TheHydrolysisofTertiary-ButylChlorideAChemicalKineticStudy

Thefieldofchemicalkineticsisconcernedwiththerateorspeedatwhichachemical

reactionoccurs.

Knowledgeofachemicalreactionandtheintegratedratelaw(anexpressionrelating

concentrationwithtime)forthereactionallowsustoaccuratelypredicthowmuchofagivenreactantorproductexistsatanytimeduringthereaction.Theratelawcanalsoprovideusefulinsightintowhichvariablescontrolareaction(temperature,reactantconcentration,catalysts)andhowthesevariablescanbeusedtomaximizetheamountofproduct(s)formedinthe

reactionorminimizethetimeinvolvedtoobtainproduct(s).Theexperimentalkineticdata

obtainedduringthereactionisusedtohelpunderstandthemechanismofareaction.The

reactionmechanismdescribeshowareactionproceedsthroughsingle(elementary)stepsonthemolecularlevel.

Althoughasignificantamountofdatahasbeencollectedinthefieldofkinetics,chemicalratelawsandreactionorderscannotbepredictedfromthestoichiometryofanychemicalreactionThus,toobtainreachconclusionsaboutaprocess,onlyexperimentationwillprovideuswithreactionkinetics.

Severalfactorsinfluencetherateofachemicalreaction.Inparticular,thefollowingshouldbeexamined:

·Theinherentreactivityoftheparticipatingmolecules,

·Theinitialconcentrationofallreactantsandtheireffects.

·Changesintheratecausedbyfluctuationsinreactiontemperature,

·Theeffectofcatalystsinthereaction

TheTheoryBehindtheExperiment

Oneoftheprimarygoalsinthisexperimentistomeasuretherateconstantkforthisreaction,performedatthreedifferenttemperatures(temperatureisheldconstantduringeachreaction).Oncetherateconstantsarefound,determiningthehalf-lifeateachtemperatureandthe

activationenergy(Ea)shouldbestraightforward.

A.TheReactionMechanism

Inthisexperimentwewillinvestigatetheeffectsofconcentrationandtemperatureonthe

hydrolysisoftertiarybutylchlorideort-butylchloride(2-chloro-2-methylpropane).Noticefromthereactionsequencebelow,waterreplaceschlorineonthemoleculetomakeanalcohol.Theterm"hydrolysis"referstothisprocess,asawatermoleculeisbrokenaparttoformthealcoholproduct

Thereactionoft-butylchloridewithwaterproceedsasshowninthetwo-stepreactionbelow:

steygug

CHS

CH?C

CH;

f-BuCl

k

CH

CHsCOCH

+Cie

Combiningsteps(1)and(2),wefindtheoverallreactionshownbelow.Noticethatt-butyl

alcoholandhydrochloricacidarethetwoproducts.

Thereactionrateforthisprocesscanbeexpressedaseitherthedisappearanceoft-BuClortheappearanceoft-BuOH(seeequationbelow).Referringbacktotheoverallnetequation,HClisalsoaproduct.Becauseofourfamiliaritywithmeasuringacidstrength(pH)inthelab,

monitoringtheappearanceofHClduringthereactionwouldbethemostconvenientmethod

tostudykineticchangesduringthereaction.MeasuringthepHofthereactioncanbedoneeasilywithapHmeterresidingintheflaskduringreaction(recallthatpH=-log[H']).

B.TheRateLawfort-BuOH

Becausestep(2)inthereactionmechanismisfasterthanstep(1),step(1)determinestherateofreaction(step(1)istheRDSorratedeterminingstep).Step(1)isunimolecular,

meaningtherateforthatreactiondependsonlyupontheconcentrationofthet-BuCl.Anyunimolecularreactionisalsoafirst-orderreaction.Thus,theratelawforafirst-order

reactionis:(noticethelastexpressionisaformoffirstorderintegratedratelaw)

rate=k[t-Bucl]!)

ds0

sotbenaegettbedifierentialeguation

[t-BuCl]=[t-BuCl]。e-kt

concentratlonconcentraton

afanptimetatlimet=uzteconst

Astheoverallreactionindicates,foreverymoleoft-BuClconsumedinthereaction,onemoleofHClisproduced(i.e.1moleofH+and1moleCl).Therefore,thenumberofmolesofHClproducedduringthecourseofthereactionisameasureofthenumberofmolesoft-BuCl

reacting.Understandthatnoteverymoleculeoft-BuClreactsinthisprocess,somonitoring

[HCl]iscrucialtodetermining[t-BuCl].Usingtheintegratedratelawforthefirstorderprocess,wecaneasilyfindtheinitialconcentrationoft-BuClinthereaction.Experimentally,thingsarenotquitethissimple.

C.AdjustingforWater-Finding[t-BuCl].

Waterisusedasareactantandpartofthesolventsysteminthisreaction(itsreactionorderiszero,meaning[H?O]hasnoeffectontherateandisessentiallyconstant).Astimeapproachesinfinity,themolesoft-BuClapproacheszeroandthemolesofHClproducedapproachesa

maximum.Bytitratingtheresultingsolutionafter48hours,agoodapproximationoftheinitialamountoft-butylchloridecanbemade.Experimentallytimestartswhenthet-BuClisaddedtothesolventmixture.Atanytime,thenumberofmolesofHClproducedinthereactionequaltothetotalnumberofmolesoft-BuClconsumedinthereaction.

NHaproduced=N-BuGlreacted

ReactionMechanism

Thehydrolysisinvolves3steps:

1.theionizationofthet-butylchloridetoformacarbocationintermediate;

2.theformationofahighenergytransitionstatebetweenthecarbocationandwater;

3.theformationofproduct,t-butylalcoholbyahydrogenionleavingthetransitionstatecomplex.

t-butylalcohol

carbocation

transitionstate

t-butylchloride

VirtualModels

Thereactantsandintermediatesrelatedtothekineticsexperiment,showingthevariousgeometriesofthemoleculesinvolvedthehydrolysisoft-butylchloride,(C=gray,

H=white,Cl=green,O=red),Moleculescanbeviewedon-lineatthefollowinglink.

/doliver/chem12B/lab/kineticsmolecules.htm

Toseeotherformatssuchasspace-fillingmodels,rightclickyourmousebuttonwhileonthepicture.ChooseanotherviewundertheDisplaysubmenu.

Half-Life

Ausefulparameterofrateequations,particularlyfirst-orderreactions,isthehalf-lifeort?r2.Thehalf-lifeissimplythetimeittakesforthereactantconcentrationinareactiontoreduceby50%.

Forexample,ifwebeginareactionwitha0.20Msolution,theninonehalf-lifethereactionwillreduceto0.10Moveraspecifictimeframe,tiz.Inasecondhalf-life,thereactionreduces

another50%to0.05M.

Half-lifeforareactionprocessisdependentonthereactionorder.Firstthefirstorderprocess,thetizisindependentonreactantconcentrationbutdependentontherateconstantk:

Note:kistemperature-dependent,andsothehalf-lifeforareactionwillchangewiththetemperature

Youshouldobtain3half-livesforthiskineticstudy.

ActivationEnergy-TheArrheniusEquation

Therateofachemicalreactionvariesdirectlywithtemperature.Thisisunderstandableintermsofthefrequencyandproductivityofcollisionsbetweenthespeciesinareaction.Thehigherthekineticenergy(directlyrelatedtoT),thegreaterthenumberofcollisions.SvanteArrheniusproposedthefollowingmathematicalrelationshipforthisphenomenon:

·k=therateconstantofthereaction

·A=thefrequencyfactor(unitless)

·Ea=activationenergy(usuallyinkJ/mol)

Arrhenius

Equation

·R=idealgasconstant(usually8.314x103kJ/mol·K)

·T=temperatureinkelvins

Takingthenaturallogarithmofbothsidesoftheequationgivesyou

y=b+mx

ThebottomequationallowsyoutocomparethedatafromreactionsrunatdifferenttemperaturesinordertodeterminevaluesforAandEa.Yes,youshouldbethinking

graphically.

Procedure

Setupthereactionvesselasshownatright.To

Thermometer

prepareatemperature-controlledreactionflask,

125.ml

ErlenmeyerFlask

Clamp

placeasmallErlenmeyerFlask(reactionflask)

intoalargebeaker(temperaturebath)asshownin

thefigure.Accuratelymeasure50mLofthe50%

water/50%isopropylalcoholv/vmixtureandplace

intotheflask.

N

Youwillperformonerunat10℃andonerunat

40°℃.Itisimperativethatthetemperatureremains

constantduringtherun,sincekisextremely

SmallSirBar

temperaturedependent(seetheArrhenius

equation).

Large

StirBar

Add5dropsofphenolphthaleintotheflask.

membertitratesthereactionmixtureandnotestimesofeachcolorchange.

Allowthetemperaturebath(beaker)andreactionflask(Erlenmeyer)tocometothermal

equilibrium(about10minutes)Temperaturecontrolisessentialforthisexperiment.To

ensureaconstanttemperatureduringthereaction,thesolutionlevelinthereactionflask

shouldbewellBELOWthewaterlevelinthetemperaturebath(beaker).Youwillneedtoadd

iceorhotwaterinsmallquantitiestocontrolthebathtemperaturewithin±1°C.Useamagnetic

memberofthepairtomaintaintheconstanttemperaturebathandrecordtimes,whiletheother

stirrerwithstirbarsintheouterbeakerandinthereactionflask.Itisoftenusefultoone

Whilewaitingforthermalequilibrium,cleanaburetteandfillwith.100MNaOH.Add1.00mloftheNaOHtotheflaskcontainingthemixtureofisopropylalcoholandwater.Prepareadata

tableinyounotebookwithtwocolumns;mlBaseAddedandclocktimeforcolordisappearance.

Ifyoudon'tknowhowtouseapipettebulb,PLEASEASKbeforeyoumessup!!!Place

1.00mloft-BuClintotheflaskandrecordthetime.Thismarkst=0.Recordtheclocktime

requiredforthepinkcolortodisappear.Assoonasthepinkcolordisappears,addanother

1.00mlofbase,andagain,notethetimerequiredforthepinkcolortodisappear.Continuethe

additionofbaseandtiminguntilthecolorremainspink.Stopperandlabelthebottleas10℃.

Setasideforthenextlabperiod.

Repeattheprocessfor40°C.HaveaBunsenburner,hotwaterandsomeiceavailabletomaintainthetemperatureat40±1°C.Whenfinished,stopperandlabelthebottleas40℃.Setasideforthenextlabperiod.(Attheinstructor'soptionathirdrunof25℃willberun.)

Duringthenextlabperiod,titratetoaphenolphthaleinendpointwithwhateveris

required(acidorbase)toverifytheinitialmolesoft-butylchloride,anddolabcalculations.

Importantthingstonote:

1.RecordyourCLOCKtimestothenearestsecond.Youwillhavetimetoconverttosecondsafterthedatacollectionperiod

2.Positionthethermometerinthewaterbathsothatthemagneticstirbardoesnotstrikeitwhilespinning.

3.Makesurethatyouarereadytotakedatabeforeaddingt-BuCl.Wheneverythingisready,useapipettoadd1.00mLofthet-BuCl.Notethatthetotalvolumeofsolutionwillbe51.0mL.

4.Assoonasthefirstdropsoft-BuClentersthereactionvessel,thereactionhasbeen

initiated.Thisistimet=0andyoushouldimmediatelybegincollectingdata.Ifyoumissacolorchange,QUICKLYaddanadditional1.00mlofbaseandmakenoteofthevolumechange.

DataProcessingandCalculations

Thevaluestobedeterminedfromthisexperimentare:

·therateconstants,k,foreachtemperatureperformed

·thecorrespondinghalf-life,trz,foreachtemperature

·theArrhenius-derivedAandEaforthereaction.

Youwillbeproducingseveralplots([t-BuCl]vs.timeandIn[t-BuCl]vs.timeforboth

reactiontemperatures+Arrheniusplot).Youmaywishtooverlaytheconcentrationplotsononegraphtosavespace.Italsoaffordsadirectcomparisonbetweenthegraphs.

YouwilluseExceltoprocessandplotthedata.Yourspreadsheetcanbesetupsomethinglikethis.NOTE:Theinitialmolesoft-butylchloridemustbecalculatedbasedonthetotal

molesofbaseaddedatt=infinityoracoupleofdays,whichevercomesfirst.Thisis

calculatedusingthevolumeofbaseaddedduringthetimingportionoftheexperimentplusthevolumeofbaseusedtotitratetotheendpointmultipliedbythemolarityofthebase.

ClockTimein

timetoseconds

colortocolormL

changechangeMBasebase

7:55:2000.1

0

7:58:261260.1

1

8:02:01etc