl reacts with alcohol?
to be more specific, I need to seperate a 1-bromobutane from butan-1-ol
so it was written in the book that I have to wash the mixture with concentrated hydrochloric acid, this will remove the unreacted alcohol and seperates into 2 layers.

thanks in advance

[Hunter2]

I think in your case it will not work. HCl will give 1-chloro-butane, what is not solouble in water and gives two layers then. You have already 1-brormo-butane. So you have mixture of both. But You could use HBr instead of HCl.

[mahrap]

If you want to get a 1-bromobutane you will have to use HBr instead of HCl. Having said that, this is how the reaction should proceed with HBr:


The Oxygen will form a bond with the Hydrogen in HBr and become H2O, leaving the butyl group. During this process the Hydrogen in HBr will donate it's electrons to Bromine making Bromine an anion. In order to reach a lower energy this Bromide will now bond with the Carbon that the O came off from and give you a 1-bromobutane.

To see this reaction written out please follow the link (If you aren't familiar with reaction mechanisms just note that the arrows denote the movement of electrons.)


http://www.curvedarrow.com/chem/bromobutane.jpg

This is a typical SN2 reaction. I suggest watching the following video - the Khan Academy video on SN2 reactions- if you want to gain a better understanding of these types of reactions in general: http://www.youtube.com/watch?v=Z_85KXnBSYc.

[TheOrganic]

I don't think using concentrated HCl or HBr washing will help in separating these two.
You will have to go for a round-about process.

Try converting the alcohol into something else other than an alkyl halide?

[Babcock_Hall]

At what temperature does the conversion from an alcohol to an alkyl chloride using HCl take place?  I think that what is intended in this purification is extraction, as opposed to chemical conversion.

[Enthalpy]

Would water separate them? Butanol has some solubility in water but bromobutane not.
Sorry if I put nonsense.

[Babcock_Hall]

I think that a strong acid (possibly sulfuric would be as good a choice as hydrochloric) would tend to make the alcohol more soluble in water (by protonating it).  That would improve the separation in an extraction.

[opsomath]

I am surprised at everyone telling you that using acid will work to separate these compounds.

You can't use acid for this separation, alcohol is not basic enough.

n-butanol is soluble up to about 7% in water: http://en.wikipedia.org/wiki/N-Butanol

Because of this, you could potentially do it by dissolving your compound in a very nonpolar solvent like hexane or methylene chloride, then extracting with water repeatedly. Don't use ether or anything else with hydrogen bonding capacity.

[TheOrganic]

I think that a strong acid (possibly sulfuric would be as good a choice as hydrochloric) would tend to make the alcohol more soluble in water (by protonating it).  That would improve the separation in an extraction.

Well, protonating an alcohol might improve solubility in water, but I do not think the solubility will be increased dramatically. The problem here is, both the alkyl halide and the protonated alcohol will have comparable solubility in water.

I think even if solubility differences between the two exist, it'd still be a bad and inefficient extraction.

In fact, I believe solvent extraction should be a second choice for this separation. Look at the boiling points- 1-Bromobutane boils at around 101°C, 1-Butanol at around 118°C.
Fractional distillation seems good for this.

[opsomath]

Butanol is essentially "soluble" in water while bromobutane is "insoluble." As I said in the earlier post, water extraction should be viable although it'll take a few repetitions to get it all. That small boiling point difference means it will be hard to do the separation that way.

I am surprised to hear everyone talking about protonated alcohol. The alcohol will not be protonated in strong acid to any significant degree. Adding strong acid to this mixture will only serve to "salt out" organics as the ionic strength of the water will go up, since ionization of the acid will surely be more favorable in pure water than in a water solution of butanol.

[TheOrganic]

Butanol is essentially "soluble" in water while bromobutane is "insoluble." As I said in the earlier post, water extraction should be viable although it'll take a few repetitions to get it all. That small boiling point difference means it will be hard to do the separation that way.

I am surprised to hear everyone talking about protonated alcohol. The alcohol will not be protonated in strong acid to any significant degree. Adding strong acid to this mixture will only serve to "salt out" organics as the ionic strength of the water will go up, since ionization of the acid will surely be more favorable in pure water than in a water solution of butanol.

I don't know. With the instruments we use at our labs, I do not think 17°C difference in boiling point will be at all "hard" or "difficult" for fractional distillation.

Okay, I was wrong in my previous post. The Bromobutane is completely insoluble in water, while Butanol happens to have some solubility. I think repeated water-extraction can hence work out, as suggested above.

What is the problem regarding strong acid protonating the alcohol to a significant degree?
Alcohol is a lewis base and strong acid means protons every-where. If reactants come in contact, an acid-base reaction should happen, and I argue, it should be reasonably fast.
I think if one uses concentrated Sulfuric acid, and appropriate temperature, the alcohol will react to give alkyl hydrogen sulfate - which will form a clear solution in sulfuric acid. The alkyl halide on the other hand will be even less soluble in this, and will separate into the oil-layer. After separating, alkyl hydrogen sulfate could be re-converted to the original alcohol simply by boiling in water.

[opsomath]

The distillation could definitely be done with a good fractionating column, but I wouldn't want to have the job myself.

It looks like my reasoning was incorrect about the effect of acid on a water/butanol solution. Probably it's true that dilute HCl would decrease the solubility, but concentrated HCl does indeed protonate and remove butanol:

http://books.google.com/books?id=deyF5OeOOvgC&lpg=RA2-PA27-IA5&ots=Olrhq9igkz&dq=solubility%20of%20butanol%20%20in%20hydrochloric%20acid&pg=RA2-PA27-IA5#v=onepage&q=solubility%20of%20butanol%20%20in%20hydrochloric%20acid&f=false

[Babcock_Hall]

In a teaching lab at my institution we do one experiment which uses sulfuric acid to help remove unreacted alcohol from an experiment.  I can look up the details if anyone is interested.

[orgopete]

In a teaching lab at my institution we do one experiment which uses sulfuric acid to help remove unreacted alcohol from an experiment.  I can look up the details if anyone is interested.

We did that experiment also, it's a standard reaction. I did not like the procedure as it called for a wash with conc sulfuric acid to remove unreacted alcohol. I expect butanol may be more soluble in it (if it matters), however the yields are pretty good anyway, so I don't remember seeing it in anyone's samples.

I did find and substitute an alternate procedure that used an HCl wash, but I don't remember the details.