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CarboxylicAcids

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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

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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.

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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

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Ka2=2.04x10-6pKa2=5.69ThepKa1ofdicarboxylicacidsissmallerthanthepKaofmonocarboxylicacidsduetostatisticalreasons,i.e.,therearetwocarboxylicacidgroupsfordicarboxylicacidscomparedtooneformonocarboxylicacids.PreparationofCarboxylicAcidsSynthesisofCarboxylicAcidsbytheOxidationofarenes:OxidationwithPotassiumPermanganate

orOxidationwithPotassiumDichromate

FollowingaremorecomprehensiveequationsthatrepresentaromaticsidechainoxidationusingKMnO4.Theoxidizingagenttransformsarenestoaromaticcarboxylicacids.

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FollowingisabalancedmolecularequationfortheoxidationofarenesusingKMnO4.

FollowingaremorecomprehensiveequationsthatrepresentaromaticsidechainoxidationusingK2Cr2O7.Theoxidizingagenttransformsarenestoaromaticcarboxylicacids.

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FollowingisabalancedmolecularequationfortheoxidationofarenesusingK2Cr2O7.

SynthesisofCarboxylicAcidsfromtheOxidationofPrimaryAlcohols

or

SynthesisofCarboxylicAcidsfromtheOxidationofaldehydes

or

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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.

41

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.