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Chemistry 618B
Nomenclature Nomenclature -- IUPACIUPAC• IUPAC names: drop the -e from the parent
alkane and add the suffix -oic acid• If the compound contains a carbon-carbon
double bond, change the infix -an- to -en-
Propenoic acid (Acrylic acid)
trans-3-Phenylpropenoic acid
(Cinnamic acid)
trans-2-Butenoic acid
(Crotonic acid)
CCO2 H
CH
H3 C H
CH2 =CHCO 2 H CCO2 H
CH
C6 H5 H
Chemistry 618B
NomenclatureNomenclature--CommonCommon
• When common names are used, the letters α, β, γ, δ, etc. are often used to locate substituents
Alanine)(αααα-Aminopropionic acid;(γγγγ-Hydroxybutyric acid)2-Aminopropanoic acid4-Hydroxybutanoic acid
4 3 2 15
δδδδ γγγγ ββββ ααααO
HOCH2 CH2 CH2 CO2 H
C-C-C-C-C-OH
CH3 CHCO2 HNH2
Chemistry 618B
Benzoic acid Benzoic acid Sodium benzoateSodium benzoate
Butyric acidButyric acid Ammonium butyrateAmmonium butyrate
Naming the SaltsNaming the Salts
• To name the salt of the carboxylic acid, name the cation followed by the name of the anion (two words).
• The anion is named by removing -oic acid and adding ate
Boiling PointsBoiling Points
• Intermolecular forces, especially hydrogen bonding, are stronger in carboxylic acids than in other compounds of similar shape and molecular weight
bpbp (1(1 atmatm)) 31°C31°C 80°C80°C 99°C99°C
OH
141°C141°COH
OO
Chemistry 618B
Physical PropertiesPhysical Properties• In the liquid and solid states, carboxylic
acids are associated by hydrogen bonding into dimeric structures
δ- δ+
δ-δ+
CO H O
COHO
CH3H3C
Chemistry 618B
Physical PropertiesPhysical Properties• Carboxylic acids have significantly higher boiling
points than other types of organic compounds of comparable molecular weight– they are polar compounds and form very strong
intermolecular hydrogen bonds
• Carboxylic acids are more soluble in water than alcohols, ethers, aldehydes, and ketones of comparable molecular weight– they form hydrogen bonds with water molecules
through both the C=O and OH groups
Chemistry 618B
Physical PropertiesPhysical Properties• Water solubility decreases as the relative
size of the hydrophobic portion of the molecule increases
hydrophobic region; decreases water solubility
hydrophilic region; increases water solubility
C HOCR
O
CH2 O C R
O
CH2 O C
O
R NaOHC OHH
CH2 OH
CH2 OH
glycerolFat
C
O
O- Na+R
Fatty Acid Salt "Soap"
R = CH3(CH2)16COOH Stearic AcidCH3(CH2)7CH CH(CH2)7COOH Oleic acid
etc.
SaponificationSaponification of Fatof Fat
Soaps and DetergentsSoaps and Detergents
CH3(CH2)16CO O - N a+
CH3(CH2)11 SO
OO'Na+
CH2 NCH3
CH3
(CH2)15CH3
Cl
Soap
Anionic detergent
Cationic Detergent
Surface Active Agents = surfactants
•• A carboxylic acid is characterized by peaks due toA carboxylic acid is characterized by peaks due toOH and C=O groups in its infrared spectrum.OH and C=O groups in its infrared spectrum.
•• C=O stretching gives an intense absorptionC=O stretching gives an intense absorptionnear 1700 cmnear 1700 cm--11..
•• OH peak is broad and overlaps with COH peak is broad and overlaps with C——H H absorptions.absorptions.
Infrared SpectroscopyInfrared Spectroscopy
Chemistry 618B
2000200035003500 30003000 25002500 1000100015001500 500500Wave number, cmWave number, cm--11
Infrared Spectrum of 4Infrared Spectrum of 4--PhenylbutanoicPhenylbutanoic acidacid
C=O
O—H and C—H stretch
monosubstitutedbenzene
C6H5CH2CH2CH2CO2H
11H NMR of Carboxylic acidsH NMR of Carboxylic acids
The acidic proton in the HOThe acidic proton in the HO-- croup of a croup of a carboxylic acid is normally the least carboxylic acid is normally the least shielded of all protons in ashielded of all protons in a 11HH nmr nmr spectrum: (d 10spectrum: (d 10--12 12 ppmppm; broad)…; broad)…
Chemical shift (Chemical shift (δδ,, ppmppm))
CCHH22CCHH22CCHH22COCOHH
OO
01.02.03.04.05.06.07.08.09.010.011.012.0
1313C NMR of Carboxylic acidsC NMR of Carboxylic acids
The Carbonyl carbon on the carboxylic acid The Carbonyl carbon on the carboxylic acid group is at low field (group is at low field (δδ 160160--185185 ppmppm), but not ), but not quite as quite as deshieldeddeshielded as the carbonyl carbon of as the carbonyl carbon of anan aldehydealdehyde or ketone (or ketone (δδ 190190--215215 ppmppm). ).
Chemistry 618B
Mass SpectrometryMass Spectrometry– The McLafferty rearrangement gives a
characteristic peak at m/z = 60
•+
•+
+
McLafferty rearrangement
m/z 60
HH2 C
H2 CCH2
C
O
COH
O
CH2
HOHH2 C
H2 C
Chemistry 618B
Acidity Acidity •• Carboxylic acids are weak acidsCarboxylic acids are weak acids
– The pKa of typical aliphatic and aromatic carboxylic acids falls within the range 4 to 5
R CO
OHC
O
O-R + H+
R CO-
O
•The greater acidity of carboxylic acids relative to alcohols, both of which have oxyanions conjugate bases is because:
•the carboxylate anion is stabilized by resonance
Chemistry 618B
Review definitions of:Review definitions of:
KaKapKapKa[H[H++]]pKapKa
Assuming equal molar concentations, which produces the solution with the lower pH, an acid with high or low pKa??
Chemistry 618B
AcidityAcidity• Electron-withdrawing substituents near the
carboxyl group increase acidity through their inductive effect
2.903.184.76 2.86 2.59
CH2 CO2 HCH2 CO2 H CH2 CO2 HCH2 CO2 HCH2 CO2 HFClBrH I
Acid Strength
Chemistry 618B
AcidityAcidity• Substitution by multiple electron-
withdrawing groups further increases acidity
Chloroacetic Trichloroacetic Dichloroacetic Acetic
pKa
: 2.86 0.701.484.7 6
H3CCO2H H2ClCCO2H HCl2CCO2H Cl3CO2H
Acid Strength
Chemistry 618B
AcidityAcidity
pKa: 4.52 3.98 2.83
• The inductive effect of an electron-withdrawing substituent falls off rapidly with its distance from the carboxyl group
4-Chlorobutanoic 3-Chlorobutanoic 2-Chlorobutanoic
Cl Cl Cl
CH2 CH2 CH2 CO2 H CH3 CH2 CHCO2 HCH3 CHCH2 CO2 H
Acid Strength
Chemistry 618B
The key to understanding acid The key to understanding acid strength is in being able to strength is in being able to
understand the stability of the understand the stability of the conjugate base, the anionconjugate base, the anion
R CO
OHC
O
O-R + H+
R CO-
O
Chemistry 618B
Reactions of AcidsReactions of Acids
• Reduction• Decarboxylation• Esterification• Formation of Acid Halides
Chemistry 618B
ReductionReduction• The carboxyl groups is one of the organic
functional groups most resistant to reduction– it is not affected by catalytic hydrogenation
under conditions that easily reduce aldehydesand ketones to alcohols, and reduce alkenes and alkynes to alkanes
– it is not reduced by NaBH4
Chemistry 618B
Reduction by LiAlHReduction by LiAlH44
• Lithium aluminum hydride reduces a carboxyl group to a 1° alcohol– reduction is carried out in diethyl ether, THF, or
other nonreactive, aprotic solvent
1 . LiAlH 4 , ether2 . H 2 O
COH
O
+ +LiOH Al(OH) 3CH2 OH
Chemistry 618B
Selective ReductionSelective Reduction
• Using the less reactive NaBH4, it is possible to reduce the carbonyl group of an aldehyde or ketone without affecting a carboxyl group
OOHCHCH2 CH2 CH2 COHCCH2 CH2 CH2 COH
O O1 . NaBH4
2 . H 2 O
Chemistry 618B
DecarboxylationDecarboxylation•• DecarboxylationDecarboxylation:: loss of COloss of CO22 from a from a
carboxyl groupcarboxyl group
• Most carboxylic acids, if heated to a very high temperature (fried), undergo thermal decarboxylation
• Most carboxylic acids, however, are quite resistant to reasonable heat and melt or even boil without decarboxylation
Chemistry 618B
DecarboxylationDecarboxylation•• Exceptions are carboxylic acids that have a Exceptions are carboxylic acids that have a
carbonyl group beta to the carboxyl groupcarbonyl group beta to the carboxyl group– this type of carboxylic acid undergoes
decarboxylation on mild heating
Acetone3-Oxobutanoic acid (Acetoacetic acid)
+warmββββ ααααO O O
CH3 -C-CH2-C-OH CH3 -C-CH 3 CO2
Chemistry 618B
DecarboxylationDecarboxylation•• DecarboxylationDecarboxylation occurs readily if there is any occurs readily if there is any
carbonyl group beta to the carboxylcarbonyl group beta to the carboxyl• Malonic acid and substituted malonic acids, for
example, also undergo thermal decarboxylation
Propanedioic acid (Malonic acid)
+140-150°CO O O
HOCCH2 COH CH3 COH CO2
Chemistry 618B
DecarboxylationDecarboxylation•• ThermalThermal decarboxylationdecarboxylation of a of a ββ--ketoacidketoacid
involves rearrangement of six electrons in a involves rearrangement of six electrons in a cyclic sixcyclic six--memberedmembered transition state transition state
(A cyclic six-membered transition state)
CH
O
C C
OH
H3 C OH
enol of a ketone
HO
CC
C
O
H3 C OH H
+
O
CH3 -C-CH 3 CO2
Chemistry 618B
DecarboxylationDecarboxylation•• ThermalThermal decarboxylationdecarboxylation ofof malonicmalonic acids acids
also involves rearrangement of six electrons also involves rearrangement of six electrons in a cyclic sixin a cyclic six--memberedmembered transition statetransition state
enol of a carboxylic acid
CHH
CH
CO
CHO
HOO
H
CO
CHO
OOH
+O
CH3 -C-OH CO2
Chemistry 618B
FischerFischer EsterificationEsterification•• Esters can be prepared by treatment of a Esters can be prepared by treatment of a
carboxylic acid with an alcohol in the presence of carboxylic acid with an alcohol in the presence of an acid catalyst, commonly Han acid catalyst, commonly H22SOSO44 or gaseousor gaseous HClHCl
Ethanol (Ethyl alcohol)
Ethanoic acid (Acetic acid)
+
OCH3 COH CH3 CH2 OH
H2 SO 4
Ethyl ethanoate (Ethyl acetate)
+
OCH3 COCH2 CH3 H2 O
Chemistry 618B
FischerFischer EsterificationEsterification• The key intermediate in Fischer esterification is
the tetrahedral addition intermediate formed by addition of ROH to the C=O group
••
••
••••
••••
C OCH3
OH
O
R
H
••
••
•• ••
C OH
O
R ••
••
•• ••
C OCH3
O
R••
••HOCH3+ ••
••HOH+
H+ H+
tetrahedral carbonyl addition intermediate
Chemistry 618B
Reaction of Esters with OH Reaction of Esters with OH --• Hydrolysis of an esters is aqueous base is called
saponification• Each mol of ester hydrolyzed requires 1 mol of base; for
this reason, ester hydrolysis in aqueous base is said to be “base-promoted” (not catalyzed)
• Hydrolysis of an ester in aqueous base involves formation of a tetrahedral carbonyl addition intermediate followed by collapse and proton transfer (Nucleophilic acyl substitution)
++
O ORCOCH3 NaOH RCO- Na + CH3 OH
H2 O
Chemistry 618B
SaponificationSaponification• Base-promoted ester hydrolysis• Irreversible due to deprotonation of the
acid
••
••
••
••
••
••
••
••
••
••
••
••
••
•• ••
••
••
••
••
•• ••
••
••
••
••
•• ••
••++
-
+
O O
OH
O CH 3
O
O H HO
O
R-C-O-CH 3 OH R-C
R-C O-CH 3 R-C O-CH 3
--
-
-
Chemistry 618B
Hydrolysis of AmidesHydrolysis of Amides
Hydrolysis of amides is also irreversible. In acid Hydrolysis of amides is also irreversible. In acid solution the amine product issolution the amine product is protonatedprotonated to to give an ammonium salt.give an ammonium salt.
++ R'R'NNHH33++
RCOHRCOH
OO
RCRCNNHR'HR'
OO
++ HH22OO HH++++
Chemistry 618B
GrignardGrignard ReagentsReagents• Addition to CO2 gives a carboxylic acid
• This is a great way to add a carbon and make an acid
Mg+2H3O+
CH3CH2- MgBr+ CH3CH2 C O- MgBr+
O
+THF dil.
CH3CH2 C OH
O
C OO
Chemistry 618B
DiazomethaneDiazomethane
• Diazomethane, CH2N2, a potentially explosive, toxic yellow gas, is best drawn as a hybrid of two contributing structures
• Treatment of a carboxylic acid with diazomethane gives a methyl ester
++ether
Diazomethane A methyl ester
O OCH2 N2RCOH RCOCH3 N2
•• •••••• + +H
H
C N N
H
N NCH
Chemistry 618B
DiazomethaneDiazomethane• Esterification occurs in two steps
Step 1: proton transfer from the carboxyl group to diazomethane
••
••
••
•• ••
••
••
++
++
A carboxylate anion
R C O H
O
N NO
NC OR N
CH2
CH3
••