December 08, 2019, 12:59:30 PM
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Topic: CCl4 radical bromination  (Read 576 times)

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Offline phth

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CCl4 radical bromination
« on: October 28, 2019, 11:31:35 PM »
Is there an alternative solvent for radical bromination of a benzylic position using NBS and 254nm than carbon tetrachloride? The yield doesn't matter because the SM is cheap

Offline hollytara

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Re: CCl4 radical bromination
« Reply #1 on: October 29, 2019, 12:06:59 AM »
Manufacturing CCl4 was banned years ago as an ozone depleting chemical, so it is really expensive.  It was the best solvent for this reaction.

I think a lot of people use chloroform as a replacement but it has problems - the trichloromethyl radical is too easily formed.  Others use 1,2-dichloroethane as it is inexpensive and less reactive.

The third but more expensive option is trifluoromethylbenzene. 

Offline kamiyu2550

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Re: CCl4 radical bromination
« Reply #2 on: October 29, 2019, 12:58:41 PM »
Is there an alternative solvent for radical bromination of a benzylic position using NBS and 254nm than carbon tetrachloride? The yield doesn't matter because the SM is cheap

Solvent for allylic bromination?? What a good question where I find it hard to suggest one that is not subjected to practical issues, rather than chemistry concerns.

CCl4 is the best, another routine choice of solvent in the literature is benzene, if you do not mind its toxicity.

Offline wildfyr

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Re: CCl4 radical bromination
« Reply #3 on: October 29, 2019, 04:17:26 PM »
chloroform or hexane usually work fine. I think I tried cyclohexane one time too.

Offline kamiyu2550

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Re: CCl4 radical bromination
« Reply #4 on: October 30, 2019, 02:15:12 PM »
chloroform or hexane usually work fine. I think I tried cyclohexane one time too.

I am wondering if hexane or chloroform themselves won't undergo radical bromination this case?

Just my two cents.

Offline wildfyr

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Re: CCl4 radical bromination
« Reply #5 on: October 31, 2019, 02:48:06 PM »
I thought the same, but they are pretty unreactive compared to a benzylic proton, especially hexane.

I did this reaction a bunch of time with difference solvents trying to monobrominate tosyl chloride. I never had any side products like that hang around. The reason I switched around between CCl4, cyclohexane, etc was because I kept getting 10% of the dibrominated tosyl chloride, and separating them was very difficult (in the end, nigh impossible). I ran columns, recrystallized carefully from heptane, no luck.

One thing I just saw, you want to do it with 254 nm light, I used AIBN or benzoyl peroxide and heat.

Another  solvent to throw out there for this, chlorobenzene.

Offline kamiyu2550

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Re: CCl4 radical bromination
« Reply #6 on: November 01, 2019, 12:50:05 PM »
I thought the same, but they are pretty unreactive compared to a benzylic proton, especially hexane.

I did this reaction a bunch of time with difference solvents trying to monobrominate tosyl chloride. I never had any side products like that hang around. The reason I switched around between CCl4, cyclohexane, etc was because I kept getting 10% of the dibrominated tosyl chloride, and separating them was very difficult (in the end, nigh impossible). I ran columns, recrystallized carefully from heptane, no luck.

One thing I just saw, you want to do it with 254 nm light, I used AIBN or benzoyl peroxide and heat.

Another  solvent to throw out there for this, chlorobenzene.

There is a way to turn di-bromo product back into mono-brominated product. You need to use phosphorus-based reagent, sorry I forgot the details, many many years ago

Offline wildfyr

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Re: CCl4 radical bromination
« Reply #7 on: November 01, 2019, 12:58:08 PM »
Its ok Kamiyu, the project died about 6-7 years ago :)

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