CEPTED ACCEPTED MANUSCRIPT 3 organic media. As the length of the alkyl groups increases, the water solubility decreases. Therefor we can tune the solubility of salts with precisely selection of alkyl or aryl groups on the cation. With a suitable choice from many different cations and related counterions, phosphonium salts can be utilized as oxidative, reductive or halogenating agents with good selectivity, mild reaction conditions and acceptable yields. Although bromination of organic substrates are usually carried out by molecular bromine but the new generation of phosphonium and ammonium tribromides are synthesized and used for this goal [2, 3, 10, 14, 21-27] so we prepared and used a new phosphonium tribromide for bromination of double bonds. Tribromides are more suitable than the liquid bromine because of their crystalline nature, hence it makes it easier to store, transport, and maintain the desired stoichiometry. Instead of bromination with nonselective elemental bromine under harsh condition, organic phosphonium tribromides as well as ammonium and immidazolium tribromides were used to brominate the C-C double bonds and aromatic rings using mild conditions and in a selective manner [10, 26, 27]. The bromination of double bonds by liquid bromine or by tribromide salts have a different mechanisms, thermodynamics and kinetics [28]. Although, bromination of cyclohexene by Br2 and Br3 - in 1, 2-dichloroethane have large negative entropies (S‡ = -62.0 eu. and S‡ = -40.9 eu. respectively), for bromination with Br2 there is a negative activation parameters and enthalpy (Ea = -7.8 kcal/ mol, H‡ = -8.4 kcal/ mol), while Br3 - has positive values (Ea = +6.6 kcal/ mol, H‡ = +6.0 kcal/ mol). Thus third -order and second -order rate constants are obtained for the reactions of free bromine and tribromide (respectively) in different solvents [28]. With respect to the kinetic and thermodynamic evidence, belluchi proposed two different mechanisms for bromination with Br2 and by tribromide anion.

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