Reagent grade chemicals were used in this sudy, which were procured from eiher Sigma-Aldrich (India), SD-Fine Chemicals (India), or Avra (India) through a local vendor, and used wihout further purification. Ultrasonically (US) assisted reactions were conducted in a benchtop laboratory sonicator bath (TCL - BIO-Technics,India). Microwave (MW) assisted reactions were carried out in (CEM-908010, benchmate model, 300W) laboratory reactor.
(A) Preparation of SiO2/KHSO4, and SiO2/HClO4catalysts:
Silica gel (4 g, 100–200 mesh) was added to a (20 mmol; 25mL) aqueous solution of KHSO4with constant stirring for about 30min at room temperature. During this time,adsorption of KHSO4 occurred on the surface of silica gel. A powder separated out after removing water under vaccum, which was dried in an oven at 120°C for 2-3hrs to afford a SiO2/KHSO4 catalyst. Procedure for the preparation of SiO2/HClO4 catalyst is same as mentioned in the previous section for the preparation of SiO2/KHSO4 catalyst by replacing HClO4 (20 mmol), inplaceof KHSO4 (20 mmol). Rest of procedure is same as described above.
Scanning electron microscopic pictures of SiO2/KHSO4 and SiO2/HClO4 catalysts (Figures 3) with 50 μmmagnification revealed non-uniform morphologies with polynomial cubic crystals and flakes embedded with grain like species. These observations are almost agreeing well with the findingsof Zeba Siddiqui [35], and also by us in our earlier papers[36-38],suggesting the heterogeneous nature of catalysts which comprise adsorbed KHSO4 and HClO4 on silica.
(C) Bromination of aromatics using SiO2/KHSO4/HClO4 in conventional solvothermal reactions
Optimum amount of the catalyst (SiO2/KHSO4 or SiO2/HClO4) was added to the reaction mixture containing (10 mmol) of KBr, (10 mmol) aromatic compound, and acetonitrile the mixture of aromatic compound (10 mmol), KBr (10 mmol) and acetonitrile. The reaction mixure is subjected to continuous stirring under reflux, till the reaction is completed, as ascertained by TLC. After filteration catalyst separated out from the reaction mixture. Then, the filterate is treated sequentially wih NaHCO3, and etylacetate. The as-obtained organic layer was dried over Na2SO4, and evaporated under vacuum to get final product. In these raections, reaction rates are attributed to the fraction of activation of molecules that are formed in the reaction mixture by random collisions occurring between reactant molecules. Rate of the reaction depends on the number of activated molecules, as stated by Arrhenius, Eyring and several other pioneers.
(D) Ultrasonically assisted Bromination of aromatics under solvothermal conditions:
After preparing the reaction mixture as described in the preceding method, reaction flask is clamped in a benchtop laboratory ultrasonic sonicator bathtill the reaction is completed, as indicated by TLC. Same work-up procdure, as described in the previous secion is followed to obtain final product. Here, rate accelerations are explained due to the collapse of cavitation bubble formed during the course of reaction in presence of sonication. Collapse of Cavitation bubble generates large amount of energy causing the activation of large number of reactive species. We know that rate of the reaction depends on the fraction of reactive species.
(E) Microwave assisted Bromination of aromatics under solvent free conditions:
Reaction mixture was prepared as described in he above protocols, taken in a previously cleaned beaker, small amount of silica gel was added in a previously cleaned beaker, mixed thoroughly and placed in a laboratory desk top (CEM-908010, bench mate model, 300W laboratory microwave reactor)micro-wave oven.till the reaction is completed. Similar work-up procedure is followed to obtain the end-product. Observed enormous rate accelerations,here in, areexplaineddue to the bulk activation of molecules bymicrowave (MW) irradiation, whichiscapable to heat the target compounds directly, and uniformly without heating the reaction vessel entire, which saves time and energy.
(F) Spectroscopic Data for Some Representative Compounds
1. 4-Bromophenol :
1HNMR: d 5.12 (s, 1H, OH), 7.3 (d, 2H, Ar), 6.66 (d, 2H, Ar).
IR(cm-1): 1400 (Ar-OH), 450 (Ar-Br).
M. P : 1630c
2. 2-Bromo 4-methyl phenol :
1HNMR: d 5.12 (s,1H, OH), 2.32 (s, 3H, CH3), 6.68 (d,1H, Ar), 6.5 (d, 1H, Ar), 6.9 (d, 1H, Ar).
IR(cm-1): 1400 (Ar-OH), 450 (Ar-Br).
M. P :1770c.
3. 4-Bromo 2-methylphenol :
1HNMR : d 5.12 (s, 1H, OH), 2.32 (s, 3H, CH3), 6.5-7.0 (m, 3H, Ar).
IR(cm-1) : 1400 (Ar-OH), 450 (Ar-Br).
M.P : 1770c.
4. 4-Bromo 2-chlorophenol :
1HNMR: d 5.12 (s, 1H, OH), 6.56-7.3 (m, 3H, Ar).
IR(cm-1):1400 (Ar-OH), 450 (Ar-Br).
M. P : 1970c.
5. 1-Bromo 2-napthol :
1HNMR : d 5.12 (s, 1H, OH), 6.8-8 (m, 6H, Ar).
IR(cm-1) : 1400 (Ar-OH), 450 (Ar-Br).
M. P : 2130c.
6. 4-Bromoaniline:
1HNMR : d 4.12 (s, 2H, NH2), 6.3-7.2 (dd, 4H, Ar).
IR(cm-1) : 1350 (Ar-NH2), 450 (Ar-Br).
M. P : 1620c.
7. 2-Bromo 4-nitroaniline :
1HNMR : d 4.1 (s, 2H, NH2), 8.1 (d, 1H, Ar), 7.8 (d, 1H, Ar), 6.66 (d, 1H, Ar).
IR(cm-1) : 1350 (Ar-NH2), 450 (Ar-Br).
M. P : 2070c.
8. 4-Bromo 3-chloroaniline :
1HNMR : d 4.1 (s, 2H, NH2), 6.2-6.3 (dd,2H, Ar), 7.2 (d, 1H, Ar).
IR(cm-1) : 1350 (Ar-NH2), 450 (Ar-Br).
M. P : 1960c.