Is the HBr was added to prevent the formation of HBr as by-product from Br2 during the reaction (minimize the wastage of Br2)? And to move the reaction forwards faster?
Actually, it's a surprisingly complex (and hence interesting) question. HBr can either accelerate OR slow down the reaction. From :
Catalysis by HBr in the bromination of o-xylene in acetic acid
E. P. Yesodharan, J. Rajaram,, J. C. Kuriacose, International Journal of Chemical Kinetics, 1976, Volume 8, Issue 2, Pages 277–283
(an excellent article which you should read every word of)
Hydrogen bromide is known to inhibit the bromination of aromatic substrates (ArH), either by fixing up bromine as HBr3 or ArH as ArH · HBr. However, there is catalysis by HBr in the bromination of mesitylene in acetic acid. The bromination of o-xylene in acetic acid in the dark is found to be autocatalytic, and the reaction is overall third order, first order in o-xylene with the orders in Br2 and HBr depending on the concentrations. A composite rate expression involving Br2 and HBr as electrophiles has been proposed and verified using iodine bromide as a catalyst where the orders are one in each of the reactants, irrespective of the concentrations used.
are you brominating aromatic system or alkene? They have different reaction types
@Eugene Yang
I am brominating the aromatic system.
What is the purpose of HBr in this step?
@Shu Er Tan . HBr is an acid so the pupose is similar to adding Fe3+ (Lewis Acid). It activates Br2 by pulling electrondensity out of the Br2 molecule.
Br2 + H+( proton) -----> HBr + Br+(bromonium) [ this is a representation of an extreme scenario)
@Bibaswan Biswas
Why don't the Br- dissociated from the HBr reacts with the Br+ (formed from Br2 + H+)?
Actually, it's a surprisingly complex (and hence interesting) question. HBr can either accelerate OR slow down the reaction. From :
Catalysis by HBr in the bromination of o-xylene in acetic acid
E. P. Yesodharan, J. Rajaram,, J. C. Kuriacose, International Journal of Chemical Kinetics, 1976, Volume 8, Issue 2, Pages 277–283
(an excellent article which you should read every word of)
Hydrogen bromide is known to inhibit the bromination of aromatic substrates (ArH), either by fixing up bromine as HBr3 or ArH as ArH · HBr. However, there is catalysis by HBr in the bromination of mesitylene in acetic acid. The bromination of o-xylene in acetic acid in the dark is found to be autocatalytic, and the reaction is overall third order, first order in o-xylene with the orders in Br2 and HBr depending on the concentrations. A composite rate expression involving Br2 and HBr as electrophiles has been proposed and verified using iodine bromide as a catalyst where the orders are one in each of the reactants, irrespective of the concentrations used.
For those who don't get a chance to read the paper, the uncatalyzed bromination is second order in bromine (initial rate, then autocatalytic, as the reaction produces HBr). In the presence of HBr it is first order in bromine and first order in HBr.
The authors invoke the following mechanism, where ArHBr+ is the traditional cationic intermediate that we draw in undergraduate chemistry.
ArH + Br2 ArH.Br2
ArH.Br2 + E --> ArHBr+ + EBr- (rate determining step)
Essentially, Br2 complexes the aromatic, and this complex is activated by an electrophile (protonated, brominated, or iodinated) to make the cationic intermediate.)
Where
E = Br2 , EBr- = Br3-, k = 2.8 10^3 M2/sec
E = HBr, EBr- = HBr + Br- k = 11 10^3 M2/sec
E = IBr, EBr- = IBr2- k = 449 10^3 M2/sec
So in addition to the very nice kinetic work, it also gives you a better catalyst. There are way too few papers like this in the literature these days...
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