Sciencemadness Discussion Board

questions about procedure of distilling ethyl bromide

chochu3 - 26-2-2006 at 18:03

Upon forming ethyl bromide from 40 % sulphuric acid , sodium bromide, and 99 % ethanol in a distillation apparatus connected to a claisen adapter in which a addition funnel was attached on top to slowly drip in the sulphuric acid. After all sulphuric acid was added the apparatus was reconnected so simple distillation could be done at 40 degrees celsuis using a sand bath. At that temperature the ethyl bromide did not distill over even though I had the reciever flask inside a ice-salt water bath. Instead the temperature flunctuated up and I would turn the heat down. When I reread the literature (practical organic chemistry by Mann I believe) it said to do a rapid distillation up to 80 degrees celsius or so. I added ice cubes inside the reciever flask and when distilling above the temperature of ethyl bromides boiling point oily dropplets began to fall to the bottom. Now maybe the whole time the ethyl bromide was there but was not noticable until the water had melted in the second attempt, but the yield looked very low probably due to evaporation as the experiment was done in a course of two or three days.

The question is why would I want to raise the temp so high from the boiling point of bromoethane? Is it because the product does not form until it hits a certain temperature (assuming ethyl bromide does not form so quickly after addition with above chemicals) or because it is easier to condense the fumes of a volatile liquid with a liquid with a higher boiling point such as ethyl bromide condesing with ethanol?

Another question is why is it so shitty to use concentrated aqueous hydrogen chloride with sodium bromide, does it not form 100% hydrogen bromide (yield wise not percentage of solution) when both reactants are mix together? My first attempt at ethyl bromide was trying to use this. Because I got very little product I gave up on this and decided to use sulphuric acid to form the hydrogen bromide from sodium bromide. Correct me if I'm wrong but the reaction goes through a SN2 reaction where the alcohol is protonated and the alcohol is substitued by backside attack with the halogen, so why doesn't:
HCl + NaBr -> HBr + NaCl
work so good?

If the latter is the case then is it wise to assume distilling any volatile liquid one should raise the temperature higher than its actual boiling point such as ether from mixture extracts so I may reobtain my solvent?

leu - 26-2-2006 at 18:21

According to the article in Cohen's Practical Organic Chemistry, one must use a long condenser with the outlet of the condenser extending into the receiver and being immersed in cold water, and the receiver immersed in ice water :cool:

mantis - 27-2-2006 at 05:21


so why doesn't:
HCl + NaBr -> HBr + NaCl
work so good?

because HCl is more volatile than HBr in a aqueous solution, but you can use acids like H3PO4.

garage chemist - 27-2-2006 at 06:07

That's not true!
HCl forms an azeotrope with water at a concentration of ca. 23%. It boils higher than water itself. Try it our yourself, Mantis: put some 23% HCl in a distillation setup and distill it, and you will see that absolutely no HCl gas comes out at the end of the condensor. The distillate is still 23% HCl acid.

The reason why a mixture of HCl and NaBr is unable to convert ethanol to ethyl bromide is because HCl is a weaker acid than HBr.
In the mixture, only little HBr exists.
I tried this out as well, and there was a distillate but it consisted mostly of ethyl chloride (it condensed only because it was very cold in my lab at that time).

I had good success in the preparation of ethyl bromide via the method in rhodium, only with more ethanol used (they use too little in the preparation).
It uses halfconcentrated sulfuric acid, NaBr and ethanol, which are refluxed with a very efficient condenser for an hour and the fractionally distilled through a short column.

The reason why you had to heat high is because the reaction hadn't taken place to a sufficient extent.
Use a column, and distill slowly, and your ethyl bromide will come over cleanly at its boiling point of 38°C.

Douchermann - 28-2-2006 at 11:02

Haha, my reaction flask with reflux column is still sitting out in my lab. Its been sitting there since sunday and I refluxed it for about half an hour last night but didn't have the time to keep going. I put a stopper in the top of the column to keep evaporation down. Personally I think it would be wise to reflux for longer than 1 hour but thats just me talking. Just make sure there is ice cold water flowing through the condenser.

garage chemist - 1-3-2006 at 03:49

I don't know whats wrong here, but in my experiment I didn't even reflux it before... I just slowly distilled the mix through a column. This substitutes the refluxing.
The yield was good, even after washing, drying and redistillation.

Maybe your H2SO4 wasn't concentrated enough... 40% doesn't sound good.
Use 70%, this should work better.

[Edited on 1-3-2006 by garage chemist]