What is the mechanism for the bromination of acetophenone with tetrabutyl ammoniumtribromide? Find the attached file and suggest a suitable mechanism? Thank you.
The picture in the question is misleading, check the real structure here: https://en.wikipedia.org/wiki/Tetrabutylammonium_tribromide
Hence the above answer is the correct one (albeit simplified), in reality the Br2 is in the tribromide ion form.
1. The simplified mechanism suggested, where the enol attacks Br2 is acceptable but I have not seen the exact mechanism showing the generation of Br2 from TBATB anywhere.
2. Are you sure you only get monobromination with TBATB?
3. At what temperature do you run the reaction?
I have brominated 4-acetybiphenyl on numerous occasions using bromine in acetic acid. The acid helps to generate an intermediate enol tautomer pictured above by Phillip Van der Peet. The enol then attacks Br2 to transfer the bromonium ion to the enol which then loses proton from the oxocarbocation.
The amazing thing to me, and I have tested this on several occasions, that the first equivalent of bromine goes exclusively to give the monobromo compound. No dibromoacetyl group is formed until the all acetyl groups have been monobrominated. I have wondered about the cause of this complete selectivity for decades, but I cannot think of a good explanaton for this chemical behavior.
The addition of highly electronegative atom (in this case bromine) in alpha atom prevent the enolization process which prevent further bromination.
..."prevent further bromination." But only until the next equivalent of bromine is added. Then the dibromoacetyl group is formed smoothly.
But I ran a reaction between acetophenone and NBS, as the bromine source, and noticed that the product was a mixture. There we two singlets in the aliphatic region in a ratio of 2H to 1H for the downfield (higher chemical) region and upfield (lower chemical shift) region, respectively. I read a paper recently in a Chinese journal where they reported getting a major product being monobromination and two possible minor products, one being dibromination at the alpha position and the second being EAS of Br at the meta position. Any thoughts?
As Felix N Ngassa pointed out there is no explanation for the generation of Br2 from TBATB. Please suggest a plausible mechanism.
Mr. Ngassa,
There are two path that the reaction can take:
http://chemwiki.ucdavis.edu/Organic_Chemistry/Reactivity_of_Alpha_Hydrogens/Alpha_Halogenation
http://www.mhhe.com/physsci/chemistry/carey/student/olc/ch18reviewhalogenation.html
For monobrominated ketone, under the acidic condition, the enolization process will run very slowly because of the electron withdrawing effect of the halogen, on the contrary, the enolization through basic condition will be faster because the negative charge in carbon alpha will be stabilized by the electron withdrawing effect of halogen.
Mr. Srikrishna,
Please check the Wikipedia link I provided above, you'll know it when you see the real structure of it.
Somehow I got the feeling that you've already see the structure and seems to be don't get it. So I'll explain thoroughly.
First of all, there are TBAB & TBATB.
TBAB = Tetrabutylammonium Bromide (That's the one in the picture on question)
TBATB = Tetrabutylammonium Tribromide (not Tetrabutylammoniumbromide tribromide, that would be TBABTB. And it is chemically impossible, because there's no way that one ammonium ion bind two anion, do your research)
Second, TBATB is made by mixing Br2 with TBAB (https://www.jstage.jst.go.jp/article/bcsj1926/60/3/60_3_1159/_pdf).
Third, after the tribromide react with ketone, TBATB will return into TBAB while producing HBr at the same time.
Dear Muhammad Radifar,
Thank you for ur miniature observation on my question. Sorry, I wrote wrong abbreviation for TBATB. But the structure I written for TBATB is correct one only. Thank you for ur nice explanation for the above process. Kindly show the mechanism in schematic way how the TBATB will return into TBAB and how the bromination occur on acetophenone.
Thank u once again.
Not on ketone, but basically the same:
http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/Articleimage/2011/GC/c1gc15359e/c1gc15359e-s3.gif
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