The synthesis of 2-Bromomethyl-5-bromothiophene can be easily carried out by treating 2-methylthiophene with NBS in CCl4 as I found in several papers. But in this article, in addition of aforesaid reagents few drops of HClO4 were added. What is the role of using acid and how it will affect the reaction?
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Addition of a small quantity of strong acid protonates the succinimide portion of NBS, This increased polarisation makes the bromine atom more electrophilic, lowering the activation energy. It may be worth running this reaction in the absence of light which may cause radical initiated byproducts.
I agreed with Harris, Strong acid generates neucleophile bromine which attacks by electrophile followed by deprotonation give the product.
Bromination at the 5 position will occur via simple electrophilic aromatic substitution. Bromination of the methyl group most likely occurs via bromination at positions 2 or 5 followed by elimination of HBr to yield a fulvene like intermediate which is the thiophene equivalent of a benzyl cation which then undergoes nucleophilic addition of Br-. It would be interesting to see if the addition of tetrabutylammonium bromide would impact the yield.
The bromination at thiophene (-5 position) will occur by electrophilic substitution reaction, while on methyl group (of thiophene) via free radical. So, I think use of benzoyl peroxide would also accelerate the reaction.
For a free radical process, ambient light or a UV lamp should be sufficient to activate the system without having to add further reagents. Benzoyl peroxide is potentially incompatible with the low pH caused by perchloric acid. Thiophenes can also oligomerise fairly easily, I would be interested to know how clean the reaction is.
I am talking about the reaction in the absence of acid. In such condition, the impact of peroxide would be positive.
Absolutely, as would UV light or AIBN; similar to the conditions used in a Wohl-Ziegler bromination. The addition of acid would not diminish radical activity but may be required kinetically in order to get a good yield without too many byproducts.
You have regioselective free-radical bromination of the methyl group in the presence of AIBN and regioselective electrophilic bromination of the 5-position by the liberated bromine. The acid will make bromine more electrophilic
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