I am conducting an electrophilic aromatic bromination reaction according to literature. The substrate and intended substitution pattern do not matter, but the reported procedure requires the dropwise addition of bromine at room temperature followed by refluxing overnight. (The solvent’s boiling point is very close to bromine’s so it may be assumed that both compounds reflux.) The reaction is not conducted under inert atmosphere and as with every organic bromination I expect HBr to be a (gaseous) by-product.

Obviously, the reaction is being conducted in a fume hood whose front opening is closed as far as possible. Nonetheless, I want to limit the amount of bromine vapour that gets released into the surrounding atmosphere. I came up with the following setup:

Two-necked flask with dripping funnel on the side neck and reflux condenser on the upright neck. The reflux condensor is topped with a drying tube (non-accurate depiction) with sodium thiosulphate.
Figure 1: A two-necked flask with a dripping funnel and reflux condenser. The glassware on top of the reflux condenser should be a drying tube (no accurate clip-art was found) containing damp $\ce{Na2S2O3}$ between cotton wool.

The dripping funnel is obviously used to add bromine dropwise to the reaction mixture and is sealed on top. The reflux condenser is open to the atmosphere through a drying tube; I filled the drying tube with sodium thiosulphate to quench any bromine vapour that makes it up there.

I did not attach the coolant flow to the reflux condenser until after the dropwise addition. This may have been a mistake, as bromine vapours had already diffused all the way up and penetrated the cotton wool and thiosulphate. It seems that these were far less effective in holding back any vapour than I had anticipated. Luckily, our coolant system is closed-circuit and the temperature adjustable. Thus, I set the cooling temperature to $\pu{0 ^\circ C}$ which seemed to reduce bromine’s vapour pressure sufficiently to clear the air in the drying tube. However, I believe there must be a better solution.

How can I run this reaction in reflux conditions while preventing bromine vapour escaping as much as possible? I had thought of sealing the entire reaction apparatus, but the reaction gives a gaseous side product (HBr) which could cause an undesired increase in pressure. I had also considered using a balloon to close off the reaction atmosphere, but I fear that both bromine and HBr will react with the rubber and destroy the balloon rapidly. What other methods exist that I may have missed?

Side note: the literature I am following was published in 1987. As expected of this era, they do not note specific precautions against bromine release.

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    $\begingroup$ Instead of the sodium thiosulfate filled dry tube you need a scrubber - a Dreschel bottle filled with thiosulfate soln would do it, though to be safe against suckback a train of 2 bottles with the first empty might be advisable as you're leaving it overnight. $\endgroup$ – Waylander Sep 2 '20 at 14:22
  • $\begingroup$ You might look for another funnel or different flask, to make sure the bromine drops directly into the solvent. Add bromine slowly. That should reduce fuming significantly. $\endgroup$ – Karl Sep 2 '20 at 14:37
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    $\begingroup$ @Karl Simply swap over the addition funnel and condensor so that the addition funnel is vertical. $\endgroup$ – Waylander Sep 2 '20 at 15:04
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    $\begingroup$ @Karl the picture is schematic. The actual flask I’m using is more pear-shaped and the dripping funnel has an outlet extending through the joint so that bromine did drop into the reaction solution. As there is quite a large amount of bromine compared to solvent, I am not sure whether more careful dripping will change any part of the general picture. $\endgroup$ – Jan Sep 2 '20 at 15:06
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    $\begingroup$ Based on your reaction solvent, you may dilute bromine 1:1 with solvent instead of using neat bromine. I was successful using $\ce{Br2/CHCl3}$ in aromatic bromination with 4-cyanophenol. $\endgroup$ – Mathew Mahindaratne Sep 2 '20 at 16:55

Check if you may skip using elemental bromine as starting material altogether. An alternative approach may be running the bromination in a heterogenic, biphasic setup with the couple $\ce{NaBrO3/NaHSO3}$ in water (e.g., 1998JOrgChem6023, 2000OrgProcRes Dev.30), or $\ce{NaBr}$ / sodium perborate (e.g., 1998SynthCommun925), or pyridinium tribomide (e.g.,2005JChemEduc306), or bromodimethylsulfonium bromide prepared by mixing aqueous $\ce{HBr}$ with DMSO (1997JOC4321, often at rt). They share a slower release of bromine, and because of this, lower amount and concentration of reactive bromine and attenuate the hassle with corrosive gases.

If staying with liquid bromine:

  • Trapping unconsumed bromine with thiosulfate per se is a good idea, but the reaction of bromine gas with solid $\ce{Na2S2O3}$ in the drying tube is both incomplete, bears the potential that the drying tube gets congested, and tends to be messy to clean. You could end up with a closed vessel, which isn't safe; and even dangerous as you generate HBr in stoichiometric amounts. Thus passing the exhaust gas across scrubbers filled with aqueous thiosulfate solution has a much higher chance of win. Don't forget the safety flask (different reaction, but same principle).

  • The the normal balloons from the toy shop does break under bromine, as well as HBr gas too quickly.

  • Beside @Mathew Mahindaratne's comment to dissolve bromine in chloroform, using acetic acid as solvent provides you a wider margin of temperature. (If your substrate can withstand the acid.)


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