Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Organic Chemistry Forum for Graduate Students and Professionals => Topic started by: nox on September 03, 2011, 05:22:36 PM
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A few days ago I was doing a large scale (>7g) bromination of indole. I stupidly used a plastic syringe to measure out the volume of Br2 needed. I noticed the steel needle acquired a brown-black coating within a minute, and became completely clogged up after about 5 minutes (luckily I had already transferred the bromine somewhere).
Is bromine supposed to attack stainless steel? Also, besides using a graduated cylinder, is there any way of accurately measuring out the volume of bromine without it eating away absolutely everything?
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Stainless steel is ok, and so are any lab syringes compatible with organic solvents. I hate trying to drip out neat bromine, though. I'm a NBS man myself, when possible.
Other than that, I do use a syringe. On occasions when I've had to brominate with Br2, I have measured volume with a syringe, but immediately transferred it into solvent (CCl4 or acetic acid) and added that dropwise to my stuff with an addition funnel.
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Dripping out neat bromine is a pain, especially since it's so volatile, and if I use a pipette, I can forget about the rubber bulb.
I did transfer my bromine into DMF and added it slowly as required by the prep. I'm just surprised the needle turned black and got clogged.
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I've never found this a problem, even adding 10 mL (~30 g) of bromine to a reaction mixture slowly (over ~10 min) with a syringe. The syringe does stain orange, but I never investigated the the needle afterwards, it all goes directly into a thiosulfate bath.
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I always use a dropping funnel. Dropwise addition works well for multiple 100ml´s of bromine.
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I wouldn't be surprised if its not stainless steel if its a cheap disposable needle. I've had a number of these corrode on me.
With the vapor pressure of Br2, I wouldn't recommend measuring it in a syringe either. That's asking for a bromine super soaker.
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Stainless steel is known to be corroded by chloride, so I'm not surprised that you've had issues with bromine especially as the quality of the steel in the needle is unknown.
I'd definitely recommend the approach of adding the bromine by dropping funnel. If you are worried about accurate measurement then put an old balance in you fumehood and weigh out the bromine.
Dave
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Bromine will attack many "stainless" steel varieties in my experience.
I did not notice anyone mentioning this but please note that
Bromine will damage Teflon stopcocks, so use a glass stopcock when adding Bromine to a reaction, especially on a large scale reaction.
Another bad accident can happen when using plastic syringes with chlorosulfonic acid - IT WILL DISSOLVE THEM! Use only glass one with it.
So please be careful using plastic syringes, they are not appropriate for all uses, and can be dangerous with very strong acids and bases like t-butyl lithium. TFA can also damage some disposable needles and syringes.
Bob
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This is how I would do it. I would pour out an amount (xs) into a tared and stoppered graduated cylinder in the fume hood. I would stopper it and weigh it. Then back in my hood, I would dilute it with my reaction solvent and then pour the portion required into an addition funnel.
This is not meant to be condescending. This is how I have begun to think from working with inexperienced chemists, especially if I may not be present to directly supervise them.
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It's possible you are actually attacking the syringe or the plastic coupling on disposable needles instead of the needle. Plasticizers in plastic syringes can leach into and react with the bromine, sometimes forming thick black tars that can clog the needles.
If I have to syringe bromine (and I do sometimes), I use a really close-fitting glass syringe with what I normally would consider a somewhat oversized needle (16 ga or larger). You have to work fairly rapidly to keep the bromine from getting past the plunger, but slowly enough that you don't get the pressure differentials which end up spraying bromine all over the inside of the hood. If your reaction conditions don't allow you to add the bromine fairly rapidly, definitely use an addition funnel.
Orgopete's method is good if you absolutely have to weigh out your bromine, but Murphy tends to bite me in the ass too much for me to try carrying bromine in anything except it's heavyweight screwtop bottle outside of the fume hood. Especially since I have to walk by three other hoods (with workers) to get to the balance in my lab. I'll handle it by volume if at all possible.
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Bromine will attack many "stainless" steel varieties in my experience.
I did not notice anyone mentioning this but please note that
Bromine will damage Teflon stopcocks, so use a glass stopcock when adding Bromine to a reaction, especially on a large scale reaction.
Another bad accident can happen when using plastic syringes with chlorosulfonic acid - IT WILL DISSOLVE THEM! Use only glass one with it.
So please be careful using plastic syringes, they are not appropriate for all uses, and can be dangerous with very strong acids and bases like t-butyl lithium. TFA can also damage some disposable needles and syringes.
Bob
I don't think that it actually damages Teflon, so much as diffuses into it. If you are very patient it will diffuse back out. I used Teflon sealed gas-tight syringes a few times (before I had a leakage incident due to a subtly damaged plunger) and the color goes away after a few weeks in a well ventilated place. This may be what is happening with polypropylene syringes too, I wouldn't expect something as electron poor as a dialkyl phthalate or as inert as a polyalkane to brominate on contact.
Proper reusable stainless needles should stand up to bromine for 10 min without clogging, unless they are very thin or already rusty. I see major slow corrosion issues while drying in the oven afterwards unless they cleaned very carefully (use plenty of thiosulfate)
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I measure using a teflon needle, and reuse it again and again, and there is no degradation. I also use a disposable plastic syringe with no problem.
Then I transfer into a dropping funnel, which has a teflon stopcock. As soon as the reaction is over (5 hours) I rinse the stopcock in alcohol and it's not orange or degraded or anything.
I typically work with 10 ml bromine at a time.