The addition and hydrogen abstraction reactions of a fluorinated radical in aqueous solution

165 Pages·2001·6.3 MB·English

by  Zhang, Li

THEADDITIONANDHYDROGENABSTRACTIONREACTIONSOFA
FLUORINATEDRADICALINAQUEOUSSOLUTION
By
LIZHANG
ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL
OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT
OFTHEREQUIREMENTSFORTHEDEGREEOF
DOCTOROFPHILOSOPHY
UNIVERSITYOFFLORIDA
2001
TomyHeavenlyFather
TomyMom,Dad,Jing,andmychurchfamily
ACKNOWLEDGMENTS
AmongmanypeoplewithwhomIhaveinteractedthroughoutthecourseofmy
graduate study at the University of Florida, I would like to extend my sincere
appreciationtothespecialfewthathavesupported,motivated,andinspiredme.
I would like to express my gratitudeto my research advisor. Dr. WilliamR.
Dolbier,Jr., forhelping metobecome aresearcherandforhiscontinuous guidance
during mytime inthe Ph.D. program. I wouldespecially like to thankhimforhis
motivationandforchallengingmetopursueexcellence.
Forthesupervisionduringmytimeasagraduatestudent,Iwouldliketothank
themembersofmysupervisorycommitteefortheiradviceandencouragement. These
includeDr.MerleBattiste,Dr.KirkSchanze,Dr.RobertKennedy,andDr.Wolfgang
Sigmund.
IwouldliketothankDr.KeithU.Ingoldforhiscollaborationonthisproject,for
his help and support, and for allowing me work in his lab to conduct the LFP
experiments.
SpecialthanksgotothestaffoftheSpectroscopicServicesLaboratoriesinthe
DepartmentofChemistry. Dr.IonGhiviriga,Mr.RobertHarker,Dr.DavidPowell,and
Dr.LidiaMatveevahavebeenveryhelpfulforgettingspectrawithspecialrequirements,
forinterpretatingofspectra,andforsolvinginstrumentalproblems. Itrulyappreciate
theirskill,kindness,andpatience.
iii
ItisalwaysapleasureformetobeintheDolbiergroup. Iwouldliketosay
special thanks to Dr. Conrad Burkholder, Dr. Jian-Xin Duan, Dr. An-Rong Li, Dr.
AlexanderB. Shtarev, Dr. SarniaAit-Mohandand Dr. Yue-LianXu fortheirhelpful
discussion and friendship; Dr. Feng Tian for his friendship and encouragement; Dr.
MichelleFletcherforherhelpduringtheearlystageofmygraduatestudyintheUnited
States;Mr.MarshallBakerandMr.BobSheltonfortheirhelpandpatiencetocheckthe
Englishofmydissertationandreports;Mr.JoeCradlebaughforhiscontributionstothe
projects; and the Dolbier group members, both past and present, for their helpful
discussionandfriendship.
TheGainesvilleChineseChristianChurchhasbeenaconstantsourceofsupport
andencouragementformeandmyacademicendeavors. Itisimpossibletonameallthe
peoplewhohavehelpedme. Iwouldliketoacknowledgeallofthefriendsfortheirlove
andprayers.
Lastanddefinitelynotleast,Ithankmyfamilyfortheendlessloveandsupport
thatmakesmydreambecometrue. Ithankmymotherandfatherfortheirbeliefinme,
especially during some difficult times. I would also thank my sister Jing for her
encouragement.
IV
TABLEOFCONTENTS
ACKNOWLEDGMENTS iii
KEYTOSYMBOLSORABBREVIATIONS vii
ABSTRACT viii
CHAPTERS
ANOVERVIEWOFRADICALREACTIONSANDTHEREACTIVITIESOF
F1LUORINATEDRADICALSINORGANICSOLVENTS 1
GeneralOverviewofAdditionandH-AbstractionReactionsofFreeRadicals 2
ReactivitiesofFluorinatedRadicalsinOrganicSolvents 15
SolventEffectsonFreeRadicalReactions 28
Conclusion 33
2ADDITIONREACTIONSOFPERFLUORO-N-ALKYLRADICALIN
AQUEOUSSOLUTION 35
SynthesisofWater-SolubleSubstrates 36
LaserFlashPhotolysisExperiments 40
ResultsandDiscussion 46
Conclusion 32
3HYDROGENATOMABSTRACTIONREACTIONS
OFPERFLUORO-N-ALKYLRADICALINAQUEOUSSOLUTION 53
AbsoluteRateDetermination-CompetitionbetweenAddition
andH-AbstractionReactions 34
AbsoluteRateConstantsofHydrogenAbstractionReactionsby
Perfluoro-n-alkylRadicalinAqueousSolution. 64
SolventEffectoftheH-abstractionReactionsby RfSOsNaRadical 80
KineticIsotopeEffectsofH-abstractionReactionsby RfSOaNaRadical
inAqueousSolution 81
Conclusion 84
v
4SUMMARY 86
AdditionReactionsofPerfluoro-n-alkylRadicalinAqueousSolution 86
HydrogenAtomAbstractionReactionsofPerfluoro-n-alkyl
RadicalinAqueousSolution 87
5EXPERIMENTAL 90
GeneralProcedures 90
ExperimentsforChapter2 91
ExperimentsforChapter3 104
APPENDIX SELECTED19FNMRSPECTRA 140
REFERENCES 145
BIOGRAPHICALSKETCH 152
vi
KEYTOSYMBOLSANDABBREVIATIONS
AcronymNamesandIUPACNames
AcronymsName IUPACName
1,3-BTB 1,3-Bis(trifluoromethyl)benzene
FI13 1,1,2-Trichloro-trifluoroethane
THF Tetrahydrofuran
AbbreviationsandExplanations
Abbreviation Explanation
R-radical Hydrocarbonalkylradical
Rrradical Perfluoro-n-alkylradical
•RfSOsNaradical •CF2CF20CF2CF2S03NaRadical
kadd Additionreactionrateconstantofaradicaltoanalkene
kH H-Abstractionreactionrateconstantofaradical
kexptl Pseudo1storderrateconstantofadditionreactionmeasuredby
LFPtechnique
kgi Globalrateconstantofaradicalandalkenereaction
kD,THF-d8 AbsoluteD-abstractionrateconstantfromTHF-d8byaradical
kHoverall OverallH-abstractionrateconstantfromsubstratesbyaradical
kHperH H-AbstractionrateconstantbyaradicalfromeachactiveH-
transfersiteinthesubstratemolecule
LFP LaserFlashPhotolysis
vii
AbstractofDissertationPresentedtotheGraduateSchool
oftheUniversityofFloridainPartialFulfillmentofthe
RequirementsfortheDegreeofDoctorofPhilosophy
THEADDITIONANDHYDROGENABSTRACTIONREACTIONSOFA
FLUORINATEDRADICALINAQUEOUSSOLUTION
By
LiZhang
December2001
Chairman: WilliamR.Dolbier,Jr.
MajorDepartment: Chemistry
Thekineticsofadditionandhydrogenabstractionreactionsofafluorinated
radicalareinvestigatedbyacombinationoflaserflashphotolysis(LFP)experiments,
competitionkineticmethodology,andproductanalysisinaqueoussolution.
Absoluterateconstantsofadditionreactionsof•CF2CF20CF2CF2S03Naradical
towater-solublealkenesaremeasuredbytheLFPtechniqueinwateratroom
temperature. Theresultsimplythatthermodynamiceffects,polareffects,andsteric
effectsallplayrolesinsuchadditionreactions. Therateconstantsobtainedinaqueous
solutionarecomparedwiththoseoftheanalogousreactionsobtainedinthenonpolar
organicsolventI,l,2-trichloro-l,2,2-trifluoroethane(FI13). Itisfoundthattheaddition
reactionsofperfluoro-n-alkylradicalstoalkenesaresomewhatfasterinwaterthanin
FI13. Theserateenhancementsaresufficienttodemonstratetheimportanceofsolvent
polareffectsintheadditionofelectrophilicperfluoro-n-alkylradicalstoalkenes.
vm
Byusingcompetitionkineticmethodology,absoluterateconstantsofhydrogen
abstractionreactionsby CF2CF20CF2CF2S03Naradicalfromavarietyofhydrogen
donorsaredeterminedinwater. Thesubstratesarealcohols,alkoxides,ethers,acetone,
carboxylicacidsandsalts,thiol,silanes,andphosphorousacid. Awiderangeof
substratereactivity(63,400fold)isobserved,aceticacidbeingtheslowestone,and
HSCH2CH2S03Nabeingthemostreactiveone.
Theresultsindicatethattheaqueoushydrogenabstractionreactionsofperfluoro-
n-alkylradicalarecontrolledbyacombinationofthermodynamic,transitionstatepolar,
andfieldeffects. Whentherateconstantsarecomparedwiththoseobtainedinthe
nonpolarorganicsolventl,3-bis(trifluoromethyl)benzene(1,3-BTB),asignificantsolvent
polareffectisrevealed. Theperfluoro-n-alkylradicalisobservedtoreactwithhydrogen
donorsfasterinwaterthanin1,3-BTB.
KineticisotopeeffectsareinvestigatedfortheH-abstractionreactionbythe
•CF2CF20CF2CF2S03Naradicalinaqueoussolution. Arelationshipbetweenthe
substratereactivityandKIEisnoticed:themorereactivethesubstrate,thesmallerthe
KIEvalue. LargeKIE’s(11-22)areobtainedforsomeunreactivecompounds,with
tunnelingeffectthoughttobeinvolved.
IX
CHAPTER
1
ANOVERVIEWOFRADICALREACTIONSANDTHEREACTIVITIESOF
FLUORINATEDRADICALSINORGANICSOLVENTS
Asoneofthereactiveintermediatesinorganicreactions,freeradicalshavebeen
extensivelyusedinorganicsynthesis.1'7Atthesametime,radicalpolymerizationhas
beenwidelyadoptedforbothindustrialandlaboratoryscalepolymersyntheses.8'10 Free
radicalreactionsoftenaretolerantoffunctionalgroups,highlyselective,andproceed
undermildconditions.11 Theycanalsogiveproductsthatarenotaccessiblebymeansof
themoreconventionalpolarroutes.
Thereareseveralelementaryreactionsoffreeradicals(Scheme1-1):12
1. Atomabstraction: R- + X-Y - R-X + Y.
2. Recombination: R. + R'. R-R'
3. Disproportionation: R-CHrCHr -RCH2CH3 + R-CH=CH
x-
4. Addition: R. +
5. ElectronTransfer: R.+ M R+ + M(n’1)+
Scheme1-1ElementaryReactionsofFreeRadicals
Most free radical reactions proceed through chain processes, where different
elementaryreactionscanoccurineachstep,andfreeradicalprocessesoftenhaveseveral
competingpathways. Itisimportanttobeabletoquantifythereactivitiesofradicals
toward different reagents under various reaction conditions, so that one can predict
expectedoutcomesbycarefullydesigningthereaction. Indeed,thekineticsofalkyl
1