Kinetics and Mechanism of the Oxidation of Glycol by Dihydroxyditelluratoargentate(Ⅲ) with Spectroph

Vol. 16.No.32000Chemical Research in Chinese Universities218 ~ 222Kinetics and Mechanism of the 0xidation of Glycolby Dihydroxyditelluratoargentate(亚)with Spectrophotometry*SHAN Jin-huanWANG Li , SHEN Shi-gang , SUN Han-wen and WANG An-zhou( Department of Chemistry , Hebei University , Baoding 071002 , P. R. China )( Reeived Oct.27 , 1999)'The kinetics of the oxidation of glycol by dihydroxylielluratoargentate( IHI ) complex( DDA ) wasstudied in alkaline medium with spectrophotometry( in a temperature range of 16.6- -40 C ). The fist-order rates with respect to glycol and Ag( II ) were all found. The rates increased with the increase in[ 0H- ] and decreased with the increase in[ TeO2 - ] No effect was found with the addition of KNO3 andno free radical was detected. In view of this , the dihydroxymonotelluratoargentat( II ) specie( DMA)isassumed to be the active species. A plausible mechanism involving a two-electron transfer is proposed ，and the rate equation derived from the mechanism can explain all experimental observations. Activationparameters of the rate-determining step and constants are evaluated .Keywords Dihyroxyditelluratoargentate( II ), Glycol , Redox reaction , Kinetics and mechanismArticle ID 1005-9040( 2000 )-03-218-05IntroductionRecently , using a transition metal in its unusual oxidation state as an oxidizing agent which canbe stabilized by co-ordinaton of it with a suitable anion such as periodate or tllurate 1-101 has been .well known in analytical chemistry in the estimation of sugars , amino acids and proteins etc. . Theoxidation of a number of organic compounds and metals in their lower oxidation states by Ag( II ) has .been reportedt 8-101 , but no further information on the kinetics is available. The reactions are rapidand the experiments are performed easily , which is expected to be an analytical method for the ana-lytical chemistry . Therefore , the further investigation on the reactions is more important. The presentpaper covers the mechanism of the oxidation of glycol by diydroyielluratoarentat( II ).Experimental1 MaterialAll the chemicals used wereofA. R. grade. The中国煤化工as prepared and stan-dardized by the method reported by Jaiswal and Yadave:MYHCNMHG:trumwasfoundtobeconsistent with that reported. All the solutions were prepared with double-distilled water. The solu-* Supported by the Natural Science Foundation of Hebei Province No. 295066 ).* *丙亦数掘rrespondence should be dresed.No.3SHAN Jin-huan , WANG Li , SHEN Shi-gang et al.219tions of DDA and glycol were always freshly prepared before use with stock solution and double-dis-tlld water. The ionice strength was maintained by adding a KNO3 solution and the concentration ofhydroxide ions in the reaction mixture was regulated with a KOH solution.2 ApparatusThe measurements of the kinetics were performed by using a Shimadzu UV-3000 spectropho-tometer( Japan ) equipped with a Shimadzu TB-85 thermosta( +0.1 C ) with a constant temperaturecell-holder.3 Kinetics MeasurementsUnder the condition of[ glycol ]≥[ Ag ] , 2 mL of a solution containing definite[ DDA ],[0H- ],[ TeO2- ] and[ KNO3 ] and 2 mL of a glycol solution of appropriate concentration weretransferred separately to the upper and lower branch tubes of a λ type two-cell reactor. After theywere thermally equilibrated in the thermostat , the two solutions were thoroughly mixed and immedi-ately transferred toa 10 mm thick quartz cell. The cell was stopped and put in the cell-holder.The reactions process was monitored automatically by recording the variation of the absorbance( A, )of the reaction mixture with time( t )at λ = 350 nm with the spectrophotometer , from which theinformation on the kinetics was obtainedf11].4 Stoichiometry and Product AnalysisThe solutions , in which have been known concentrations of Ag( II ) and[ 0H ] , were mixedwith an excess of glycol . The completion of the reaction was marked by the complete disappearance ofAg II ) color. After the completion of the reaction , the product was identified 12 J as formaldehyde ,which was transformed into a precipitate 2 A-dinitro-phenyldrazine derivative. It was found that onemole of glycol consumed one mole Ag I ) by weight.Results and Discussion1 Evaluation of pseudo-First-order Rate ConstantsUnder the condition of[ glycol b>x[ DDA] , the plots of ln( ArAx )vs. time t for more thanthree half-lives of the reaction were good straight lines , indicating the order with respect to DDA to beunity , where A, and Ao stand for the absorbances at time t and infinity , respectively. The pseudo-first-order rate constants , kobs , were calculated by using the least- squares method r≥0. 999 ). Therate constants reported here were averages of three independent runs. Deviations in duplicate determi-nations were generally less than 5% .2 Effect of Varying Concentrations of glycolAt constant[ DDA],[ 0H- b ,[ TeO4- ] , ionic strength μ and temperature , kobs values in-creased with the increase in concentration of glycol and the plot of hobs Us. concentration of glycol wasa straight line passing through the origin( r≥30.995 )中国煤化工he first order with re-spect to glycoK Fig.1 ).YHCNMHG20Chemical Research in Chinese UniversitiesVol. 161505. ot1304. 0110Q 3.0902.0F701.05012 1430121620 2210*[glyool]/(mol . L-1)1/[OH ]/(mol-1. L)Fig.1 Plots of kos vs. [ glycol ].Fig.2 Plots of 1/kobs vs. 1/[ OH~ ] at differentt/C :a. 16.6;b. 25.5;c. 27 ;d. 30.6;e. 34.5.[ Agtemperatures .( II )]=3.922x 10- 4 mol/L;[ Te0F- ]=2.000x 10-3 mol/ 1/C, r: a17.5, 0.995; b. 25.5, 0.999; c. 29.8，L ;μ=0.150 mol/L ;[ 0H- ]= 0. 100 mol/L.0.994;d. 35.3 ,0.994;e.39.8 ,0.996.[ Ag II )]= 3.922x 10-4 mol/L;[ glycol]= 0.060 mol/L; μ=0.220 mol/L;[ TeOf- ]= 2.000x 10-3 mol/L.3 Effect of Varying Concentration of OH-Under the condition of[ glycol ]>[ DDA ] ,at constant [ glycolb ,[ TeO2 - ] , ionic strengthμ and temperature，kobs values increased with the increase in concentration of OH~ and the orderwith respect to 0H was found to be fractiona( in the temperature range of 17.5- 39.8 C ηap=0.19- -0.32). The plots of 1/kabs vs. 1/[ 0H- ] at different temperatures were all straight lineswithout passing through the origir( Fig.2).4 Effect of Varying[ TeO:- ]At constant[ DDA] ,[ glycol] ,[ 0H- ]and p( 1=25.5 °C ), kobs values decreased with theincrease in[ Te04 - ] and the order with respect to TeO4- was found to be inverse fractionaX ηap= -0.67). The plot of 1/hkobo vs. [ TeO2 - ] was a straight line without passing the origin( Fig.3 ).oFof60oL2.00 3. 004.00 5. 0010'[TeOT ]/(mol●L-中国煤化工Fig.3 Plot of 1/kobsYHCNMHG.[ glycol]=0.08 mol/L;[ 0H- ]= 0.110 mol/Liμ=0.150 mol/L;t=25.5 C.5 Effect of Varying lonic Strength and Free Radical DetectionTable 1 reveals that μ effect on the rates is negligible , indicating an ion-dipole type reaction.The addit夜帮操rylonitrile or acrylamide to the reaction mixture under nitrogen atmosphere neitherNo.3SHAN Jin-huan , WANG Li , SHEN Shi-gang et al.221changed the rate nor initiated any polymerization , showing no free radical in the reaction.Table 1 Rate dependence on ionic strength *p( mot L-1)0. 1000. 1500.2000. 2500.30010kan/s-11.684 .1.6671.6401.6801.650* [Ag II )]=3.922 x 10-4 mol/L ;[ glycol ]= 0.06 mol/L ;[ 0H- ]=0.10 mol/L ;[ Te02- ]=2.00x 10-3 mol/L;l=29C.6 DiscussionIn the alkaline medium( 0.05- -0.22 mol/L KOH ), the dissociative equilibrium of the easilyhydrous TeO2 - was given earlier I3]:HsTeO + 0H= = HTeOg- + H20lgβ1 = 10.049H4TeOg- + 0H~ H3TeO- + H2OlgB2 =- 1.00Hence the main tellurate species was HTeOg- . According to the report 891 , the main DDA specieswas[ Ag OH )( H4TeO6)了一over the experimental range of[ 0H- ].The fractional and inverse fractional order dependences of kobs on[ 0H- ] and[ TeO2- ] respec-tively indicate that 0HT ions would be present in a preequlibrium with DDA and then DDA losing aH4TeO- ligand from its coordination sphere 14] , foming a deprotonated[ Ag( 0H )( H;TeO6 )]-. ( DMA ) as reactive species which react with glycol , forming a transition complex , so taking[ Ag( 0H )( H4TeO)] to be the reactive species is unreasonable 89]. In view of this ,a mechanism in-volving an inner- sphere two-electron transfer was proposed , which may be written as follows :K[ Ag OH )( HTeO6λJ- + 0H=-[ Ag 0H)( H,TeO。)F- + HTeO2- + H0( 1)DDADMA[ Ag OH )( H,TeO。)F- + HOCH2CH2OH→[ Af 0CH2 )( OHXH;TeO,)F- + H2O+ H+ (2)complexcomplex-s Ag I )+ 2HCHO(3)As the rate of the disappearance of[ Ag( II )] has been monitored and[ Ag( II )]=[ DDA l +[ DMA 1=[ DMA1{[ HTe0q- ]+ K[ 0H- ]) K[ 0H - ]}, the rate law derived is-dAgII)]/dt=K[DMA1[glycol](4)=”kK[ 0H- I glycolI As II )]= kon[Ag II )](5)[HTeO-]+K[0H-]Subscripts t and e stand for the total and equilibrium concentrations respectively .The other two forms of eq.( 5 ) are as follows :[ H4TeO2- ]_11hodo =瓦glycol]+ kk[ glycol]x[0H ](6)1/kobo =h[ glycol]+ kK[ glycol I 0H-x[ H4TeOf- ](7)Equations(4)- ( 7 ) suggest that the orders with resp(中国煤化工) were all fist-orderrate and those with repect to 0HT and H4TeOf- wereYHCNMHGgactional respectively ,which conform to the observation. All above mentioned indicate that the mechanism we have proposedis more plausible. A plot of 1/kobs us. 1/[ 0H - ] derived from eq.( 6 ) at constant [ glycol ] and[ HTeOg - ] is linear and the rate constants of rate-determining at different temperatures are obtainedfrom the伍厄路据of the straight lin( See Table2). The plot of lnk us.1/ T is also linear. From the222Chemical Research in Chinese UniversitiesVol. 16slope and intercept of the line , activation energy and the thermodynamic parameters can be evaluat-ed14]. Those at t= 25 C were evaluated.Table 2 Rate constants and activation parameters't/C17.525. 529.835.339.810k/(mol-' E s-1)1. 6512.4473.6404.7166.089Activation parameter( 25 C )E。= 44.36 kJ/ mol△H# =41.58 kJ/mol△S#= - 14.04 J( K mol)* The slope and intercept of the plot Inkus. 1/T are - 5 336 , 16.56 respectively( r=0.999).References[ 1 ] Murthy ,C. P. ,Sethuram,B. ,Rao,T. N. ,Z. Phys. Chem. ( leipig), 267 6),1 21X 1986 )[ 2 ] Khan,J. A. , Chandraiah,U. ,Kumar ,B. 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Soc. ,69 ,( 127 )1992[11] SHAN Jin-Huan , LIU Tie- Ying , Acta Chemical Sinica,52 ,1 140( 1994 )[12] Feigl ,F. ,Spot Tests in Organic Analysis , Elsevier Publishing Co. , New York , 208( 1956 )[13] The Teaching and Research Section of Analytical Chemistry in Zhongnan Mining Instiute ，Handbook of Analytical Chemistry ,Science Publishing Co. , Bejing , 56X 1984)[ 14] Tang Ao-Qing , An Introduction to Chemical Kinetics , Jilin People' s Publishing Co. , Changchun , 183- -184 ;257- -258 1982 )中国煤化工MHCNMHG