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Topic: Brominating 4-methylbenzoic acid  (Read 4010 times)

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Offline Nescafe

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Brominating 4-methylbenzoic acid
« on: June 30, 2014, 01:43:55 AM »
Hello,

If I planned to monobrominate the methyl position would I have to first protect the acid to avoid any radical formation in that position or would it be too unstable for that specie to form? i.e. I wouldnt have to worry about it?

thanks,

Nescafe.

Offline Hunter2

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Re: Brominating 4-methylbenzoic acid
« Reply #1 on: June 30, 2014, 06:20:53 AM »
The problem would be how to protect the aromatic ring and also to stop the reaction after one bromine is substituted. Normally it goes through until tribromine derivate.

Offline Nescafe

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Re: Brominating 4-methylbenzoic acid
« Reply #2 on: June 30, 2014, 09:41:17 AM »
would I even need to protect it though?

Offline discodermolide

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Re: Brominating 4-methylbenzoic acid
« Reply #3 on: June 30, 2014, 10:09:29 AM »
The problem would be how to protect the aromatic ring and also to stop the reaction after one bromine is substituted. Normally it goes through until tribromine derivate.


So how do you protect an aromatic ring? Did you mean the acidic group?
To the OP, you can always make an ester, it adds a step but may be worth the time. Give the bromination of the free acid a try and see what happens.
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Offline Hunter2

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Re: Brominating 4-methylbenzoic acid
« Reply #4 on: June 30, 2014, 10:32:26 AM »
Do I get bromination on the Methylgroup or do I get it on the ring.

Which is the method to do. add bromine and Aluminiumbromide? Or use light to get it in the methylgroup.

Offline Nescafe

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Re: Brominating 4-methylbenzoic acid
« Reply #5 on: June 30, 2014, 10:42:37 AM »
I dont think you can brominate the methyl of the methyl ester, or at least it will be unlikely, I think since it is next to an Oxygen which has two electron pairs the radical formed is too unstable (short lived) compared to radical on methyl which is stablized through resonance, I would imagine

Offline discodermolide

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Re: Brominating 4-methylbenzoic acid
« Reply #6 on: June 30, 2014, 10:48:49 AM »
The methyl of the methyl ester won't be brominated. You can readily brominate the methyl group attached to the ring. You may see some ring bromination, but as Hunter2 asked, what conditions are you going to use?
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Offline Nescafe

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Re: Brominating 4-methylbenzoic acid
« Reply #7 on: June 30, 2014, 10:51:07 AM »
CCl4, NBS, BPO(cat)

Offline Hunter2

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Re: Brominating 4-methylbenzoic acid
« Reply #8 on: June 30, 2014, 10:59:38 AM »

Offline orgopete

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Re: Brominating 4-methylbenzoic acid
« Reply #9 on: June 30, 2014, 11:32:53 AM »
This looks as though this should be known chemistry, known conditions, yields, isolation, etc. Why wouldn't you just follow a lit procedure.

I always heard two opposite sayings, five minutes in the library can save five days in the lab, and of course, five minutes in the lab can save five days in the library (or something like that). Experience tells us which rule to follow.
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Offline clarkstill

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Re: Brominating 4-methylbenzoic acid
« Reply #10 on: June 30, 2014, 01:29:11 PM »
I don't think you'll need to protect the carboxylic acid.  Radical bromination under the conditions you have given shouldn't mess with the acid, and I wouldn't worry about brominating the ring either. There are loads of examples in the literature in the presence of an unprotected acid (e.g. J Med Chem 2007, 5882).

 You definitely shouldn't use Friedel-Crafts conditions, as this will certainly brominate the ring and wouldn't touch the methyl group.

Offline discodermolide

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Re: Brominating 4-methylbenzoic acid
« Reply #11 on: June 30, 2014, 01:43:51 PM »
When I said some ring bromination I mean about 0.5-1% or less. It's something you may only pick up when doing the reaction on scale, >5-10 kg or so.
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Offline TwistedConf

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Re: Brominating 4-methylbenzoic acid
« Reply #12 on: June 30, 2014, 06:44:37 PM »
If I planned to monobrominate the methyl position

You shouldn't plan to brominate the methyl position.

You should plan to find a catalog and buy some... it's not very expensive. If you want the ester, you can buy that too.


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