InterviewSolution
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InterviewSolution
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Writeshortnotes on (i) Negativecatalyst (ii) Phasetransfer catalyst |
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Answer» Solution :(i) Negativecatalyst : In certain reactions , presenceof certainsubstances, decreases , the rate of the reaction. Ethanol is anegative catalystfor the followingreactions. Eg : (i) ` 4CHCl_(3) + 3O_(2) to 4COCl_(2)+ 2H_(2)O+ 2Cl_(2)` Ethanaldecreasesthe rateof the reaction (ii) ` 2H_(2)O_(2) overset("Glycerine")to 2H_(2)O + O_(2)` Int he decomposition of hydrogen peroxide,diluteacidor glycerol acts as a negative catalyst (iii) PhaseTransfercatalysis :Supposethe reactant of a reaction is presentin onesolventand the other reactant is presentin an another solvent. The reaction between them is very slow,if the solventsare immisible . As thesolventsfrom separate phases, the REACTANTS have to migrateacross the boundary to react. But migration of reactants acrossthe boundaryis noteasy.For such SITUATIONS a third solventis added which is miscible with both . So, the phaseboundary is eliminated reactantsfreelymix react fast.but forlargescaleproductionof anyproduct, use of thirdsolvent is not convenient as it MAYBE exprensive.for suchproblems phasetransfer catalyst( a phosetransferreagent) to facilitatetransportof a reactantin onesolventto the othersolventwherethe second reactant is present . As the reactantsare now brought together, theyrapidlyreactand fromthe product. Examples : Substitution of ` CI^(-) and CN^(-)` in thefollowingreaction. ` underset(" organic phase")(R-CI) + underset("AQUEOUS phase")(NaCN) to underset( " orgainc phase")` R-Cl = 1- chloroocatane R -CN=1 - cyanooctane Bydirect heatingof two phase mixture of organic1- chlorooctane with aqueoussodium cyanide for several days, 1 -cyanoocatane is not obtained. However,if a small amountof quaternary ammoniumsaltliketetraalkyl ammoniumchloride is added, a rapid transition of 1-cyanoocatane occursin about100% yieldafter1 or 2hours.in this reaction,the tetraalkylammoniumcation, whichhashydrophobic and hydrophilicends, TRANSPORTS CN fromthe aqueous phaseto the organicphasethe aqueousphaseto the orgainc phase usingits hydrophilic ends and facilitates the reaction with 1- chloroocatne as shown below : `underset( "aqueous phase")NaCH +R_(4)N^(+)CI^(-) tounderset(" It movesto organicphase ")(R_(4) N^(+)CN^(-) + CI^(-)) ` ` underset( " Bothin organicphase") ( R_(4)N^(+)CN^(-)+ R-CI) tounderset( " organic phase") ( R- CN)+ underset(" It movesto aqeouesphase,releases Cl- again picksup CN- and transports it")( R_(4) N^(+)CI^(-))` so phasetransfer catalyst,speeds up thereaction by transportingone reactantfromone phaseto another. |
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