Carboxylic acids, Building Bridges to Knowledge
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1
CarboxylicAcids
BuildingBridgestoKnowledge
Photo a Freeway in Shanghai, China
StructureandNomenclature
Thecarboxylicacidgroupisoneofthemostimportantfunctionalgroupsinorganicchemistry.Carboxylicacidsarenamedbyincludingcarboxylicacidfunctionaspartofthelongestcontinuouscarbonchain.Oncethelongestcontinuouschainincludingthecarbonylofthecarboxylicacidfunctionhasbeenidentified,theattachedsubstituentsarenumberedinsuchamannerthatthecarbonylcarboniscountedasnumber1.Thenameoftheparentstructureisdeterminebydroppingthe“e”ofthealkanefromwhichthecarboxylicacidoriginatedandadd“oicacid,”i.e.,alkane-oicacid.TheIUPACnomenclatureofcarboxylicacidcanbeillustratedinthefollowingmanner.
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4-methylheptanoicacidAnothermethodfornamingacidsistousetheGreekalphabettoidentifysubstituentsattachedtothelongestcontinuouschainthatincludesthecarbonylcarbonofthecarboxylicacid.4-Methylheptanoicacidis
γ-methylheptanoicacid
Followingaresomecommonnamesofcarboxylicacids.
Unsaturatedcarboxylicacidsfollowthesamesystemofnomenclature.Identifythelongestcontinuouschaincontainingthecarbonylofthecarboxylicacidandthecenterofunsaturation.
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(E)-2-hexenoicacid
(E)-4-octenoicacidSeveralorganicacidsarefoundinnature.Formicacid,foundinants,canbeobtainedfromthedestructivedistillationofants.Aceticacidisformedinsouringwine.Butyricacidisformedinrancidbutter.Malicacidisformedinapples.Oleicacidisfoundinolives.
Malicacid
Oleicacid (Z)-9-decaoctenoicacid
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PhysicalPropertiesCarboxylicacidshavehighermeltingpointsandboilingpointsofcomparablemolecularmassalkanes,alkenes,aldehydes,ketones,andalcohols.Thisisduetotheabilityofcarboxylicacidstohydrogenbond:
Carboxylicacidswith1-4carbonatomsaresolubleinwater.AcidityofCarboxylicAcidsElectronwithdrawingsubstituentsattachedtotheαcarbonatomincreasetheacidityofthecarboxylicacid.Theacidityofcarboxylicacidsisrelatedtotheavailabilityofhydroniumionswhenthecarboxylicacidisaddedtowater.
At298KaceticacidhasapKaequalto4.7
ΔGo=-RTlnKaΔGo=-(8.314J/Kmole)(298K)ln(2.0x10-5)ΔGo=+2.7x104J/mol=+27kJ/mole
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Tounderstandacidityofcarboxylicacids,let’scomparetheacidityofaceticacidwiththecomparablemolecularmassalcohol1-propanol,CH3CH2CH2OH.
At298K1-propanolhasapKaisapproximately16ΔGo=-RTlnKaΔGo=-(8.314J/Kmole)(298K)ln(1.0x10-16)ΔGo=+9.1x104J/mol=+91kJ/moleThevaluesofthestandardGibbsfreeenergy,ΔGo,giveinsightintotheabilityofreactionstobespontaneousornonspontaneous.ThemorenegativetheGibbsfreeenergy,themorespontaneousthereaction.ThehigherthepositivevaluefortheGibbsfreeenergy,thelessspontaneousthereaction.Thestandardfreeenergyforthedissociationofaceticacidhasalowerpositivevaluethanthestandardfreeenergyforthedissociationof1-propanolinwater.Thismeansthataceticaciddissociatestoagreaterextentthan1-propanol;therefore,aceticacidismoreacidic,i.e.,hasmorehydroniumions,H3O+,inaqueoussolution,than1-propanol.Acidityisbasedontheinductiveeffectofthecarbonylgroupandresonanceoftheincipientcarboxylateanion.Thecarbonylgroupofthecarboxylicacidiselectronwithdrawingandwouldattractelectronsfromthenegativelychargedoxygenoftheacetateanion.
Oncetheacetateionisformedinaqueoussolution,itcanberesonancestabilizedasillustratedbelow.
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ResonanceofthecarboxylateanioncanbeverifiedbymeasuringthebonddistancesoftheC-Obondsincarboxylicacids.Forexample,inaceticacid,theC=Obonddistanceis121pmandtheC-Obondis136pm.
Ontheotherhand,theC-Obondsintheacetateionare125pmeach.
Whenaceticaciddissociatesinaqueoussolutiontoformhydroniumionsandacetateions,thepHoftheresultingsolutiondependsontheinitialconcentrationoftheaceticacid.Forexample,100.mLofa0.100Msolutionofaceticacidat298KwouldhaveapHequalto2.9.Numberofmolesofaceticacid:
100. mL x 1 L1000 mL
x 0.100 molL
= 0.0100 mol
0.0100–xxx
Ka = CH3COO−⎡⎣ ⎤⎦ H3O
+⎡⎣ ⎤⎦CH3COOH[ ] =
x0.100 L
⎛⎝⎜
⎞⎠⎟
x0.100 L
⎛⎝⎜
⎞⎠⎟
0.0100 − x0.100 L
⎛⎝⎜
⎞⎠⎟
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Ka = CH3COO−⎡⎣ ⎤⎦ H3O
+⎡⎣ ⎤⎦CH3COOH[ ] =
x2
0.0100 ⎛⎝⎜
⎞⎠⎟
0.0100 − x0.100
⎛⎝⎜
⎞⎠⎟
approximation0.0100>x
Therefore,
Ka = CH3COO−⎡⎣ ⎤⎦ H3O
+⎡⎣ ⎤⎦CH3COOH[ ] =
x2
0.0100 ⎛⎝⎜
⎞⎠⎟
0.01000.100
⎛⎝⎜
⎞⎠⎟
Ka = CH3COO−⎡⎣ ⎤⎦ H3O
+⎡⎣ ⎤⎦CH3COOH[ ] = x2
0.0100 x 0.100
0.0100
2.0 x 10−5 = x2
0.0100 x 0.100
0.0100 2.0 x 10−5 x 0.0100 x 0.0100
0.100 = x2
2.0 x 10−5 x 0.0100 x 0.0100 0.100
= x
x=1.4x10-4molwherexisthenumberofmoleshydroniumionsandacetateionsThemolarityofthehydroniumionandacetateionis
x mol0.100 L
[H3O+ ] = [CH3COO− ] = x mol
0.100 L
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[H3O+ ] = [CH3COO− ] = 1.4 x 10−4 mol
0.100 L
[H3O+ ] = [CH3COO− ] = 1.4 x 10−3 mol
L pH=-log[H3O+]pH=-log(1.4x10-3)pH=2.9ThepHofa0.000100Msolutionofaceticacidat298Kwouldb4.4.
0.000100–xxxwherexisthenumberofmoles,butsincewehave1Lofsolutionthemolarityandthenumberofmolescanberepresentedbyx
Xmustbesolvedusingthequadraticequation:
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pH=-log[H3O+]pH=-log(3.6x10-5)pH=4.4Ifsodiumacetatewereintroducedintoanaceticacidsolution,theresultingsolutionwouldbeabuffer.AbufferisasolutionthatresistschangestopH.Forexample,ifoneadded0.82gofsodiumacetateto100.0mLofa0.100Msolutionofaceticacid,theresultingbuffersolutionwouldhaveapHof4.7.ThepHofthisbuffersolutionstaysthesameifasmallamountofbaseorasmallamountofacidisaddedtothesystem.Forexample,if1mLofa0.10Mofbaseisaddedtothesystem,thepHremains4.7,becausetheacidneutralizesthebase.ThepHoftheaceticacid/acetatebuffersolutioncanbecalculatedinthefollowingmanner:
10.0mmolxmmolxmmol
10.0mmol10.0mmol10.0mol[CH3COO-Na+]=10.0mmol/100.0mL;[CH3COO-]=10.0mmol/100.0mL;[Na+]=10.0mmol/100.0mL
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[CH3COO-Na+]=0.100M;[CH3COO-]=0.100M;[Na+]=0.100M
or
Total CH3COO-⎡⎣ ⎤⎦ = 0.100 M + x mol0.1000 L
CH3COOH[ ] = 0.100 M - x mmol100.0 mL
or
CH3COOH[ ] = 0.100 M - x mol0.1000 L
Ka = CH3COO−⎡⎣ ⎤⎦ H3O
+⎡⎣ ⎤⎦CH3COOH[ ] =
0.100 M + x100.0 mL
⎛⎝⎜
⎞⎠⎟
x100.0 mL
⎛⎝⎜
⎞⎠⎟
0.100 M - x100.0 mL
⎛⎝⎜
⎞⎠⎟
or
Ka = CH3COO−⎡⎣ ⎤⎦ H3O
+⎡⎣ ⎤⎦CH3COOH[ ] =
0.100 M + x mol0.1000 L
⎛⎝⎜
⎞⎠⎟
x mol0.1000 L
⎛⎝⎜
⎞⎠⎟
0.100 M - x mol0.1000 L
⎛⎝⎜
⎞⎠⎟
0.100M> x mol0.1000 L
Therefore,
Ka = CH3COO−⎡⎣ ⎤⎦ H3O
+⎡⎣ ⎤⎦CH3COOH[ ] =
0.100 M ( ) x mol0.1000 L
⎛⎝⎜
⎞⎠⎟
0.100 M ( )
Total CH3COO-⎡⎣ ⎤⎦ = 0.100 M + x mmol100.0 mL
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2.0 x 10−5 = 0.100 M ( ) x
0.1000 L⎛⎝⎜
⎞⎠⎟
0.100 M ( )
2.0 x 10−6 = x
H3O+⎡⎣ ⎤⎦ = x
0.1000 L
H3O+⎡⎣ ⎤⎦ = 2.0 x 10−6 mol
0.1000 L
H3O
+⎡⎣ ⎤⎦ = 2.0 x 10−5 M pH=4.7Ifasmallamountofbaseisaddedtothebuffersolution,thepHwillremainthesame.Forexample,if1.0mLof0.100Mstrongbase,e.g,NaOHisaddedtothebuffersolution,theaceticacidwouldreactwiththesodiumhydroxidetoform0.10mmolofacetate.The0.10mmolofacetateformedwouldhaveminimalaffectonthebuffersolution.CH3COOH+-OH→CH3COO-+H2O10.0mmol-0.10mmol0.10mmol0.10mmol0.10mmol[CH3COOH]=
[CH3COO-]=
0.101M> x mol0.10010 L
and0.099M> x mol0.10010 L
Ka = CH3COO−⎡⎣ ⎤⎦ H3O
+⎡⎣ ⎤⎦CH3COOH[ ] =
0.101 M ( ) x mol0.1010 L
⎛⎝⎜
⎞⎠⎟
0.099 M ( )
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2.0 x 10−5 = 0.101 M ( ) x mol
0.1010 L⎛⎝⎜
⎞⎠⎟
0.099 M ( )
2.0 x 10−5 x 0.099 = x 1.98 x 10−6 mol = x 1.98 x 10−6 mol
0.1010 = H3O
+⎡⎣ ⎤⎦
2.0 x 10−5 M = H3O
+⎡⎣ ⎤⎦ pH=-log[H3O+]pH=-log(2.0x10-5)pH=4.7If1mLofa0.10MofacidisaddedtothesystemthepHremains4.7,becausetheacetateneutralizestheacid.CH3COO-+H3O+→CH3COOH+H2O10.0mmol-0.10mmol0.10mmol0.10mmol0.10mmol[CH3COO-]=
[CH3COOH]=
Ka = CH3COO−⎡⎣ ⎤⎦ H3O
+⎡⎣ ⎤⎦CH3COOH[ ] =
0.099 M + x mol0.1010 L
⎛⎝⎜
⎞⎠⎟
x mol0.1010 L
⎛⎝⎜
⎞⎠⎟
0.101 M - x mol0.1010 L
⎛⎝⎜
⎞⎠⎟
0.099M> x mol0.1010 L
and0.101M> x mol0.1010 L
13
Ka = CH3COO−⎡⎣ ⎤⎦ H3O
+⎡⎣ ⎤⎦CH3COOH[ ] =
0.099 M( ) x mol0.1010 L
⎛⎝⎜
⎞⎠⎟
0.101 M ( )
2.0 x 10−5 = 0.099 M( ) x mol
0.1010 L⎛⎝⎜
⎞⎠⎟
0.101 M ( )
2.0 x 10−5 x 0.101 x 0.10100.099
= x mol
2.06 x 10−6 = x mol 2.06 x 10−6 mol
0.1010 L = H3O
+⎡⎣ ⎤⎦
2.0 x 10−5 mol L
= H3O+⎡⎣ ⎤⎦
pH=-log[H3O+]pH=-log(2.0x10-5)pH=4.7Thesaltofthecarboxylicacidisaconjugatebase,andtheconjugatebaseandthecarboxylicacidformthebuffer.Anequationforthisbuffersolutioncanbederived.ThisequationisreferredtoastheHenderson-Hasselbalchequation.TheHenderson-HasselbalchequationcanbeusedtodeterminethepHofabuffersolution.
Acidconjugatebase
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Theaceticacid/acetatebuffersolutionwherepH=4.7andthepKais4.7wouldbe
Therefore,inthisbuffersolutiontheconcentrationoftheconjugatebase,[conjugatebase],isequaltotheconcentrationoftheacid,[acid].LacticacidhasapKa=3.9.TheratiooflactateconcentrationtolacticacidconcentrationatpH=7.4canbecalculatedinthefollowingmannerusingtheHenderson-Hasselbalchequation.
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Thetotalconcentrationofacetatewouldbetheacetatefromthedissociationoflacticacidandtheconcentrationoftheacetatefromthe100%dissociationofsodiumacetate.
Therefore,theconcentrationofsodiumlactatewouldbe3,200timestheconcentrationoflacticacid.ThesedatamaybeusedtoprepareabuffersolutionwithadesiredpHfromaweakbaseanditsconjugateacidoraweakacidanditsconjugatebase.TheselectionofingredientstomakethebuffersolutiondependsonthedesiredpH.ThedesiredpHisdeterminedbyselectingthecorrectweakacid/conjugatebasepairortheweakbase/conjugateacidpair.Theselectionoftheappropriateacid-basepairwouldapproximatethethepHplusorminus1.Forinstance,toprepareabuffersolutionwithapH=4.5,thepKaoftheacidusedshouldbebetween3.5and4.5.Aceticacid-sodiumacetatesolutionwouldbeanappropriatebuffertouse,becausethepKaofaceticacidis4.7.Ifastudenthad100.0mLofa0.100Msolutionofaceticacidand100.0mLofa0.0500Msolutionofsodiumacetate,whatvolumeoftheweakacidsolutionwouldshehavetomixwiththeconjugatebasesolutioninordertoprepare50.0mLofabuffersolutionwithapHof4.5?Thesolutionisrelativelysimple.UsingtheHenderson-Hasselbalchequationinthefollowingformat:
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Thevolumeoftheacidandthebasemaybecalculatedinthefollowingmanner.Letxequalthevolumeof0.0500Mofthebaseneededtomake50.0mLofbuffer,withapHequalto4.5.Sincethetotalvolumedesiredis50.0mL,then50.0-xequalsthevolumeofacidneededtomake50.0mLofbuffer,withapHequalto4.5.Thenumberofmillimolesofsalt(conjugatebase)wouldequaltheconcentrationofsalttimesthevolumeofsalt,andthenumberofmolesofacidwouldequaltheconcentrationoftheweakacidtimesthevolumeoftheacid.
miilimolesofsalt=x(0.0500)mmolesmillimolesofacid=(50.0-x)0.100mmolesConsequently,themathematicalexpressionforsolvingxwouldbe
x,thevolume(inmL)of0.0500Msodiumacetate,wouldbe28mL50.0mL-28mL=22mL,thevolumeof0.100MaceticacidsolutionTherefore,adding28mLof0.0500Msodiumacetateto22mLof0.100MaceticacidsolutionwillcreateabuffersolutionwithapHequalto4.5.Thesevaluescanbecheckedbysubstitutingtheappropriatenumberofmillimolesper50.0mLintotheHenderson-Hasselbalchequation.
pH=4.5
pH = pKa + log Csalt
Cacid
⎡
⎣⎢
⎤
⎦⎥
millimoles of salt = VmLx Mmmol/mL
millimoles of acid = VmLx Mmmol/mL
4.5 = 4.7 + log
x(0.0500) mmol50.0 mL
(50.0-x) 0.100 mmol50.0 mL
⎡
⎣
⎢⎢⎢
⎤
⎦
⎥⎥⎥
pH = 4.7 + log
1.4 mmol50.0 mL2.2 mmol50.0 mL
⎡
⎣
⎢⎢⎢
⎤
⎦
⎥⎥⎥
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SoapsSoapsaresaltsoflongchainfattyacids.Thelongchainhydrocarbonportionofasoapisthehydrophobic(lipophilic)portionofthesoap,andthehydrophobicportionissolubleinnon-polarsubstancesandtheCOO-Na+group,thehydrophilicportionofthesoap,issolubleinwater.Sodiumstearateisanexampleofasoap.
sodiumstearate
Whenahydrophilicgroupandahydrophobic(lipophilic)groupareinthesamemolecule,themoleculeisdefinedasbeingamphiphilic.Whenthesaltofalongchainfattyacidisplacedinwater,acolloidaldispersioncalledmicelleisformed.Micellesareformedwhentheconcentrationofthecarboxylateexceedsacertainminimum(thecriticalmicelleconcentration).Micellesareaggregatesof50-100carboxylatemolecules(withdiametersofapproximately50angstroms).Thepolarpartofthemicelle,theCOO-Na+groups,isdirectedtotheoutsideofthemicelle.ThehydrocarbonportionisdirectedtowardtheinsideofthemicelleandisheldtogetherbyVanderWaalforces.Thesemicellesarespherical.Thearrangementcausesthecleansingactionofsoapswheredirtistrappedintheinteriorofthemicelle,andthedirtiswashedawayfromthewater.
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http://www.chemgapedia.de/vsengine/media/vsc/en/ch/12/oc/c_acid/fatty_acid/micelle_gif.gifDetergentsareanalogoustosoaps,butthehydrophilicendisOSO3-Na+insteadofCOO-Na+
sodiumstearylsulfateStrengthofacidsElectronwithdrawinggroupssuchashalogens,methoxy,nitro,andcyanoincreasethestrengthofcarboxylicacids.
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Acid Structure pKabutanoicacid CH3CH2CH2COOH
4.8
α-chlorobutanoicacid CH3CH2CH2CH(Cl)COOH 2.8β-chlorobutanoicacid CH3CH(Cl)CH2COOH
4.1
γ-chlorobutanoicacid ClCH2CH2CH2COOH
4.5
Acid Structure pKaAceticacid CH3COOH 4.7fluoroaceticacid FCH2COOH 2.6chloroaceticacid ClCH2COOH 2.9bromoaceticacid BrCH2COOH 2.9dichloroaceticacid Cl2CHCOOH 1.3trichloroaceticacid Cl3CCOOH 0.9Acid Structure pKaMethoxyaceticacid CH3OCH2COOH 3.6cyanoaceticacid NΞCCH2COOH 2.5nitroaceticacid NO2CH2COOH 1.7Electronreleasinggroupssuchasalkylgroupshaveatendencytodecreasethestrengthofacids. Acid Structure pKapropanoicacid CH3CH2COOH 4.92-methylpropanoicacid CH3CH(CH3)COOH 4.82,2-dimethylpropanoicacid CH3C(CH3)2COOH 4.1TheFieldEffectontheStrengthofanAcidThefieldeffectiscloselyrelatedtotheinductiveeffect.TheFieldEffectinvolvessolventsratherthanchemicalbonds.Theelectronegativityofthegroupattachedtothecarboxylicacidpolarizesthesurroundingsolventmoleculesandthispolarizationistransmittedthroughothersolventmoleculestothehydrogenatomattachedtothecarboxylicacidgroup.
20
TheFieldEffectaffectstheentropymorethantheenthalpy.Therelationshipbetweenenthalpyandentropyisgivenbyequation16.1.Equation16.1ΔGo=ΔHo-TΔSoInthefieldeffect,TΔSo>ΔHo,becauseΔHoisclosetozeroandinthesesystems.ΔSoisnegative;therefore-TΔSoispositiveThemorenegativetheΔSo,thelessspontaneousthesystem.Thelessorderedthesystem,themorenegativetheentropy.Themorenegativethevaluefortheentropy,themorepositivethevalueforTΔSo.ThemorepositivethevalueofTΔSo,themorepositivethevalueforΔGoandthelessspontaneousthedissociation.Consequently,themoreorderedthecarboxylicacidcausedbytheFieldEffect,thelessacidictheacidorthemoredisorderedthecarboxylicacidcausedbytheFieldEffect,themoreacidictheacid.IonizationofsubstitutedBenzoicacidsisaffectedbygroupsintheortho,para,andmetapositions.Theortho-position
ThefollowingtableliststhepKasofsomeortho-substitutedbenzoicacids.X pKaH 4.2CH3 3.9F 3.3Cl 2.9Br 2.8I 2.9CH3O 4.1NO2 2.2Themeta-positionThefollowingtableliststhepKasofsomemeta-substitutedbenzoicacids.
21
X pKaH 4.2CH3 4.3F 3.9Cl 3.8Br 3.8I 3.9CH3O 4.1NO2 3.5TheparapositionThefollowingtableliststhepKasofsomeortho-substitutedbenzoicacids.
DicarboxylicAcidsDicarboxylicacidshavetwodissociationconstants,Ka1andKa2.Forexample,malonicacid,anacidthathastwocarboxylicacidgroups,dissociatesintwosteps.Thefirstdissociationconstantisequalto1.48x10-3andtheseconddissociationconstantis2.04x10-6.
Ka1=1.48x10-3pKa1=2.83
22
Ka2=2.04x10-6pKa2=5.69ThepKa1ofdicarboxylicacidsissmallerthanthepKaofmonocarboxylicacidsduetostatisticalreasons,i.e.,therearetwocarboxylicacidgroupsfordicarboxylicacidscomparedtooneformonocarboxylicacids.PreparationofCarboxylicAcidsSynthesisofCarboxylicAcidsbytheOxidationofarenes:OxidationwithPotassiumPermanganate
orOxidationwithPotassiumDichromate
FollowingaremorecomprehensiveequationsthatrepresentaromaticsidechainoxidationusingKMnO4.Theoxidizingagenttransformsarenestoaromaticcarboxylicacids.
23
FollowingisabalancedmolecularequationfortheoxidationofarenesusingKMnO4.
FollowingaremorecomprehensiveequationsthatrepresentaromaticsidechainoxidationusingK2Cr2O7.Theoxidizingagenttransformsarenestoaromaticcarboxylicacids.
24
FollowingisabalancedmolecularequationfortheoxidationofarenesusingK2Cr2O7.
SynthesisofCarboxylicAcidsfromtheOxidationofPrimaryAlcohols
or
SynthesisofCarboxylicAcidsfromtheOxidationofaldehydes
or
25
SynthesisofaCarboxylicAcidfromGrignardreagents
FollowingisanexampleofthesynthesisofacarboxylicacidfromaGrignardreagent.
26
CarboxylicAcidscanbesynthesizedfromNitriles.Conversionofaprimaryalkylhalidetoacarboxylicacidwithanadditionalcarbonatomcanbeaccomplishedbyasubstitutionnucleophilicreactionbetweenaprimaryalkylhalideandsodiumcyanide.Acidhydrolysisoftheresultingnitrilewouldproducethedesiredcarboxylicacid.Forexamplecyclohexylaceticacidcanbesynthesizedinatwostepprocessbyanucleophilicsubstitutionreactionbetweenbromomethylcyclohexanewithsodiumcyanide.Acidificationoftheresultingnitrilewouldproducethedesiredcarboxylicacid.Step1
27
Step2
The acid hydrolysis of nitriles to produce carboxylic acids can be rationalized by the following six-step general mechanism and a specific mechanism involved in the synthesis of cyclohexylacetic acid from cyclohexylmethylcarbonitrile.(1)
(2)
28
(3)
(4)
(5)
29
(6)
ReactionsofCarboxylicAcidsCarboxylicacidswillreactwiththionylchloridetoformacylchlorides.
Carboxylicacidwillreactwithlithiumaluminumhydridetoproduceprimaryalcoholsbyatwo-stepprocess.Lithiumaluminumhydrideisdestroyedinwater;therefore,anaproticsolvent,e.g.tetrahydrofuran,isusedasasolventforthisreaction.Thefirststepistheformationoflithiumtetracyclopentylmethoxyaluminate
30
lithiumtetracyclopentylmethoxyaluminateThesecondstepinvolvesthehydrolysisofthelithiumtetraalkoxyaluminatecomlextoproducefourmolesofthedesiredalcohol.
CarboxylicacidsundergothereversibleFisherEsterificationreaction.
31
ThemechanismfortheFischerEsterificationreactionhasbeenresolvedusingkineticexperimentsandlabelingexperimentswith18Omethanol.WhenCH318OHreactswithacarboxylicacid,amethylesterisformed,andtheresultingmethylesterisrichin18O.Thefollowingsevenelementarystepsexplaintheformationofthemethylesterthatisrichin18O.(1) CH3
18 OH + H3O+! CH3
18 O+H2 + H2O (2)
(3)
(4)
(5)
(6)
32
(7)
IntramolecularEsterificationfollowsasimilarmechanism.Suggestamechanismforthefollowingreaction.
δ-valerolactone (IUPACNomenclature:5-pentanolide)Intramolecularesterificationformslactones,andlactonesarenamedbyreplacingthe-oicacidendingoftheparentcarboxylicacidby–olideandwiththenumberonthecarbonthatattackedthecarbonylcarbon.Forexample,thefollowingmoleculecanundergointramolecularesterificationtoproducetheδ-lactone5-pentanolide.
5-pentanolideReductionofγandδketoacidswillleadtoγandδlactonesasthemajorproducts.Thereactiontakesplaceinatwo-stepprocess.(1)
33
(2)
5-pentanolide (aδlactone)TheHell-Volhard-ZelinskyReactionTheHell-Volhard-Zelinskyreactionisusedtoprepare
OtherreagentsusedtoaccomplishthisreactionareBr2andphosphorus.Ammoniawilldisplacethebromineofthealphacarbonatomtoproducealphaaminoacids.
β-halocarboxylic acids
34
Followingaretheseriesofelementarystepsthatexplaintheformationofthealphabromocarboxylicacid,theproductoftheHell-Volhard-ZelinskyReaction.(1)
(2)
35
(3)
(4)
(5)
(6)
36
(7)
(8)
(9)
37
(10)
(11)
38
(12)
Anα-bromocarboxylicacid Thesumofthetwelveelementarystepsrationalizestheformationoftheα-bromocarboxylicacid,andexplainsthestoichiometryofthemolecularequation.
Analternatepathwaytothesynthesisofanα-bromocarboxylicacidistotreatthecarboxylicacidwiththionylchloride,followedbytreatmentwithN-bromosuccinimide,andfinallyfollowedbyhydrolysis.
39
DecarboxylationofDicarboxylicAcidsThemolecularstructureofmalonicacidcanundergodecarboxylationtoformcarbondioxideandtheenolformofaceticacid.Thisreactioncanbeaccomplishedbyheatingmalonicacidtoitsmeltingpoint(150oC).
Thefollowingtwostepsrepresentapathwayforthedecarboxylationofmalonicacidtoaceticacid.(1)LossofCarbonDioxide
40
(2)Tautometrism
Suggestasynthesisforthefollowingmoleculefromtheindicatedstartingmaterial.
heptanedioicacidcyclopenteneAnswerThesynthesiscanbeaccomplishedinsevensteps.Step1istheoxidativecleavageofcyclopentenetoformadicarboxylicacid,adipicacid.Thiscanbeaccomplishedwithpotassiumpermanganate.
Steps2and3involvethereductionofadipicacidtoformthelithiumaluminumalkoxidecomplexandthehydrolysisoftheresultinglithiumaluminumalkoxidetoform1,5-pentandiol.
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Step3isthehydrolysisofthelithiumaluminumalkoxide.
Step4istheformationofthe1,5-dihalopentanecompoindfrom1,5-pentandiol.
Step5ispreparationoftheGrignardreagentfrom1,5-dichloropentane.
Step6istheformationofsaltofthecarboxylicacidwithcarbondioxide.
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Thefinalstepisacidificationofthedicarboxylatetoformheptanedioicacid(pimelicacid).
heptanedioicacid(pimelicacid)
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Problems
CarboxylicAcids
1. Arrangethefollowingacidinorderofincreasingacidity:propanoicacid;2-fluoropropanoicacid,3-fluoropropanoicacid,2,2-difluoropropanoicacid,3,3-difluoropropanoicacid
2. Whichcompound,propanoicacidor2-phenylpropanoicacid,wouldhavethehigherpKavalue?Givearationaleforyourselection.
3. Suggestasynthesisforbenzoicacidfrombenzeneastheonlysourceoforganicstartingmaterialandanyothernecessaryinorganicmaterials.
4. Suggestasynthesisfor4-phenylbutanoicacidfrombenzeneandanyothernecessaryorganicandinorganicmaterials.
5. Whichofthefollowingcompoundswouldhavethehighestboilingpoint?Givearationaleforyouranswer.
(a) CH3(CH2)4CH3
(b) CH3(CH2)3CH2OH
(c) CH3(CH2)2COOH
(d) CH3(CH2)2CH2SH
6. Ifastudenthad100.0mLofa0.050Msolutionofaceticacidand100.0mLofa0.100Msolutionofsodiumacetate,whatvolumeoftheweakacidsolutionwouldshehavetomixwiththeconjugatebasesolutioninordertoprepare75.0mLofabuffersolutionwithapHof5.0?ThepKaforaceticacidis4.7.
7. BenzaldehydereactedwithchromicacidtoformacompoundthatreactedwithpropylalcoholinaDean-Starkapparatustoformacompoundthathasantimicrobialproperties.Thecompoundalsohasanuttyodorandanut-liketaste.Suggestastructuralformulaforthissweetandfruitytastingcompound.
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8. ListthefollowingacidsinorderincreasingpKavalues.
(a)
(b)
(c)
(d)
9. Suggestasynthesisforthefollowingfromthegivencompoundandanyothernecessaryinorganicandorganicmaterials.
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10. Suggestasynthesisforthefollowingfromtheindicatedstartingmaterialandanynecessaryinorganicreagents.
11. Suggestasynthesisforthefollowingfromtheindicatedstartingmaterialandanyothernecessaryinorganicororganicmaterials.
12. Suggestasynthesisforthefollowingfromtheindicatedstartingmaterialandanyothernecessaryinorganicororganicmaterials.
13.2,5-Diethyl-1,1-cyclopentanedicarboxylicacidwasisolatedas opticallyinactivecompoundA(aracemicmixture)andoptically inactivecompoundB.AandBhavedifferentmeltingpoints.
CompoundAyieldstwo2,5-diethylcyclopentanecarboxylicacidswhenitisheated.CompoundByieldsone2,5-diethylcyclopentanecarboxylicacidswhenitisheated.(a) SuggeststructuresforAandB(b) Suggeststructure(s)fortheproduct(s)formedfromheating compoundA.(c) Suggeststructure(s)fortheproduct(s)formedfromheating compoundB.(d) Writemechanismstoaccountfortheseobservations.