Download CHE2060 Lecture 5: Acid-base chemistry CHE2060 Lecture 5: Acid

4/10/17
CHE2060Lecture5:Acid-basechemistry
5.1Acids&bases:overview&basics
5.2Acid&basestrength
5.3Equilibriumacid-basereactions
5.4Thelevelingeffectofsolvents
5.5Estimationofaciditybyconceptualknowledge
5.6 Classesoforganicacids&bases
5.7Functionalgroups:acid-basenature
Daley&Daley
Chapter5:
Acid-basetheory
CHE2060Lecture5:Acid-basechemistry
5.1Acids&bases:overview&basics
5.2Acid&basestrength
5.3Equilibriumacid-basereactions
5.4Thelevelingeffectofsolvents
5.5Estimationofaciditybyconceptualknowledge
5.6 Classesoforganicacids&bases
5.7Functionalgroups:acid-basenature
Daley&Daley
Chapter5:
Acid-basetheory
1
4/10/17
Acids&bases
Overview&definitions
Basicacid-basereaction
Electrophiles&neutrophiles
Basesvs.Nu
Anorbitalview
Acids&bases:ashiftinperspective
1884  Arrhenius AcidsaresourcesofH+ions;basessourceofOH-ions
1923
1916
Brønsted-Lowry AcidsareH+donors;basessourceofH+acceptors
G.N.Lewis Acidsacceptelectronpairs(:);basesdonate:
M&Bp.6-7
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Acids&bases:ashiftinperspective
1884  Arrhenius AcidsaresourcesofH+ions;basessourceofOH-ions
1923
1916
Brønsted-Lowry AcidsareH+donors;basessourceofH+acceptors
G.N.Lewis Acidsacceptelectronpairs(:);basesdonate:
Notethatthisreactionmovesapairofelectronsinasingle-stepreaction.
•  Afreepairfromthehydroxideoxygenformsadativebondwith
thehydrogeniontoformwater.
M&Bp.6-7
Acids&bases:ashiftinperspective
1884  Arrhenius AcidsaresourcesofH+ions;basessourceofOH-ions
1923
1916
Brønsted-Lowry AcidsareH+donors;basessourceofH+acceptors
G.N.Lewis Acidsacceptelectronpairs(:);basesdonate:
Notethatthisreactionmovesapairofelectronsinasingle-stepreaction.
•  Afreepairfromthehydroxideoxygenformsadativebondwith
thehydrogeniontoformwater.
M&Bp.6-7
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Acids&bases
Acid-basereactionsarecentraltobiochemistrybecausemostbiological
reactionsarecatalyzedbyacidsorbases.Sowemustunderstandthebasics
ofacid-basereactionsandbeabletoanswerthesequestions:
•  Where’stheacid?
•  Where’sthebase?
•  Howcanthetworeact?
•  Whatwillhappen?
Brønsted-Lowryacids&bases:acidsdonateprotons(H+1)&basesacceptthem
Conjugatepairs:before&afterlossofproton
N’sfreepairformsadativebond;πbecomefreepair,breakingbond.
M&Bp.6-7
Examples:drawtheproducts
Karty
4
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Examples:donor?acceptor?products?
Step1.Line-bondstructures,showingallfree:
M&Bp.6-7
Examples:donor?acceptor?products?
Step1.Line-bondstructures,showingallfree:
Step2.IdentifyLA&LB
M&Bp.6-7
5
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Examples:donor?acceptor?products?
Step1.Line-bondstructures,showingallfree:
Step2.IdentifyLA&LB
Step3.Free:fromtheLBmovetocreateadative
bondwiththeLA
M&Bp.6-7
Examples:donor?acceptor?products?
Step1.Line-bondstructures,showingallfree:
Step2.IdentifyLA&LB
Step3.Free:fromtheLBmovetocreateadative
bondwiththeLA
Step4.Createtheproducts
M&Bp.6-7
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Examples:donor?acceptor?products?
Step1.Line-bondstructures,showingallfree:
Step2.IdentifyLA&LB
Step3.Free:fromtheLBmovetocreateadative
bondwiththeLA
Step4.Createtheproducts.
Step5.Identifytheconjugateacid&conjugatebase
Conjugateacid-basepair:twomoleculesthatdifferonlybyoneH!
NotethatthesereactionsmoveoneH,andoronepairofelectrons.
M&Bp.6-7
Examples:donor?acceptor?products?
Whichproductisthe
conjugateacid&
conjugatebase?
conjbaseconjacid
M&Bp.6-7
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Lewisacids&bases
Criticalfororganicchemistry…andacidsdon’talwayshaveH!
Lewisacid: acceptsane-pairfromaLewisbase…..Doesn’thavetobeH+1
Lewisbase: donatesane-pairfromtoLewisacid
TCAcycleexample:
HereFeistheLewisacidin
theenzyme’scofactor.
Metalcationsoftenact
cofactoracids/basesin
enzymes.
M&Bp.10
Electrophiles&nucelophiles
Electrophiles&nucleophilesarewhatorganicchemistscallLewisacids&
Lewisbases.
Electrophile:+charged&thereforee-loving;lookingforane-richnucleophile
E+=acids(H+1);trialkylsulfonium(R3S+1);carbonyls(δ+C=O)
Nucleophile: e-rich(sometimes-)&lookingforane-seekingelectrophile
Nu:=amines(:NH2);water(H2O);hydroxideion(OH-1);
alkoxideions(RO-1);thiolateions(RS-1)
Nu:-
Nu:
E+
E
M&Bp.11
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Who’swho?
Acommonthemeoforganicreactionsisnucleophilicattack:anucleophile
usesitselectronpairstoattackanelectrophile,achemicalwithlow
electrondensity.
It’scriticaltobeabletoidentifyreactingchemicalsasNu:orE+.
1.  Identifyeachplayer.
2.  Predicttheoutcomeoftheattack.
..
:O:
δ-
E+
δ+
:OH
..
Nu:
..
:O:
δ- E+
δ+
Nu:
E+
..
Cl:
..
..
OH
..
Nu:
..+
OH2
..
Kleinp.178
Basesvs.Nu:
MostorganicrxnsinvolveeitherbasesorNu:&studentsoftenconfusethem.
Thedifferenceispurelyfunctionalbecauseanion,moleculeorgroupmay
functionasboth.
Base: thegroupremovesaprotonandtakesitaway
Nu:attackstheotherreactant&isaddedtoit
HydroxidecanactaseitherabaseoraNu:.Whichisitdoingintheseexamples?
OHactingasabase:removesproton&becomesH2O
OHactingasaNu;it’saddedtotheotherreactant
Kleinp.179
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Examples:basesvs.Nu:
Foreachexamplebelow,determinewhetherhydroxideorwaterisactingas
abaseoraNu:,andpredicttheoutcome.
base
Nu:
..
:O:
..
Cl:
..
:OH
..
Nu:
base
base
:O:
||
..
..
H2O+:O–S–O–H
..
..
||
:O:
:O:
H3O+1+
Kleinp.180-1
Anorbitalperspective
Let’slookatelectrophiles,nucleophiles&howtheyreactatthelevelof
obitals:
sp3
•  base
•  unbonded:
•  Nu:
emptyp
•  acid
•  notenoughe-
•  NH3sp3overlapswithBF3p
•  NH3donatesits:toform
thebond
•  BF3becomessp3hybridized
•  Thenewmoleculehasno
overallcharge,butdoeshave
formalcharges
Dativebondsoccupyemptyorbitalsbyfusingthemwithfullorbitals.
D&Dp.212-3
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Acidityofalkanesvs.alkenesvs.alkynes
Therelativeacidityofalkanes,alkenesandalkynesisaresultoftheshapes
&geometryoftheirhybridorbitals.
alkane
sp3
alkene
sp2
alkyne
sp
acidityincreases
Why?Theshorter,moresphericalshapeofthespalkyeneorbitalsplaces
electronsclosertothenucleus.Themoretightlyelectronsareheldtothenucleus,
thelesstheyarepulledtowardhydrogenatomstheyarebondedto.
Alkynesarethereforebetteracidsthanalkenesandalkanes….
...becuasetheirhydrogenarelesstightlybound.
D&D,p.213
Inbiochemistry…
Biochemicalexamplesofacid-base(akanucleophile-electrophile)reactions.
M&Bp.11
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CHE2060Lecture5:Acid-basechemistry
5.1Acids&bases:overview&basics
5.2Acid&basestrength
5.3Equilibriumacid-basereactions
5.4Thelevelingeffectofsolvents
5.5Estimationofaciditybyconceptualknowledge
5.6 Classesoforganicacids&bases
5.7Functionalgroups:acid-basenature
Daley&Daley
Chapter5:
Acid-basetheory
Acid&basestrength
Ionization&pH
Strength:Ka&pKa
Commonorganicacids&bases
Predictingdirectionofequilibrium
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Ionizationofwater&pH
ThestrengthofaBrønsted-Lowryacidisdeterminedbytheextentto
whichitionizesinwater:autoionization.
hydronium
ion
Becauseboththe“acid”and“base”arewater,thedegreeofdissociationor
auto-ionizationisveryslight.
Forwater,theconcentrationofthedissociatedproductsareconstant,Kw.
Kw=[H3O+1][OH-1]=Kw=1.00x10-14
WhileKwisconstant,theconcentrationofhydroniumioncanchangegreatly.
Itisthis[H3O+1]thatchemistsmeasureaspH!
pH=-log[H3O+1]…oftenexpressedaspH=-log[H+1]
M&Bp.6-8
AcidstrengthismeasuredasKaorpKa
Anacid’sstrengthismeasuredasitsdissociationconstant,Ka.
So,imagineanacid,Ka:
hydronium
ion
ThelargertheKavaluestrongertheacid’sstrength.
StrongacidshavepKavaluesbetweenatzeroorbelow.
MostorganicacidsarequiteweakandhaveKavaluesbetween10-2and10-60.
Tomakeiteasiertocomparethisverywiderangeofvalues,Kaisoftenconverted
topKa,alogarithmicscale:
pKa=-log(Ka)
Sincethisscaleisinverselogarithmic,butlowerthepKathestrongertheacid.
D&D,p.215-7
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CommonpKavalues
weakest
“acid”
Strongestacid
D&D,p.217
pKa’sofcommonorganicacids
M&Bp.6-8
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Examples:usingpKatogaugeacidstrength
Whichofeachpairisthestrongeracid?
•  Andwhatarethemoleculesnamed?
p-methylphenol
p-chlorophenol
Howmuchstrongeristhep-chlorophenol?
ThedifferenceinpKavalues=10.26–9.43=0.83.
Thedifferenceinacidstrength=100.83=6.8.
Sop-chlorophenolis6.8timesasstronganacid.
Kartyp.300
Examples:pKaformostacidicproton?
Foreachmolecule,identifythemostacidicproton(hydrogen).
methyl48
carboxylic
acid4.75
ethanol16
thiol10.3
phenol10
benzoicacid4.2
hydronium-1.7
methyl
ammonium
10.63
amine38
Kartyp.300
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Strongbasesareweakacids.
YoucanpredictthestrengthofabaseifyouknowtheKaorpKavaluesof
thebase’sconjugateacid.
Sobasesarestrongerwhentheirconjugateacidshave:
•  lowerKavalues;or
•  higherpKavalues.
Sowhichisthestrongerbase,hydroxide(OH-1)orammonia(NH3)?
•  Theconjugateacidofhydroxide(OH-1)iswater:pKa15.7
•  Theconjugateacidofammoniaisammonium:pKa9.4
So,hydroxideisthestrongerbase.
Howmuchstronger?
Differenceinpka: 15.7–9.4=6.3
Differenceinstrength: 106.3=2.0E6=2milliontimesstronger!
Kartyp.301
Example:predictthestrengthofabase
Whichisthestrongerbase,chlorideionorphenoxideion(C6H5O-1)?
•  Theconjugateacidofchlorideionishydrochloricacid:pKa~-7
•  Theconjugateacidofphenoxideionisphenol:pKa10
So,phenoxideionisthestrongerbase.
Howmuchstronger?
Differenceinpka: 10–(-7)=17
Differenceinstrength: 1017=1E17timesstronger! Kartyp.301
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pKa’sofcommonorganicbases
Stengthofbasesismeasuredbytheacidityoftheirconjugateacid.
pKa+pKb=14->pKb=14-pKa
Sostrongerbaseshave
higherpKavalues.
M&Bp.8-9
CHE2060Lecture5:Acid-basechemistry
5.1Acids&bases:overview&basics
5.2Acid&basestrength
5.3Equilibriumacid-basereactions
5.4Thelevelingeffectofsolvents
5.5Estimationofaciditybyconceptualknowledge
5.6 Classesoforganicacids&bases
5.7Functionalgroups:acid-basenature
Daley&Daley
Chapter5:
Acid-basetheory
17
4/10/17
Equilibriumreactions
UsingpKatopredictwhether
reactantsorproductspredominate
Equilibriumofconjugateacid-basepairs
Whenareactionisreversible,the“side”withtheweakerconjugateacid
(higherpka)isfavored.Arereactantsorproductsarefavoredhere?
pKa=15.1
pKa=36
pKa=10.66
pKa~4.76
pKa=35
pKa~26
pKa=4.76
pKa~-7.5
D&D,p.218
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Whyistheweakerconjugateacidfavored?
It’sasifthetwoacids,onthereactant&productsides,ofthereactionarein
competitionwithoneanother.
•  Inessence,thestrongeracidwinsandactslikeanacid,acceptingan
electronpair,anddrivingthereactiontoproducingtheweakeracidand
theconjugatebaseofthestrongeracid.
B:-+H–A
B–H+:A-
baseacidconjugateconjugate
acidbase
Kartyp.302
Amoredetailedlookthedrivingacid
Let’slookataspecificexamplewithnumbers.
•  Thestrongacidpushesequilibriumtotheright
aceticacidmethylacetatemethyl
amine ion ammonium
pKa4.76
pKa10.63
stronger
equilibrium
WecanseehowstronglythereactionfavorsproductsbycalculatingΔpKa.
ΔpKa=10.63–4.75=5.88
Sotheequilibriumconstantforthisreactioncanbecalculated:
Keq=105.88=7.6E105
Inotherwords,productsarefavoredbyafactorof760,000.
Kartyp.302
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Example:quantitatethepush
Predictwhichsideisfavoredandbyhowmuch.
2-methyl-
2-propoxide
propanone
pKa20
2-methyl-
2-propanol
pKa19
propanone
anion
equilibrium
WecanseehowstronglythereactionfavorsreactantsbycalculatingΔpKa.
ΔpKa=20–19=1
Sotheequilibriumconstantforthisreactioncanbecalculated:
Keq=101=1o
Inotherwords,reactantsarefavoredbyafactorof10.Barelypushed.
Kartyp.302
CHE2060Lecture5:Acid-basechemistry
5.1Acids&bases:overview&basics
5.2Acid&basestrength
5.3Equilibriumacid-basereactions
5.4Thelevelingeffectofsolvents
5.5Estimationofaciditybyconceptualknowledge
5.6 Classesoforganicacids&bases
5.7Functionalgroups:acid-basenature
Daley&Daley
Chapter5:
Acid-basetheory
20
4/10/17
The‘leveling’effect
Solventsmayhijackintendedreactions
Remembersolvents?
Solventsarecompoundsthatare:
•  Presentinlargeamounts(largerthansolvents);
•  Usedtocreateasolubleenvironmentforsolutereactants;and
•  Sometimesplaysaroleinsolutereactions.
Everydayexample:wateristhesolventforextractionofcaffeineandother
coffeeorteacompoundsfrombeansorleaves.
Ideally,solventseitherhavenoimpactonreactionsoractuallyenhancethem.
However,ifsolventsarenotselectedcarefullytheycaninterferewith–or
hijack–ourintendedreactions.
Karty,p.304
21
4/10/17
So,what’s‘leveling’?
Levelingisareactionofthesolventwithoneofthereactants.Thisreaction
usesupsomeormostofthatreactantandrestrictstheintendedreaction.
•  Acids&basesareoftenimpactedbylevelingbecausetheyreactwitha
widevarietyofsolvents,likewater.
pka~-7
pka~-1.7
SoHClisapowerfulaciduntilitisdilutedinwater.OnceHClreactswithwater,theonly
acidremainingishydroniumwhichis1.99E5timesweakerthanHCl.
•  Water‘levels’(orreducesorrestricts)thestrengthofHCl.
Karty,p.304
Basestrengthcanalsobeleveledbysolvents.
Deprotonateddimethylamineisabase.Noteitstwofreeelectronpairs.
pKa15.7
pKa38
Deprotonateddimethylamineisapowerfulbase.Itsconjugateacid,dimethylamine,
hasapKaof38!
•  Forbases,theequilibriumispushedtowardstheacidwiththehigherpKa.
•  Thestrengthofdeprotonateddimethylaminepushesequilibriumtowardsproducts
byafactorof2.oE22!
•  However,ifwaterisusedasasolventwhendeprotonateddimethylamineisusedas
abase,waterwillactasanacid,reactwithand‘neutralize’thepowerfulbase.
•  Reactionwiththewatersolventproduceshydroxideastheremainingbase.
•  Andhydroxideisaweakerbasethandeprotonateddimethylamine.
Karty,p.304
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Conclusionsaboutleveling
•  Ifwaterisusedasthesolvent,noacidstrongerthanhydronium,andno
baseweakerthanhydroxide,canexisttoappreciableextents.
Forothersolvents:
•  Thestrongestacidthatcanexistinsolution,toappreciableconcentrations,
istheprotonatedsolvent.
•  Thestrongestbasethatcanexistinsolution,toappreciableconcentration,
isthedeprotonatedsolvent.
Take-homemessage?
Solventsmustbechosencarefully!
Karty,p.304
Example:wisechoiceofsolvent
Ifyouwanttousedeprotonateddimethylamineasabase,diethyletherisa
bettersolventthanwater.
Usepkavaluestoshowthatdiethyletherisabettersolvent!
pKa45
pKa38
Forbases,theequilibriumispushedtowardstheacidwiththehigherpKa,sowhen
deprotonateddimethylamineisplacedindiethylethersolventthetwobarelyreact.
Karty,p.304
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Example:youchoosethesolvent!
Wouldeachofthesesolventsbeappropriateforthereactant?Why?
Reactant:
pKa25foritsconjugatedacid
Solvents:
pKa:15.716173545
Deprotonatedethylyne(acetylene)isabase.
•  Forbases,theequilibriumispushedtowardstheacidwiththehigherpKa.
•  Topreservethe‘baseness’ofthereactantwewanttopushequilibriumbacktowards
it;totheleft.
•  Sousesolventsthatarestrongeracidsthantheconjugateacidofthereactant.
•  DMSO(dimethylsulfoxide)anddiethyletherhavehigherpKavaluesthenacetylene.
Karty,p.304
CHE2060Lecture5:Acid-basechemistry
5.1Acids&bases:overview&basics
5.2Acid&basestrength
5.3Equilibriumacid-basereactions
5.4Thelevelingeffectofsolvents
5.5Estimationofaciditybyconceptualknowledge
5.6 Classesoforganicacids&bases
5.7Functionalgroups:acid-basenature
Daley&Daley
Chapter5:
Acid-basetheory
24
4/10/17
Estimatingacidity
…byusingconceptualknowledgeof
thefactorsthatincreaseordecrease
acidity
Howdoweestimateacidity
Andwhatmakessomeprotons(Hatoms)moreacidicthanothers?
1. 
EstimatepKabyfindingasimilargroupinatableorchartofpKavalues.
2.  LookattheENvaluesofcloseneighbors(inductiveeffect).
3.  Lookfornearbymultiplebonds(πbonds).
4.  IstheHattachedtoapositivelychargedatom?
5.  WhatistherowpositionoftheatomtheHisboundto?
6.  WhatisthecolumnpositionoftheatomtheHisboundto?
7.  WhatistheorbitalhybridizationoftheatomtheHisboundto?
8.  WhatisboundtotheatomthattheHatomisboundto?(pattern)
Karty,p.304
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1.EstimatingpKa
Functionalgroupsarethereactivepartsoforganicmolecules,andpKavalues
areonegreatwaytopredicttheoutcomeofreactions.
•  SomefunctionalgrouppKatablescanbefoundinpKatables.
•  Whatifyoucan’tfindapKa?
1.YoucanestimatepKabyfindingasimilargroupinatableorchartofpKavalues.
•  EstimatethepKavaluesofthesetwomolecules.
Similartoethanol:pKa16
•  ActualisopropanolpKais16.5
Similartoaceticacid:pKa4.75
•  ActualbenzoicacidpKais4.20
Karty,p.304
2.InductiveeffectofhighENneighbors
Electronegativeatom(s)orgroup(s)closetothehydrogenofinterestwill
increaseaciditybypullingelectronsthroughthemoleculeandtowardsthe
electronegativegroup,makingthebondtoHmorepolarandthereforemore
reactive.
•  AnionsarestabilizedbyENgroupsnearthenegativecharge.
•  CationsaredestabilizedbyENgroupsnearthepositivecharge.
pKa0.77
Theelectronegativeπbondspullelectrons
throughthestructureandtowards
themselves.Thisincreasesthepolarityof
theO–Hbondandthereforeincreases
acidity;pKaissignificantlylowered.
pKa10
pKa4.76
Theelectronegativechlorineatomspull
electronsthroughthestructureandtowards
themselves.Thisincreasesthepolarityofthe
O–Hbondandthereforeincreasesacidity;pKa
issignificantlylowered.
pKa16
Karty,p.304
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3.Nearbyπbondsstabilizeconjugateanions
Acidsaremorepowerful(lowerpKa)whentheirconjugatebasesarestable.
Resonancestabilizesanionsformedbyreactionofacids.
pKa16
Ethanoicacid(akaaceticacid)formstheacetate
anionwhendeprotonated.Theacetateanionis
stabilizedbyresonance.
Notethatthenegativechargeisdelocalizedover
severalatomsintheresonancehybrid.
Delocalizationofchargeincreasesstability.
pKa4.76
Ethanolisdeprotonatedtoform
itsconjugatebase,ethoxide.The
negativelychargedoxygenatom
isfairlystableduetoitshighEN.
Karty,p.304
3.Nearbyπbondsstabilizeconjugateanions
Acidsaremorepowerful(lowerpKa)whentheirconjugatebasesarestable.
Resonancestabilizesanionsformedbyreactionofacids.
EPM
p.321Karty
Karty,p.304
27
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4.Honpositivelychargedatoms
Hydrogenatomsboundpositivelychargedatomsaremoreacidicthanthose
boundtounchargedatoms.
H2O
pKa: H3O+1
15.7 -1.7 NH3
36 NH4+1
9.4
Why?Generally,chargedatomsarelessstable,sohigherenergy.
Theproductsareboth
chargedbutthe
reactantsarenot.
Thereactionisnot
energeticallyfavorable.
pKa: 15.7
pKa: 9.4
-1.7
15.736-1.7
Onereactant&oneproduct
arecharged.Thereactionis
lessenergetically
unfavorable.
OIsmoreelectronegative&
betterabletocarrythe
charge.
Karty,p.312-14
5.Effectofrowposition(EN)onpKa
Thefurthertotherightanatomis,themoreacidicareitshydrogenatoms.
•  Becauseatomsbecomemoreelectronegativefromlefttoright.
•  Moreelectronegativeatomsholdontotheirelectronsmorestrongly,polarizingthe
bondtohydrogenandweakeningit.
CH4
pKa48 NH3
36 OH2
15.7
Thestabilityofanionsincreasesasthechargedatom’selectronegativityincreases.
•  Becausemoreelectronegativeatomsareabletoholdfreeelectronpairs.
..
..
..
H3C:
H2N:- HO:-
:F:-
..
..
electronegativity
anionstability
Thestabilityofcationsdecreasesasthechargedatom’selectronegativityincreases.
•  Becauselesselectronegativeatomsareabletogiveupfreeelectronpairs.
electronegativity
H4N+
H3O+
H2F+
cationstability
Karty,p.312-14
28
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6.Effectofcolumnposition(size)onpKa
Thefurtherdownacolumnanatomis,themoreacidicareitshydrogenatoms.
•  Becauseatomsbecomelargerfromtoptobottom.
•  Largeratoms(ions)havemoreroomforfreeelectronpairs:lowerchargedensity.
WhichhasalowerpKavalue,CH4orSiH4?
Carbonisinrow2ofcolumnIVAwhileSiisinrow3ofcolumnIVA;
Siisthereforelargerthancarbon.
pKa=48
pKa=35
Sotheconjugatebase(-:SiH3)formedbydeprotonationofSiH4ismorestable
thantheconjugatebase(-:CH3)formedbydeprotonationofCH4.
Karty,p.312-14
7.Effectoforbitalhybridization
OrbitalhybridizationoftheatomtheHisbondedtoaffectsthatatoms‘effective
electronegativity’.
Effectiveelectronegativity:sp>sp2>sp3
•  ThestabilityofachargedmoleculeincreasesastheeffectiveENofnegatively
chargedatomsincreases.
•  ThestabilityofachargedmoleculedecreasesastheeffectiveENofpositively
chargedatomsincreases
Whichisastrongeracid?(CH3)2C=OH+or(CH3)2CH-OH2+?
Lessstable
cation,so
strongeracid.
Morestable
cation,so
weakeracid.
pKa=20
Check?
•  Strongeracidshave
weakerconjugate
bases.
•  Strength0fbase
increaseswithpKa.
pKa=16.5
Karty,p.312-14
29
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8.Effectof1°,2°,or3°ions
Moleculargeometrycanaffectthestabilityofcarbocationsandcarbanions.
Carbocationsaremoleculesinwhichcarbonispositivelycharged.
Carbanionsaremoleculesinwhichcarbonisnegativelycharged.
• 
• 
• 
• 
Methyl:carbonboundonlytohydrogenatoms
Primary:carbonboundtoonealkylgroup
Secondary:carbonboundtotwoalkylgroups
Tertiary:carbonboundtothreealkylgroups
Delocalizationincreases
ionstability.
Leaststable
Moststable
Karty,p.312-14
Quickchecklist?
TrythisquicksetofquestionstoassessrelativeacidstrengthorpKa.
1.Dothemoleculeshavedifferentcharges?
+chargesincreaseacidity
-chargesdecreaseacidity
2.Dothemoleculescarrychargesondifferentatoms?
size
EN
3.Doeseithermoleculehaveresonance?
Resonancestabilizeschargeof
conjugatebasesbydelocalization.
hybridization
1°,2°,3°pattern
4.Doinductiveeffectsoccurineithermolecule?
NearbyENatomsincreaseacidity
Karty,p.312-14
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Example
EstimatethepKaforthehydrogenatomsshowninthesemolecules.
Theringdoesn’t
includesignificantly
electronegative
atomsorbonds.
So,pKaissimilar
mostamineslike
dimethylamine.
pKa~38
Thisringhas
electronegativeπbonds.
So,pKawillbelower
(moreacidic)than
propanone.
pKa<20
Thisistheprotonated
versionofdiethylether.
Protonationanda
positivechargeincrease
acidity,reducingpKa.
Comparehydroniumto
water.
pKa<45
Karty,p.310-11
Example
Estimatetherelativeaciditiesof:
•  Ethane
•  Ethene
•  Ethyne(akaacetylene)
Leastacidic
Thestabilityoftheconjugatebase
(anion)increasesfromsp3tosp.
NotethatthemostacidicHisattachedto
thecarbonwiththemostπbonds.
Mostacidic
Karty,p.310-11
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Example
Whichmoleculeismoreacidicbasedonstructure?
Moreacidic:
ReplacementofHwithmoreENCl
destabilizesthepositivelycharged
molecule.Loss(donation)ofH
removesthepositivechargeand
increasesstability.
Inductiveeffect
Karty,p.312-14
Example
Whichalcoholismoreacidicbasedonstructure?
EPM
Karty,p.324
Moreacidic:
ReplacementofHwithmoreENCl
stabilizesthenegativelycharged
conjugatebase.Amorestablebase
allowstheHtoleave.
Inductiveeffect
Karty,p.312-14
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Example
Ranktherelativeaciditiesofthesemolecules.
Leastacidic(4)
TheENFis
locatedfarfrom
thepolarOH
hydrogen.
(3)
TheENFis
locatedcloserto
thepolarOH
hydrogen.
Mostacidic(1)
Positivecharge
causesthis
moleculetobe
leaststable.
LosingaHwill
removethe
chargeand
stabilizethe
molecule.
(2)
TwoENgroups
(Fand=O)
withdraw
electronsand
increasethe
polarityofthe
OHhydrogen.
Karty,p.312-14
CHE2060Lecture5:Acid-basechemistry
5.1Acids&bases:overview&basics
5.2Acid&basestrength
5.3Equilibriumacid-basereactions
5.4Thelevelingeffectofsolvents
5.5Estimationofaciditybyconceptualknowledge
5.6 Classesoforganicacids&bases
5.7Functionalgroups:acid-basenature
Daley&Daley
Chapter5:
Acid-basetheory
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4/10/17
Classesoforganicacids&bases
Organicacids&bases
Organicacids&baseshavecarbonskeletons.We’lllookatthesetypes:
•  Neutralorganicprotonacids
•  Neutralorganicbases
•  Positivelychargedcarbonacids
•  Negativelychargedcarbonbases
D&D,p.225-6
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Neutralorganicprotonacids:carboxylicacid
Threemaintypes:
•  Carboxylicacids
•  Phenols
•  Alcohols
OH-1functionalgroup
OHbondpolaritymakesthese
groupsacidic&reactive.
Degreeofaciditydepends
onthestabilityofthe
conjugatebase.
Carboxylicacidsarethemostacidicofthethreealthoughtheyareweak
acids.Aceticacids(2carbons)hasapKaof4.8.
Anionhas3unhybridized
porbitalsw/totalof4e-.
Theyforma3-atomπ
molecularorbitalsystem;
½πconnectsCtoeachO.
Theconjugatebase(anion)isstabilizedby
resonance.The(-1)isdelocalizedover2oxygen
atomsinthehybrid.
Thereforestrongeracidthanalcohol!
D&D,p.225-7
Neutralorganicprotonacids:phenol
Phenolsaremuchlessacidicthancarboxylicacids:pKaof10.
Thephenolateanionisstabilizedbyresonance&thereforehassomeacidity.
Butsomeofphenolateanion’sresonancestructuresdisruptaromaticresonance&
creates(-)carbon.
Becausephenolate’scarboncanbecharged:
1.  thephenolateionislessstablethanthecarboxylateion;and
2.  phenolislessacidicthancarboxylicacid.
D&D,p.225-7
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Neutralorganicprotonacids:alcohols
AlcoholsarealkylmoleculeswithOH-1functionalgroups:R–OH.
•  Alcoholsaremuchlessacidicthanphenols.
•  Alcoholsarejustabitmoreacidicthanwater.
•  ThepKarangeofalcoholsare~15–18.
Isthereanyresonancestabilizationforthisalcohol?Nope.
D&D,p.228-9
Neutralorganicbases:amines
Neutralorganicbasescontainoneormorepairsofnonbondingelectrons.
•  WhenactingasLewisbasesthesee-pairsaredonated.
•  Themoreavailablethe:,themorepowerfulthebase.
•  Anymoleculewithapair(:)canactasabase.
We’lllookatjusttwoofthemanyneutralorganicbases:
•  Amines;and
•  Ethers.
Aminesarederivativesofammonia,NH3,andactasweakbasesinwater.
methyl
amine
methyl
pKa10.6
ammonium
relatively
ion
strongbase
AminesarestrongbasesbecauseNholdsits:
lessstronglythanO,S,halogens.
D&D,p.228-9
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(+)acids&(-)bases
Positivelychargedacidsaree-deficient;carbonsdon’thaveanoctet
•  Carbocationisacarbonwithafull(+)charge
•  Highlyreactiveandseldomseensincetheactasintermediates
•  CarbocationsareLewisacids(hardacids)
•  ReactwiththefirstLewisbasethey“see”
•  Preferreactingwithhardbases
Carbocationssp2
withemptyp
orbitalcarryingthe
+charge.
Negativelychargedbasesarecarbonsbondedonlytothreeotheratoms*
carryinganunbondedelectronpair(:).
•  Carbanionisacarbonwithafull(-)charge
•  Looksverymuchlikeanamine
•  Carbonholdsthe:loosely
sinceCisnotveryelectronegative
•  Thuscarbanionsarestrongbases
D&D,p.228-9
Example
Ranktheacidityoftheserelatedcarboxylicacids:
pKa:4.75 2.86
1.26
0.63
AcidityincreasesasthenumberofENsubsitutentsincreases.
Moresubstituentshelptocarrythe:(orthenegativecharge)ofthe
acid’sconjugateanion.
D&D,p.233-4
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Examples
Randtheaciditiesofthesetwosetsofmolecules.
pKa:5.2vs.11.1
Resonancestabilizesthe(+)&
holds:closertothenucleus
makingthemlessavailablefor
donation.(11.1isthebetterbase)
•  Moststablecation
sp2
sp3
pKa:-10sphybridizationkeeps:veryclose;lessavailable
•  Leaststablecation
•  Morestablecationbecausethe+is
delocalizedovertheCH3group
pKa:9.4 10.6
TheCH3alkylgrouphelpstodelocalize(orshare)the(+)charge;betterbase
However,addingmorealkylgroupscandecreasebasicitybysterically
crowdingtheN&destabilizingtheion.
D&D,p.233-4
CHE2060Lecture5:Acid-basechemistry
5.1Acids&bases:overview&basics
5.2Acid&basestrength
5.3Equilibriumacid-basereactions
5.4Thelevelingeffectofsolvents
5.5Estimationofaciditybyconceptualknowledge
5.6 Classesoforganicacids&bases
5.7Functionalgroups:acid-basenature
Daley&Daley
Chapter5:
Acid-basetheory
38
4/10/17
Functionalgroups
Acidity&reactivity
Functionalgroups
M&Bp.4
39
4/10/17
Functionalgrouppolarity
M&Bp.5-6
Graphicalrepresentation
Notethatthisscaleshowsbothmolecules&somefunctionalgroups.
D&D,p.218
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CHE2060Lecture5:Acid-basechemistry
5.1Acids&bases:overview&basics
5.2Acid&basestrength
5.3Equilibriumacid-basereactions
5.4Thelevelingeffectofsolvents
5.5Estimationofaciditybyconceptualknowledge
5.6 Classesoforganicacids&bases
5.7Functionalgroups:acid-basenature
Daley&Daley
Chapter5:
Acid-basetheory
KeyconceptsfromLecture5
•  AccordingtotheBrønsted-Lowrydefinitionofacidsandbases,anacid
isaprotondonorandabaseisaprotonacceptor.
•  Acidstrengthisrelatedtothestabilityoftheacid’sconjugatebase.The
strongertheacid,themorestableisitsconjugatebase.
•  Ameasureofacidstrengthisthevalueoftheaciddissociation
constant,Ka.HigherKa=strongeracid
•  AccordingtotheLewisdefinitionofacidsandbases,anacidisan
electron-pairacceptorandabaseisanelectron-pairdonor.
•  Thecharacteroftheacidicorbasicfunctionalgroup,aswellasthe
inductiveeffectofanyadjacentfunctionalgroup,affectsacidorbase
strength.
41