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化学专业英语之有机化学中的氧化与消解反应OXIDATIONANDREDUCTIONINORGANICCHEMISTRYTheconversionofalcoholsintocarbonylcompoundsisanimportantreactionofprimaryandsecondaryalcohols,andisoneofmanyexamplesinorganicchemistryofoxidation.Howdoweknowwhenanorganiccompoundhasbeenoxidized?Inthelastsection,werecognizedthatconversionofanalcoholtoaketoneisanoxidationbecauseitisbroughtaboutbythereductionofCr(VI).Butthereareotheroxidationsinwhichtheoxidizingagentislessobvious.Ourgoalinthissectionistobeabletorecognizeanoxidationorreductionmerelybyexaminingthetransformationoftheorganiccompounditself.Theprocedurefordoingthisinvolvesthreesteps;Step1.Assignanoxidationleveltoeachcarbonatominreactantandproduct.(Itisonlynecessarytoassignanoxidationleveltocarbonsthatundergosomechemicalchangeduringthetransformation;othercarbonsmaybeignored.)Theoxidationlevelofaparticularcarbonisassignedbyconsideringtherelativeelectronegativitiesofthegroupsboundtothecarbon,asfollows.(a)Foreverybondtoanelementlesselectronegativethancarbon(includinghydrogen),andforeverynegativechargeonthecarbon,assigna-1.(b)Foreverybondtoanothercarbonatom,andforeveryunpairedelectrononthecarbon,assignazero,(c)Foreverybondtoanelementmoreelectronegativethancarbon,andforeverypositivechargeonthecarbon,assigna+1.(d)Addthenumbersassignedin(a),(b),and(c)toobtaintheoxidationlevelforthecarbonatomunderconsideration.Letusapplythisfirststeptothetransformationofisopropylalcoholtoacetone.Sincethecarbonatomsofthetwomethylgroupsdonotchange,wedonotneedtoassignoxidationlevelstothesecarbons.NoticeinthetreatmentofacetonethattheC=Odoublebondiscountedastwobonds;+lforeachbondgivesatotalof+2forthe'doublebond.Step2.TheoxidationnumberNOIforeachcompoundiscomputedbyaddingtheoxidationlevelsofallcarbons.Inthestructuresabove,onlyonecarbonhaschangeditsoxidationlevel,sotheNoxvaluesofthereactantandproductaresimplyequaltotherespectiveoxidationlevelsofthiscarbon.Therefore,theoxidationlevelofthereactantis0andthatoftheproductis+2.Inotherreactionsinvolvingmorethanonecarbonatom,No,iscomputedbysummingtheoxidationlevelsofallcarbonatomsthatundergoachemicalchange.Step3.Computethedifference.Nox(product)—Nox(reactant)Ifthisdifferenceispositive,thetransformationisanoxidation.Ifthisdifferenceisnegative,thetransformationisareduction.Ifthedifferenceiszero,neitheranoxidationnorareductionhastakenplace.ForthereactionofEq.17—1thisdifferenceis+2–0=+2.Thistransformationisthusanoxidation.Althoughtheoxidation-numberformalismisveryuseful,weshouldnotlosesightofthefollowingtwogeneralcharacteristicsoforganicoxidationsandreductions.Thesetwopointscanenableustospotanoxidationorreductionataglance.1.Inmostoxidationsoforganiccompounds,eitherhydrogeninaC—HbondorcarboninaC—Cbondisreplacedbyamoreelectronegativeelement,suchashalogenoroxygen.Theconverseistrueforreductions.2.Theoxidationstateofamoleculeisdeterminedfromtheoxidationstatesofitsindividualcarbonatoms.Theoxidationnumberconceptcanbesimplyrelatedtoadefinitionofoxidationthatisoftenusedininorganicchemistry.Accordingtothisdefinition,oxidationisthelossofelectronsandreductionisthegainofelectrons.Toseehowthisdefinitionappliestoorganiccompounds,letusconsiderasanexampletheoxidationofethanoltoaceticacid:Wecanwritethisoxidationasabalancedhalf-reactionusingH2Otobalancemissingoxygens,protonstobalancemissinghydrogens,anddummyelectronstobalancecharges.Accordingtothishalf-reaction,fourelectronsarelostfromtheethanolmoleculev/henaceticacidisformed.(Sincethisisonlyahalf-reaction,acorrespondingnumberofelectronsmustbegainedbythespeciesthatbringsabouttheoxidation.)Itcanbesaidthattheoxidationofethanoltoaceticacidisafour-electronoxidation.Thistypeofterminology,whichisfrequentlyusedinbiochemistry,comesfromthehalf-reactionformalism.Ifwecomputetheoxidationnumbersofethanolandaceticacid,wecanseethatthe.changeinoxidationnumberforEq.17—2is+4(verifythisstatement).Thisexampleillustratesthefollowingpoint:thechangeinoxidationnumberisequaltothenumberofelectronslost.Ifthechangeinoxidationnumberisnegative,thereactionisareduction,andthenumbercorrespondstoelectronsgained.Oxidationsandreductions,likeacid-basereactions,alwaysoccurinpairs.Therefore,wheneversomethingisoxidized.somethingelseisreduced.Whenanorganiccompoundisoxidized,thereagentthatbringsaboutthetransformationiscalledanoxidizingagent.Likewise,whenanorganiccompoundisreduced,thereagentthateffectsthetransformationiscalledareducingagent.Forexample,supposethatchromateion(CrO42-)isusedtobringabouttheoxidationofethanoltoaceticacidinEq.17—2inthisreaction,chromateionisreducedtoCr3+.ThreeelectronsaregainedinthereductionofchromatetoCr3+.Sincefourelectronsarelostintheoxidation(Eq.17—3),stoichiometryrequiresthatforeverythreeethanolmoleculesoxidizedtoaceticacid(twelveelectronslost),fourCrO42-arereduced(twelveelectronsgained).Byconsideringthechangeinoxidationnumberforatransformation,wecantellwhetheranoxidizingorreducingagentisrequiredtobringaboutthereaction.Forexample,thefollowingtransformationisneitheranoxidationnorareduction(verifythisstatement):Althoughonecarbonisoxidized,anotherisreduced.Eventhough