CARBOXYLIC ACIDS
STRUCTURE
They are made of a carbonyl group (C=O) and a hydroxyl group (OH)
bonded to the same carbon.
Carboxyl carbon is sp2 hybridized. Bond angles are close to 120.
Often represented as RCOOH, RCO2H, or ArCOOH.
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NOMENCLATURE
A. Common Names
Many aliphatic acids have historical names (based on Latin).
Positions of substituents on the chain are labelled with Greek letters.
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B. IUPAC
Find longest chain with the –COOH group as the parent.
Drop the –e in parent name, replace with –oic acid.
The carbon of the carboxyl group is numbered carbon 1.
methanoic acid
propanoic acid
2-hydroxypropanoic acid
2,3-dibromo-3-phenylbutanoic acid
pentanedioic acid
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C. Cyclic Acids
Compounds with –CO2H bonded to a ring are named as alkane carboxylic acids.
Aromatic acids are named as benzenecarboxylic acid (IUPAC) or as benzoic acids (common).
cyclohexanecarboxylic acid
2-cyclohexenecarboxylic acid
3-chloro-2-nitrobenzenecarboxylic acid
3-chloro-2-nitrobenzoic acid
benzenecarboxylic acid
benzoic acid
2-chlorobenzenecarboxylic acid
2-chlorobenzoic acid
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D. Acid Salts
Name the cation, then name the anion by replacing the –oic acid with –oate.
sodium ethanoate or sodium acetate
ammonium butanoate or ammonium butyrate
magnesium propanoate or magnesium propionate
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PHYSICAL PROPERTIES
Carboxylic acids have polar C–O and O–H bonds. They form
intermolecular hydrogen bonds, existing as cyclic dimers held
together by two hydrogen bonds.
Carboxylic acids have higher boiling points and melting points than other compounds of
comparable molar mass.
Water solubility decreases with the length of the carbon chain. Up to 4 carbons, acid is miscible
in water.
Also soluble in relatively nonpolar solvents like chloroform because it dissolves as a dimer.
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ACIDITY OF CARBOXYLIC ACIDS
Carboxylic acids have mildly acid protons.
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They are the most acidic class of compounds that contain only C, H and O. They are weak acids
when compared to strong ones (HCl, H2SO4).
Ka
pKa
H2SO4
∞
−10
HCl
∞
−7
1.8 x 10−4
3.75
HCOOH
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The acidity of carboxylic acids is due to stability of the conjugate base, which is resonancestabilized.
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The presence of electron-withdrawing substituents has an impact on acidity of carboxylic acids.
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PREPARATION OF CARBOXYLIC ACIDS
A. Oxidative Methods
1° alcohols can be oxidized to a carboxylic acid using chromic acid (H2CrO4), which can be
formed either from chromium trioxide (CrO3) or from sodium dichromate (Na2Cr2O7) in aqueous
acidic solution.
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Aldehydes are particularly sensitive to oxidation. A variety of oxidizing agents can be used,
including CrO3, Na2Cr2O7, K2Cr2O7, and KMnO4.
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They can also oxidized selectively in the presence of other functional groups using silver(I) oxide
in aqueous ammonium hydroxide (Tollens reagent).
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Alkylbenzenes can be converted to aromatic carboxylic acids using dichromate or permanganate
reagents, R group must have at least one H atom on C atom attached to the ring.
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B. Addition of Carbon Dioxide to a Grignard Reagent
CO2 (g) can be passed through the Grignard solution, or CO2 (s) can be slowly added to flask
containing Grignard reagent.
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How would you prepare benzoic acid from benzene?
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C. Hydrolysis of Nitriles
Acidic or basic hydrolysis of a nitrile (cyano compound) produces a carboxylic acid.
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An example
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How can you use this synthesis to make 2-phenylethanoic acid from toluene?
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Br2
light
NaCN
REACTIONS OF CARBOXYLIC ACIDS
A. With Bases
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B. Reduction
Reduction of carboxylic acids to 1° alcohols using lithium aluminum hydride – not the weaker
sodium borohydride.
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C. Hell-Volhard-Zelinsky (HVZ) reaction
Alpha-halogenation of the carboxylic acid using elemental phosphorus (or PCl3) as a catalyst;
halogen may be Cl2 or Br2.
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Synthetically useful, halogen can then be replaced in a nucleophilic substitution reaction.
CH3CH2CH2COOH
Br2
CH3CH2CH COOH
P
Br
CH3CH2CH COOH
1. OH -, heat
2. H 3O+
Br
aq NH 3
aq KI
CH3CH2CH COOH
CH3CH2CH COOH
OH
I
CH3CH2CH COOH
NH2
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D. Electrophilic Aromatic Substitution
EAS on benzoic acid occurs more slowly than on benzene; -COOH being a deactivating group and
a meta director.
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E. Conversion to Functional Group Derivatives
Important part of carboxylic acid chemistry (next...).
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SPECTROSCOPY
A. Mass spectrometry
Parent peak is normally easy to identify.
Common fragmentation patterns are α-cleavage of the carboxyl group to give [COOH]+.
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B. IR spectroscopy
C=O stretch at 1710-1760 cm−1 (free monomer: 1760 cm−1; dimer: 1710 cm−1).
O-H stretch at 2500-3300 cm−1 (broad).
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C. UV-Visible spectroscopy
Weak absorption (200 nm), in the absence of any other active functional groups.
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D. Proton NMR
The CO2H proton absorbs as singlet near 12 δ.
The CO2H proton can be replaced by deuterium (with D2O), causing signal to disappear.
The CO2H group affects the chemical shift value of neighbouring H atoms.
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E. Carbon NMR
Carboxyl carbon atoms absorb at 165-185 δ (strongly deshielded absorption).
δ value higher for carbonyl carbon of aldehyde or ketone.
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