carboxylic acids, building bridges to knowledge
DESCRIPTION
This paper discusses carboxylic acids and the important role of the "COOH" functional group in organic chemistry. Also, the paper discusses the nomenclature of carboxylic acids, the syntheses of carboxylic acids, and the reactions of Carboxylic acids with their associated mechanisms.TRANSCRIPT
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.
3
(E)-2-hexenoicacid
(E)-4-octenoicacidSeveralorganicacidsarefoundinnature.Formicacid,foundinants,canbeobtainedfromthedestructivedistillationofants.Aceticacidisformedinsouringwine.Butyricacidisformedinrancidbutter.Malicacidisformedinapples.Oleicacidisfoundinolives.
Malicacid
Oleicacid (Z)-9-decaoctenoicacid
4
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
⎛⎝⎜
⎞⎠⎟
7
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.
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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.