klein2e lecture ch03 - University of...

Organic ChemistrySecond Edition

Chapter 3Acids and Bases

David Klein

3.1ConjugateAcidsandBases• Brønsted-Lowrydefinition

– Aconjugateacidresultswhenabaseacceptsaproton– Aconjugatebaseresultswhenanacidgivesupaproton

• RecallfromGeneralChemistrythisclassicexample

• Labeltheacid,base,andtheconjugatesinthereactionbelow

3.1AcidsandBases• Drawreasonableproductsforthefollowingacid/basereaction,andlabeltheconjugates.

• Waterisconsideredneutral.Doesthatmeanitisneitheranacidnorabase?

3.2CurvedArrowsinReactions• Consideraspecificacid/baseexample

• Youcouldsaythebase“attacks”theacid• Theacidcannotloseitsprotonwithoutthebasetakingit.Allacid/basereactionsoccurinonestep

• Themechanismshowstwoarrowsindicatingthattwopairsofelectronsmovesimultaneously

3.2CurvedArrowsinReactions• Provideproductsandcurvedarrowsforthefollowingacidbasereaction

3.2CurvedArrowsinReactions• Identifytheacid/basestepsinthefollowingmechanism

• PracticewithSkillBuilder 3.1

3.3QuantifyingAcidity• Quantitativestrengthanalysis– usingnumericaldatatocomparehowstrongacidsare.

• ReviewfromGeneralChemistryKa andpKa• Ka istheequilibriumconstantforthereactionbetweenanacidandWATER

• Iftheacidisstrong,howwillthataffectthemagnitudeofKa,theequilibriumconstant?

3.3QuantifyingAcidity• TherearemoreacidsandpKa valuesintable3.1

• EachpKa unitrepresentsanorderofmagnitudeorapowerof10.

• Whichisstronger,HClorH2SO4,andbyexactlyHOWMUCH?

Klein, Organic Chemistry 2e 3-8

3.3QuantifyingBasicity• YoucanalsousepKa valuestocomparethestrengthsofbases

• Thestrongertheacidtheweakeritsconjugatebase.WHY?

3.3UsingpKas toanalyzeEquilibria• WiththerelevantpKa values,youcanpredictwhichdirectionanacid/baseequilibriumwillfavor

• Whyistheequilibriumarrowbiggerontopthanonbottom?

3.3UsingpKas toanalyzeEquilibria

• SubtractingthepKa values,(50- 15.7≈34)alsotellsyouthattherewillbe≈1034moreproductsthanreactants.

• It’snotreallymuchofanequilibrium• PracticewithSkillBuilder 3.4andconceptualcheckpoint3.12

Figure 16.3 Strengths of conjugate acid-base pairs.

The stronger the acid is, the weaker its conjugate base. When an acid reacts with a base that is farther down the list, the reaction proceeds to the right (Kc > 1).

3.4QualifyingAcidity• Qualitativeanalysis– comparestructuralstabilitytodeterminewhichisastrongeracid

• Formalchargecanaffectstability– Themoreeffectivelyareactionproductcanstabilizeitsformalcharge,themoretheequilibriumwillfavorthatproduct

• Whichisastrongeracid,andWHY?

HCl or

3.4QualifyingAcidity• Themoreeffectivelyaconjugatebasecanstabilizeitsnegativecharge,thestrongertheacid

• Whatfactorsaffectthestabilityofanegativeformalcharge?– Thetypeofatom thatcarriesthecharge– Resonance– Induction– Thetypeoforbital wherethechargeresides

• Thesefactorscanberememberedwiththeacronym,ARIO

• ARIO - Thetypeofatom thatcarriesthecharge– Moreelectronegativeatomsarebetteratstabilizingnegativecharge.WHY?

– Comparetheacidityofthetwocompoundsbelow

– LookupthepKa valuesforsimilarcompoundsintable3.1toverifyyourprediction

3.4QualifyingAcidity

• ARIO - Resonance cangreatlystabilizeaformalnegativechargebyspreadingitoutintopartialcharges

• Comparetheacidityofthetwocompoundsbelowbycomparingthestabilitiesoftheirconjugatebases.Howdoesresonanceplayarole?

• LookupthepKa valuesintable3.1toverifyyourprediction

3.4QualifyingAcidity• ARIO - Induction canalsostabilizeaformalnegativechargebyspreadingitout.Howisinductiondifferentfromresonance?

• Comparetheacidityofthetwocompoundsbelowbycomparingthestabilitiesoftheirconjugatebases.Howdoesinductionplayarole?

• Doesinduction alsoplayaroleinexplainingwhyaceticacidisstrongerthanethanol?

3.4QualifyingAcidity• ExplainthepKa differencesbelow

3.4QualifyingAcidity• ARIO - Thetypeoforbital alsocanaffectthestabilityofaformalnegativecharge

• Isanegativechargemorestableorlessstableifitisheldcloselytoanatom’snucleus?WHY?

• Ranktheabilityoftheseorbitals(2s,2p,sp3,sp2,andsp)tostabilizeelectrons,andexplain.

3.4QualifyingAcidity• Comparetheacidityofthecompoundsbelowbycomparingthestabilitiesoftheirconjugatebases.Howdoesthetypeoforbital playarole?

• ExplainthepKa differencesabove

3.4QualifyingAcidity• Whenassessingtheacidityofprotons,wegenerally useARIO asourorderofpriority1. Thetypeofatom thatcarriesthecharge2. Resonance3. Induction4. Thetypeoforbital wherethechargeresides

• Compareethanolandpropylene.Whichhasamorestableconjugatebase?WHY?

pKa =16 pKa =43

3.4QualifyingAcidity• ARIOisagoodgeneral guideline,butitsometimesfails• Compareacetyleneandammonia.UsingARIO,whichshouldbeastrongeracid?

• Todeterminetheactual acidities,thepKa valuesmustbeexperimentallymeasuredandcompared– seenextslide

3.4QualifyingAcidity• ARIO isagoodgeneral guideline,butitsometimesfails• UsingthepKa values,youcannevergowrong.Whichacidistrulystronger?Whichdirectionwillthefollowingequilibriumfavor?

3.4QualifyingAcidity• Foracidsthatcarryaformalpositivecharge,wecanuseARIOtocomparethestabilityoftheacidsdirectlytoseewhichisbestatstabilizingitspositivecharge

• Rankthefollowingacidsinorderofincreasingstrength

3.5PredictingEquilibriumPosition• IfpKa valuesareknown,theyareasure-fire waytopredictthepositionofanequilibrium

• IfpKa valuesarenotknown,relativestabilityofconjugatesshouldbeused

3.5PredictingEquilibriumPosition• IfpKa valuesarenotknow,relativestabilityofconjugatesshouldbeused

• Isthereactionbelow,reactantorproductfavored?

3.5ChoosingaReagent• Anotherimportantskillistobeabletochooseanappropriatereagent foraacid/basereaction

• Chooseanacid fromtable3.1thatcouldeffectivelyprotonate eachofthefollowingmolecules

Table 3.1

3.5ChoosingaReagent• Anotherimportantskillistobeabletochooseanappropriatereagent foraacid/basereaction

• Chooseaconjugate base fromtable3.1thatcouldeffectivelydeprotonate eachofthefollowingmolecules

3.7Solvation• Becausetheyaresosimilar,ARIOcannotbeusedtoexplainthepKa differencecomparingethanolandtert-Butanol

• ConsideringthatpKa valuesaremeasuredinsolution,howmightthesolventacttomakeethanolaslightlystrongeracid?Thinkabouthowthesolventcouldstabilizeitsconjugatebase.

3.7Solvation• Thesolventmustformion-dipoleattractionstostabilizetheformalnegativecharge

• Ifthetert-Butoxideisstericallyhindered,itwon’tbeaswellsolvatedastheethoxide

3.7Solvation• Solvation iscriticallyimportantinreactions.Thesolventisoftenneededtostabilizetransitionstates,intermediates,and/orproductstoallowareactiontooccur

• ExplainwhythepKa foraceticacidis4.75inwaterwhileitis23.5inCH3CN

• Practicewithconceptualcheckpoint3.33

3.9LewisAcidsandBases• Lewisacid/basedefinition

– ALewisacidacceptsandsharesapairofelectrons– ALewisbasedonatesandsharesapairofelectrons

• AcidsundertheBrønsted-LowrydefinitionarealsoacidsundertheLewisdefinition

• BasesundertheBrønsted-LowrydefinitionarealsobasesundertheLewisdefinition

• Explainhowthisreactionfitsbothdefinitions

3.9LewisAcidsandBases• Lewisacid/basedefinition

– ALewisacidacceptsandsharesapairofelectrons– ALewisbasedonatesandsharesapairofelectrons

• SomeLewisacid/basereactionscannotbeclassifiedusingtheBrønsted-Lowrydefinition

• ExplainhowthisreactionfitstheLewisdefinitionbutnottheBrønsted-Lowrydefinition

AdditionalPracticeProblems• Provideproductsandcurvedarrowsforthefollowingacidbasereaction

AdditionalPracticeProblems• RankthefollowingbasesinorderofincreasingstrengthandranktheirconjugatesinorderofincreasingpKa.

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