有机化学英文课件chapter4.ppt

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ConjugateAcids&Bases
Manyorganicmoleculeshavetwoormoresitesthatcanactasprotonacceptors
inthischapter,welimitourdiscussiontocarboxylicacids,esters,andamides
inthesemolecules,thefavoredsiteofprotonationistheoneinwhichthechargeismoredelocalized
question:whichoxygenofacarboxylicacidisprotonated?
ConjugateAcids&Bases
forprotonationonthecarbonyloxygen,wecanwritethreecontributingstructures
twoplacethepositivechargeonoxygen,oneplacesitoncarbon
A-1andA-3makethegreatercontributionbecauseallatomshavecompleteoctets
thepositivechargeisdelocalizedoverthreeatomswiththegreatershareonthetwoequivalentoxygens
ConjugateAcids&Bases
forprotonationonthehydroxyloxygen,wecanwritetwocontributingstructures
B-2makesonlyaminorcontributionbecauseofchargeseparationandadjacentpositivecharges
therefore,weconcludethatprotonationofacarboxylicacidoccurspreferentiallyonthecarbonyloxygen
ConjugateAcids&Bases
?
PiElectronsAsBasicSites
Proton-transferreactionsoccurwithcompoundshavingpielectrons,asforexamplethepielectronsofcarbon-carbondoubleandtriplebonds
thepielectronsof2-butene,forexample,reactwithHBrbyprotontransfertoformanewC-Hbond
theresultisformationofacarbocation,aspeciesinwhichoneofitscarbonshasonlysixelectronsinitsvalenceshellandcarriesachargeof+1
PiElectronsAsBasicSites
-methyl-2-butene
Acids&BaseStrengths
Thestrengthofanacidisexpressedbyanequilibriumconstant
theaciddissociationofaceticacidisgivenbythefollowingequation
WeakAcidsandBases
Wecanwriteanequilibriumexpressionforthedissociationofanyunchargedacid,HA,as:

wecancombinetheseconstantstogiveanewconstant,Ka,calledanaciddissociationconstant
Acid-BaseEquilibria
Equilibriumfavorsreactionofthestrongeracidandstrongerbasetogivetheweakeracidandweakerbase