Sciencemadness Discussion Board

Ethyl/Isopropyl Bromide preparation

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chucknorris - 13-12-2012 at 12:59

What happens to the sulfuric acid in this reaction? When my friend did this and he poured the leftovers to the sewer, it sounded like the whole sewer pipe started to boil, and huge mushroom cloud of steam released from the sewers.

Does this indicate that the sulfuric acid can be used at least once more? Sounded like my friend just wasted .5 kilos of good quality H2SO4.. :/

sargent1015 - 14-12-2012 at 16:34

Does anyone have a better work up of the 2-bromopropane synthesis? I am having some difficulties. :(

UC235 - 12-10-2015 at 10:09

Just bumping this old, but useful thread with a prep I ran yesterday. (Writeup text is almost completely stolen from my post on page 4 for ethyl bromide).

I ran a prep of Isopropyl Bromide based on the orgsyn.org prep (http://www.orgsyn.org/orgsyn/orgsyn/prepcontent.asp?prep=cv1...)


Instead of HBr, I substituted NaBr with excess H2SO4. A 1L RBF was charged with 114g of "91% Isopropyl Rubbing Alcohol" (equivalent to 1.65mol, the 91% is percent by volume. It is 87.7% by weight). 150ml of distilled water and a stirbar was added. With stirring, 154.34g of NaBr (1.5mol) was then added. Without agressive stirring, the sodium bromide forms a hard lump in the bottom of the flask that is difficult to dissolve. The NaBr causes the isopropanol to salt out as an upper phase

A standard distillation apparatus was assembled with ice water supplied to the condenser and the recieving flask (250ml) sitting in an ice bath. In place of a thermometer adapter, a pressure-equalizing addition funnel was added containing 330g of ~92% drain cleaner sulfuric acid (~3.1mol). This was slowly added to the ethanol/water/NaBr mixture. A lot of (presumably HBr) fumes were produced in the flask initially, but did not pass over into the condenser. The reaction mixture became quite warm warm and turned orange. Toward the end, more sulfuric acid was causing boiling to occur, so it was allowed to sit and cool before the rest was added.

After the sulfuric acid addition was complete, the addition funnel was replaced with a thermometer adapter. Distillation was carried out on a moderate heat setting (4 on my Corning PC-351). Distillate initially came over fairly rapidly at about 55C (an azeotrope?) and continued for some time, but less rapidly. Heat was eventually increased and the stillhead rose to 61C. After about 2 hours, the heat was raised again as flow had slowed to a crawl. The stillhead rose to 70C slowly with very little distillate, at which point heating was stopped.

The distillate was a cloudy but colorless organic phase with a small aqueous phase on top. The entire contents was transferred to a 250ml sep funnel and washed with 50ml of saturated sodium bicarbonate solution. No obvious bubbling from CO2 occured on shaking indicating absolutely minimal or no HBr contamination.

The organic phase was removed, the funnel cleaned and dried, and the organic phase returned to it. 50ml (90g) of concentrated H2SO4 was added and shaken with the iPrBr repeatedly. This caused an unusual faint cloudiness in the organic phase which split light and made it very colorful when illuminated. The mixture became slightly warm and the sulfuric acid turned yellow.

The lower H2SO4 layer was drained off (it had gained about 6g) and the organic phase was washed with a 50ml portion of saturated sodium bicarbonate solution. Initial CO2 release is dramatic from H2SO4 adhering to the sep funnel's walls. Agressive shaking and venting is needed to expel CO2 from solution. A second 50ml wash produced significantly less bubbling indicating neutralization.

The lower phase was drained into an RBF. A small amount of anhydrous CaCl2 is added which causes rapid clearing of the hazy-looking solution when swirled. It is allowed to stand overnight. The dry product was decanted into a clean amber glass bottle and weighed 156.30g (1.27mol), an 84.7% yield from NaBr.

The earlier writeup for iPrBr in this thread claims 90% yield, but doesn't use a sulfuric acid wash. The procedure is also anhydrous which favors the production of the ether. Without the acid wash, there is likely several percent of junk in the product. On diluting the sulfuric acid from my wash, the smell of whatever was absorbed was abominable whereas the finished product is quite pleasant smelling.

[Edited on 13-10-2015 by UC235]

byko3y - 13-10-2015 at 08:18

You had no HBr in the distillate because you had some water in the reaction. Most writeups recommend manually adding a small amount of water.
Before adding H2SO4 to your bromide, you need to ensure it has only minor amount of water, otherwise the bromide will be hydrolysed. Density of EtBr is 1.46 g/ml, so you need some stuff that has either much higher or lower density to be separated from EtBr while carrying away water.
But in fact you need to skip the H2SO4 step completely. It is especially useless if you can't use it properly.
"dry product was decanted into a clean amber glass bottle and weighed 156.30g (1.27mol), an 84.7% yield" of crude product - you'd better mention that explicitly. It may contain some crap like isopropanol, or ethanol, or who knows what alcohol you've used.

gdflp - 13-10-2015 at 08:53

Nice write-up UC. I find it interesting that you didn't reflux the mixture at all, and still got high yields. Just out of curiosity, what color was your pot after the acid addition was finished? I have run this reaction on a variety of alcohols, and the primary alcohols all seem to give a light orange color, while the reaction with 1,3-propanediol gave a dark red color.

Also, I would distill the product after the washing and drying steps; every time I have distilled it I find that it leaves behind a small amount of tar. I'm guessing that this may be due to the sulfuric acid ripping apart some of the alcohol/ether molecules during the washing, but I'm not sure. If you look carefully, I find that the crude product always has a slight yellowish tinge after the acid wash and prior to distillation.

Quote: Originally posted by byko3y  
You had no HBr in the distillate because you had some water in the reaction. Most writeups recommend manually adding a small amount of water.
Before adding H2SO4 to your bromide, you need to ensure it has only minor amount of water, otherwise the bromide will be hydrolysed. Density of EtBr is 1.46 g/ml, so you need some stuff that has either much higher or lower density to be separated from EtBr while carrying away water.
But in fact you need to skip the H2SO4 step completely. It is especially useless if you can't use it properly.
"dry product was decanted into a clean amber glass bottle and weighed 156.30g (1.27mol), an 84.7% yield" of crude product - you'd better mention that explicitly. It may contain some crap like isopropanol, or ethanol, or who knows what alcohol you've used.

And how exactly is an alkyl halide going to undergo an acid hydrolysis? The only mechanism would be a nucleophilic substitution, and hydronium ions are terrible nucleophiles; that's not going to happen to an appreciable extent. And even if it did, the sulfuric acid would dissolve the alcohol, rather than forming alcohol contamination in the product. In my experience with running this reaction, using a sulfuric acid wash, the final product distills completely within a 2°-3° range, leaving behind a small amount of tar. I have seen absolutely no evidence of any alcohol or ether contamination.

UC235 - 13-10-2015 at 09:58

The stillpot residue was a light orange for me, but I also used drain cleaner sulfuric acid which is brownish-orange to start with. The product is completely colorless and the Ethyl Bromide I made previously successfully performed a grignard reaction as-is.

@byko3y Excuse me? Alkyl halides are highly insoluble in concentrated sulfuric acid. Hydrolysis of alkyl halides occurs in the presence of nucleophiles. Sulfuric acid is not it. Water has extremely poor solubility in halides so only traces are present. Essentially all possible contaminants of the alkyl halide are soluble in the sulfuric acid by protonation. Ether, residual alcohol, water, and alkenes are all removed. The sulfuric acid is 1.83g/ml and readily forms a lower phase, but a teeny fraction of it seems to get dispersed in the halide. If the following bicarbonate wash releases the contaminants from the 1% of acid that was left in the funnel, it's still 99% cleaner.

Fantasma4500 - 20-7-2022 at 12:16

ethyl bromide made with modern "bromine tabs"

the reaction is very very much willing to happen
i took 3 tabs and dumped into a plastic container, and covered it with EtOH
covered it with foil and put it back up on shelf
few minutes later im noticing some strange smell, turns out i now have shockboiling teargas in a plastic container. quenched it with water and tried to flush it as fast as i could, barely any of the tabs were left

i have seen mentions of.. bromyl hydrate, the bromine version of chloral hydrate being formed by EtOH and Br2, its a solid at room temperature
i noticed also a strong scent of acetaldehyde, tribromoacetaldehyde? and undoubtedly volumes of ethyl bromide and most likely also ethyl chloride

i did find also chlorine in these tabs earlier, but the specific formula i have no idea about, likely bromine tabs all react somewhat the same

now im thinking of putting this reaction mixture, maybe with some water to calm it down a bit with a bunch of ice so i dont get runoff, and then simply coming back to it next day to hopefully collect a juicy amount of ethyl bromide, which boils at 39*C, where ethyl chloride is 12*C - rather annoying to handle, especially for a lachrymator

i plan on turning this into the amine by letting it react in a plastic bottle, ive looked up compatibility and PVC + EtBr is quite bad, but lets see what PolyEthyleneTerephthalate feels about this chemical.

this may be the most comically easy way to make a lachrymator- afaik the next easiest would be TCCA with some HCl, catalytic? and then some acetone

Tsjerk - 20-7-2022 at 13:12

Aren't you just oxidizing the ethanol? Why would ethyl bromide be formed here?

[Edited on 20-7-2022 by Tsjerk]
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