Nico
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Synthesis of n-alkyl bromides from corresponding alcohols
Hello guys,
let me briefly introduce myself. My name is Nico, I am 15 years old and from Germany. I recently read more and more in your great board, a bunch of
interesting topics helped me a lot.
For the purpose of just enlarging this great knowledge, I want to start with supplying a simple experiment here. Even though it's pretty trivial, the
product can be used in several further syntheses like general alkylation processed and Grignard reaction.
Unfortunately, I haven't took pictures of it, but nothing special occurred that would be worth it to take a picture of.
_________________
Synthesis of 1-bromopropane
The alkyl bromide mentioned above is a highly interesting compounds as it is suitable to be employed in many organic syntheses like for instance the
William-Ethersynthesis and general alkylation procedures.
Here this compound is produced by the reaction of 1-Propanole with hydrogen bromide under acidic catalysis with 0.5 equivalent of sulphuric acid.
First of all, the alcohol becomes protonated, whereby an alkyl oxoniumion forms and water results as a prominent leaving group. Afterwards a
nucleophilic substitution with bromide takes place and the product is obtained alongside water.
Actually the pure hydrogenbromic acid can be replaced by in situ generated hydrogenbromide, therefore an aqueous solution of sodium bromide combined
with an excess of sulfuric acid is sufficient as well.
Experimental procedure :
69.0 g of anhydrous sodium bromide were placed in a 250 mL two-neck round bottom flask fitted with a reflux condenser. Subsequently, both 50 mL of
water and 37.5 mL of 1-propanole are added while magnetic stirring is started.
As soon as a homogeneous solution is obtained, 50 mL of concentrated sulfuric acid are added through a dropping funnel so that the mixture is boiling
calmly. The reflux conditions are maintained for 20 minutes, whereupon the reaction mixture is allowed to cool down to room temperature. Though it is
pretty important to continue stirring for a 4 hours since sodium hydrogen sulfate precipitates as a crystalline solid.
In order to obtain the crude product the mixture is distilled, whereby the entire distillate up to a steam-temperature of 95 °C has to be collected.
The cloudy liquid, predominantly consisted of 1-bromopropane, is initially washed with 20 mL of diluted(3 %) NaCl-solution for the purpose of removing
unchanged alcohol. Following this, the organic phase is dried and delivered from formed ether by washing two times with 10 mL concentrated sulfuric
acid and distilled at normal pressure.
To this final product, a little amount of calcium carbonate(for removing remaining HBr) as well as anhydrous magnesium sulfate are added. So as to
clearly identify the product the refractive index should be in accordance to 1.434 or slightly less at room temperature.
Yield : ~ 32.0 mL / 70 % of the theory.
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Magpie
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Hello Nico and welcome to the forum.
Thanks for the nice synthesis. You are very well educated for a 15 year old, in my opinion. Great command of English too.
The single most important condition for a successful synthesis is good mixing - Nicodem
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Sedit
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I generated my EtBr with insitu HBr, I slowly added the H2SO4 to the Alcohol then slowly feed in the NaBr. This worked very well and I believe I got
arouni 74% yields(its posted on the forum here somewhere).
I had found a MUCH slower distillation will greatly increase yields, that was the main downfall is rushing the distillation. Dilution of the acid also
aids in keeping the HBr vapors from escaping before they react. Nice synthesis btw.
Knowledge is useless to useless people...
"I see a lot of patterns in our behavior as a nation that parallel a lot of other historical processes. The fall of Rome, the fall of Germany — the
fall of the ruling country, the people who think they can do whatever they want without anybody else's consent. I've seen this story
before."~Maynard James Keenan
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Nico
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Thanks for this good review to you guys.
Well, actually this way carries out high yields if you employ short chain alkyl bromides.
The reason for keeping the temperature as low as possible is, that I've used an excess of sulfuric acid, respectively two equivalents. Under usual
circumstances, sodium hydrogensulfate and hydrogen bromide are formed by the reaction of H2SO4 and NaBr - however, I did not now whether the
hydrogensulfate is still able to react with NaBr or even 1-Propanole. And as y'all(my favorite word adapted from Southern slang) know sulfuric acid
reacts with primary alcohols to dialkylethers at high temperatures(or even alkenes). And yeah, obviously, the slower you distill, the longer your
mixture has time to react before the water evaporates. Ethyl-substrates are generally reacting very fast, so that this reaction is pretty convenient
for their syntheses from my point of view.
Finally, I really appreciate the "welcome" to this forum. You guys can be fairly certain, that more complex and thus more awesome experiments will
follow as I find some time to translate properly. My current project is a synthesis of Bicyclo[4.1.0]heptane employing the
Simmons-Smith-cyclopropination. But for that I have to make Diiodomethane out of Methylenchlorid with Aluminiumiodide. And for sure the
aluminiumiodide has to be prepared in advance - so a lot of work ahead for the christmas/winter break.
EDIT : I just saw that you adapted a talk about the potassium synthesis from versuchschemie.de . That's also the page I am predominantly active,
whereby my username is the same as here.
[Edited on 11-12-2011 by Nico]
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sternman318
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Very nice! Very convenient as most of the organic chemistry I have been learning so far and itching to start doing utilizes haloalkanes! One question,
what is a good source of propanol? I sound that acidification of calcium propanoate yields propanol, but that does not seem to make sense to me.
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Magpie
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That should give propanoic acid in good yield (pKa = 4.86).
What is your evidence that you are getting propanol?
The single most important condition for a successful synthesis is good mixing - Nicodem
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sternman318
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Quote: Originally posted by Magpie  |
That should give propanoic acid in good yield (pKa = 4.86).
What is your evidence that you are getting propanol? |
It was a post I had found on SM, it didn't make sense but heck maybe I was missing something ( EDIT: I misread the thread, silly me). And I dont have
any spare LiAlH4, so I guess calcium propionate is not a good starting place haha.
But it appears then that most n-alcohols are not readily available, but must be ordered? If only propene was a liquid at RT, then one could do
isopropanol->2-bromopropane -(E2)-> propene -( HBr and some organic peroxide which I realize would be difficult/expensive)-> bromopropane.
I guess there is no easy route, huh? And many reactions utilize costly/hard to get reagents. Well, 2-bromopropane might be fun to make, even if I have
no use 
[Edited on 14-12-2011 by sternman318]
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