Hydrogen Atoms and Hydroxyl Radicals

CriticalReviewofRateConstantsforReactionsofHydratedElectrons,

..

HydrogenAtomsandHydroxylRadicals(OH/O )

inAqueousSolution

GeorgeV.Buxton

UniversityofLeeds,

CookridgeRadiationResearchCentre,

Leeds,LS166QB,England

and

CliveL.Greenstock

MedicalBiophysicsBranch,

WhiteshellNuclearResearchEstablishment,

AtomicEnergyofCanadaLimitedResearchCompany,

Pinawa,Manitoba,Canada,ROE1LO

and

W.PhillipHelmanandAlbertaB.Ross

RadiationChemistryDataCenter,

RadiationLaboratory,UniversityofNotreDame,NotreDame,IN46556

...

KineticdatafortheradicalsHandOHinaqueoussolution,andthecorrespondingradicalanions,O andeaq ,havebeencriticallyreviewed.Reactionsoftheradicalsinaqueoussolutionhavebeenstudiedbypulseradiolysis, ashphotolysisandothermethods.Rateconstantsforover3,500reactionsaretabulated,includingreactionswithmolecules,ionsandotherradicalsderivedfrominorganicandorganicsolutes.

Keywords:Aqueoussolution;chemicalkinetics;criticalreview;datacompilation;hydratedelectron;hydrogenatom;hydroxylradical;rateconstants.

1.GeneralIntroduction

Thedevelopmentofaqueousradiationchemistry,1andespeciallythepulseradiolysistechnique,2hasprovidedchemistswithaverysimpleandcleanmethodofgeneratingandstudyingthereactionsofunstablespeciesinaqueoussolutionsuchasorganicandinorganicfreeradicalsandmetalionsinunusualoxidationstates.Thisreviewcomprisesacriticallyevaluatedcompilationoftherateconstantswhichhavebeenmeasuredforthereactionswithsolutesofthehydratedelectron,eaq ,the

...

hydrogenatom,H,thehydroxylradical,OH,anditsbasicform,theoxideradicalion,O .Aknowledgeofthesedataenablesthechemisttochoosetheappropriateconditionsforgeneratingthespeciesofintereststartingwiththesefreeradicals,andthisknowledgehasmanyapplicationsinthewhole eldofchemistryinaqueoussolution.

Manyreviews,datacompilations,andmonographshavebeenpublishedonthereactivityoftransientsfromwater.3-10

Thiscompilationcontainsdatapublishedthrough1986andisacompletereevaluationandupdateoftheearlierNSRDS-NBSreports8-10.

...

1.1.Propertiesofeaq ,H,OHandO

ThepropertiesofthesespeciesarelistedinTable1andtheiropticalabsorptionspectraareshowninFig.1.

1.1.1.TheHydratedElectron

Thehydratedelectronmaybevisualizedasanexcesselectronsurroundedbyasmallnumberoforientedwatermoleculesandbehavinginsomewayslikeasinglychargedanionofaboutthesamesizeastheiodideion.Itsintenseabsorp-tionbandinthevisibleregionofthespectrummakesitasimplemattertomeasureitsreactionrateconstantsusingpulseradiolysiscombinedwithkineticspectrophotometry.

Asexpectedfromitsstandardreductionpotential22,29of 2.9V,thehydratedelectronreactsrapidlywithmanyspecieshavingmorepositivereductionpotentials.Itsmodeofreactioncanbegenerallyrepresentedastheone-electrontransferpro-cess

nn 1

e aq + S ¡ S

(1)

wherenisthepositivechargeonthesolute,althoughinsomecasestheelectronadductimmediatelydissociates.Itsratecon-stantsrangefrom～101Lmol 1s 1uptothediffusioncontrolledlimitbuttheactivationenergiesareinvariablysmallandlie

intherange6-30kJmol 1.Thissuggeststhatthedominantkineticparameteristheavailabilityofavacantorbitalofthesoluteintowhichtheelectroncantransfer.

Thehydratedelectronactsasanucleophileinitsreactionswithorganicmolecules,anditsreactivityisgreatlyenhancedbyelectron-withdrawingsubstituentsadjacenttoalkenedoublebondsorattachedtoaromaticrings.Increasedreac-tivityisalsoobservedwhenorganicmoleculescontainsubstituenthalogenatoms,inwhichcasethenegativeionformedinEq.(2)rapidlyeliminatesthehalideion.

.

e aq + RX ¡ RX ¡ R + X

1.1.2.TheHydrogenAtom

(2)

Thehydrogenatomistheconjugateacidofeaq ,anditisthemajorreducingspeciesinacidicsolution,Eq.(3).

+

e aq + H3O ¡ H

(3)

.

ItabsorbsonlyweaklyintheultravioletsothatitsreactionsarenotreadilymeasuredbyobservingHitself.Withareductionpotential22,29of 2.3Vthehydrogenatomisaslightlylesspowerfulreducingagentthaneaq ,butitschemistryisoftenquitedifferent.Itreadilyreducesinorganicionshavingmorepositivereductionpotentialsthanitself,butoftenatslowerratesthaneaq .Insomecases,inhighlyacidicsolution,iteffectivelyreactsasanoxidant,formingahydrideintermediatewhichdecom-posestomolecularhydrogenandtheoxidizedsolute40,41,forexample,

H+.2+3+ ___¡Fe3+ + H2H + Fe ¡ FeH

H+I . _¡HI2 ___. _¡H2 + I H + I ¡ HI 22

(4)(5)

ThehydrogenatomreactswithorganiccompoundsbyabstractingHfromsaturatedmoleculesandbyaddingtocenters

ofunsaturation,forexample,

..

H + CH3OH ¡ CH2OH + H2..

H + CH2=CH2 ¡ CH2CH3.

(6)(7)

Inthisrespectitresemblesthehydroxylradical,althoughthelatterismorereactiveandlessselectiveinabstractionreactions.

.

AgoodillustrationofthedifferentchemistryofHandeaq isprovidedbytheirreactionswithchloroaceticacid,asshownbyEqs.(8)and(9),

..

H + ClCH2CO2H ¡ CHClCO2H + H2

.

e aq + ClCH2CO2H ¡ CH2CO2H + Cl.

(8)(9)

1.1.3.TheHydroxylRadical

Thehydroxylradicalisapowerfuloxidant,havingastandardreductionpotential31-33of2.7Vinacidicsolutionand1.8

.

VinneutralsolutionwherethefreeenergyofneutralizationofOH byH+isnotavailable.ThereactionofOHwithionsisoftenrepresentedasasimpleelectrontransfer,

.

OH + Sn ¡ Sn+1 + OH

(10)

wherenisthechargeontheion,butsuchasimplestepisunlikelybecauseofthelargesolventreorganizationenergyinvolvedinformingthehydratedhydroxideion.Instead,itissuggested42thatanintermediateadductisformed.Suchanadductisobservedintheoxidationofhalideandpseudo-halideions,

.

OH + X ¡ HOX .

(11)

.

AlthoughthereareseveralexamplesofOHreactingwithinorganicionsatthediffusioncontrolledrate,rateconstantsforoxidationofmanyaquatedmetalcationsseemtobenomorethan～3×108Lmol 1s 1.Asuggestedreason43forthisis

.

t …… 此处隐藏：43155字，全部文档内容请下载后查看。喜欢就下载吧 ……