Organic ChemistrySecond Edition
Chapter 3Acids and Bases
David Klein
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3.1ConjugateAcidsandBases• Brønsted-Lowrydefinition
– Aconjugateacidresultswhenabaseacceptsaproton– Aconjugatebaseresultswhenanacidgivesupaproton
• RecallfromGeneralChemistrythisclassicexample
• Labeltheacid,base,andtheconjugatesinthereactionbelow
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3.1AcidsandBases• Drawreasonableproductsforthefollowingacid/basereaction,andlabeltheconjugates.
• Waterisconsideredneutral.Doesthatmeanitisneitheranacidnorabase?
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3.2CurvedArrowsinReactions• Consideraspecificacid/baseexample
• Youcouldsaythebase“attacks”theacid• Theacidcannotloseitsprotonwithoutthebasetakingit.Allacid/basereactionsoccurinonestep
• Themechanismshowstwoarrowsindicatingthattwopairsofelectronsmovesimultaneously
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3.2CurvedArrowsinReactions• Provideproductsandcurvedarrowsforthefollowingacidbasereaction
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3.2CurvedArrowsinReactions• Identifytheacid/basestepsinthefollowingmechanism
• PracticewithSkillBuilder 3.1
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3.3QuantifyingAcidity• Quantitativestrengthanalysis– usingnumericaldatatocomparehowstrongacidsare.
• ReviewfromGeneralChemistryKa andpKa• Ka istheequilibriumconstantforthereactionbetweenanacidandWATER
• Iftheacidisstrong,howwillthataffectthemagnitudeofKa,theequilibriumconstant?
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3.3QuantifyingAcidity• TherearemoreacidsandpKa valuesintable3.1
• EachpKa unitrepresentsanorderofmagnitudeorapowerof10.
• Whichisstronger,HClorH2SO4,andbyexactlyHOWMUCH?
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• PracticewithSkillBuilder 3.2
Klein, Organic Chemistry 2e 3-8
3.3QuantifyingBasicity• YoucanalsousepKa valuestocomparethestrengthsofbases
• Thestrongertheacidtheweakeritsconjugatebase.WHY?
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• PracticewithSkillBuilder 3.3
Klein, Organic Chemistry 2e 3-9
3.3UsingpKas toanalyzeEquilibria• WiththerelevantpKa values,youcanpredictwhichdirectionanacid/baseequilibriumwillfavor
• Whyistheequilibriumarrowbiggerontopthanonbottom?
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3.3UsingpKas toanalyzeEquilibria
• SubtractingthepKa values,(50- 15.7≈34)alsotellsyouthattherewillbe≈1034moreproductsthanreactants.
• It’snotreallymuchofanequilibrium• PracticewithSkillBuilder 3.4andconceptualcheckpoint3.12
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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).
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3.4QualifyingAcidity• Qualitativeanalysis– comparestructuralstabilitytodeterminewhichisastrongeracid
• Formalchargecanaffectstability– Themoreeffectivelyareactionproductcanstabilizeitsformalcharge,themoretheequilibriumwillfavorthatproduct
• Whichisastrongeracid,andWHY?
HCl or
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3.4QualifyingAcidity• Themoreeffectivelyaconjugatebasecanstabilizeitsnegativecharge,thestrongertheacid
• Whatfactorsaffectthestabilityofanegativeformalcharge?– Thetypeofatom thatcarriesthecharge– Resonance– Induction– Thetypeoforbital wherethechargeresides
• Thesefactorscanberememberedwiththeacronym,ARIO
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• ARIO - Thetypeofatom thatcarriesthecharge– Moreelectronegativeatomsarebetteratstabilizingnegativecharge.WHY?
– Comparetheacidityofthetwocompoundsbelow
– LookupthepKa valuesforsimilarcompoundsintable3.1toverifyyourprediction
• PracticewithSkillBuilder 3.5
3.4QualifyingAcidity
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• ARIO - Resonance cangreatlystabilizeaformalnegativechargebyspreadingitoutintopartialcharges
• Comparetheacidityofthetwocompoundsbelowbycomparingthestabilitiesoftheirconjugatebases.Howdoesresonanceplayarole?
• LookupthepKa valuesintable3.1toverifyyourprediction
• PracticewithSkillBuilder 3.6
3.4QualifyingAcidity
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3.4QualifyingAcidity• ARIO - Induction canalsostabilizeaformalnegativechargebyspreadingitout.Howisinductiondifferentfromresonance?
• Comparetheacidityofthetwocompoundsbelowbycomparingthestabilitiesoftheirconjugatebases.Howdoesinductionplayarole?
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• Doesinduction alsoplayaroleinexplainingwhyaceticacidisstrongerthanethanol?
3.4QualifyingAcidity
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3.4QualifyingAcidity• ExplainthepKa differencesbelow
• PracticewithSkillBuilder 3.7
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3.4QualifyingAcidity• ARIO - Thetypeoforbital alsocanaffectthestabilityofaformalnegativecharge
• Isanegativechargemorestableorlessstableifitisheldcloselytoanatom’snucleus?WHY?
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• Ranktheabilityoftheseorbitals(2s,2p,sp3,sp2,andsp)tostabilizeelectrons,andexplain.
Klein, Organic Chemistry 2e 3-20
3.4QualifyingAcidity• Comparetheacidityofthecompoundsbelowbycomparingthestabilitiesoftheirconjugatebases.Howdoesthetypeoforbital playarole?
• ExplainthepKa differencesabove
• PracticewithSkillBuilder 3.8
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3.4QualifyingAcidity• Whenassessingtheacidityofprotons,wegenerally useARIO asourorderofpriority1. Thetypeofatom thatcarriesthecharge2. Resonance3. Induction4. Thetypeoforbital wherethechargeresides
• Compareethanolandpropylene.Whichhasamorestableconjugatebase?WHY?
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pKa =16 pKa =43
3-22
3.4QualifyingAcidity• ARIOisagoodgeneral guideline,butitsometimesfails• Compareacetyleneandammonia.UsingARIO,whichshouldbeastrongeracid?
• Todeterminetheactual acidities,thepKa valuesmustbeexperimentallymeasuredandcompared– seenextslide
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3.4QualifyingAcidity• ARIO isagoodgeneral guideline,butitsometimesfails• UsingthepKa values,youcannevergowrong.Whichacidistrulystronger?Whichdirectionwillthefollowingequilibriumfavor?
• PracticewithSkillBuilder 3.9
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3.4QualifyingAcidity• Foracidsthatcarryaformalpositivecharge,wecanuseARIOtocomparethestabilityoftheacidsdirectlytoseewhichisbestatstabilizingitspositivecharge
• Rankthefollowingacidsinorderofincreasingstrength
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3.5PredictingEquilibriumPosition• IfpKa valuesareknown,theyareasure-fire waytopredictthepositionofanequilibrium
• IfpKa valuesarenotknown,relativestabilityofconjugatesshouldbeused
• PracticewithSkillBuilder 3.10
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3.5PredictingEquilibriumPosition• IfpKa valuesarenotknow,relativestabilityofconjugatesshouldbeused
• Isthereactionbelow,reactantorproductfavored?
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3.5ChoosingaReagent• Anotherimportantskillistobeabletochooseanappropriatereagent foraacid/basereaction
• Chooseanacid fromtable3.1thatcouldeffectivelyprotonate eachofthefollowingmolecules
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Table 3.1
3.5ChoosingaReagent• Anotherimportantskillistobeabletochooseanappropriatereagent foraacid/basereaction
• Chooseaconjugate base fromtable3.1thatcouldeffectivelydeprotonate eachofthefollowingmolecules
• PracticewithSkillBuilder 3.11
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3.7Solvation• Becausetheyaresosimilar,ARIOcannotbeusedtoexplainthepKa differencecomparingethanolandtert-Butanol
• ConsideringthatpKa valuesaremeasuredinsolution,howmightthesolventacttomakeethanolaslightlystrongeracid?Thinkabouthowthesolventcouldstabilizeitsconjugatebase.
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3.7Solvation• Thesolventmustformion-dipoleattractionstostabilizetheformalnegativecharge
• Ifthetert-Butoxideisstericallyhindered,itwon’tbeaswellsolvatedastheethoxide
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3.7Solvation• Solvation iscriticallyimportantinreactions.Thesolventisoftenneededtostabilizetransitionstates,intermediates,and/orproductstoallowareactiontooccur
• ExplainwhythepKa foraceticacidis4.75inwaterwhileitis23.5inCH3CN
• Practicewithconceptualcheckpoint3.33
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3.9LewisAcidsandBases• Lewisacid/basedefinition
– ALewisacidacceptsandsharesapairofelectrons– ALewisbasedonatesandsharesapairofelectrons
• AcidsundertheBrønsted-LowrydefinitionarealsoacidsundertheLewisdefinition
• BasesundertheBrønsted-LowrydefinitionarealsobasesundertheLewisdefinition
• Explainhowthisreactionfitsbothdefinitions
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3.9LewisAcidsandBases• Lewisacid/basedefinition
– ALewisacidacceptsandsharesapairofelectrons– ALewisbasedonatesandsharesapairofelectrons
• SomeLewisacid/basereactionscannotbeclassifiedusingtheBrønsted-Lowrydefinition
• ExplainhowthisreactionfitstheLewisdefinitionbutnottheBrønsted-Lowrydefinition
• PracticewithSkillBuilder 3.12
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AdditionalPracticeProblems• Provideproductsandcurvedarrowsforthefollowingacidbasereaction
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AdditionalPracticeProblems• RankthefollowingbasesinorderofincreasingstrengthandranktheirconjugatesinorderofincreasingpKa.
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