Carboxylic Acids

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Carboxylic Acids
carboxyl group
Nomenclature see text Properties High
Boiling Points due to H-bonding
dimer
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Melting points - high Most acids with gt8
carbons are solids at room temperature Solubility
in water Acids lt 5 carbons are miscible
in water Acids gt 5 carbons exhibit limited
solubility
Acidity of Carboxylic Acids
?
?
carboxylate anion
pKa 3-5 Compare alcohols pKa 15-19
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Conclusion Carboxylic acids, although
considered weak acids, are much more acidic than
alcohols. Why? The carboxylate anion is
resonance- stabilized, favoring its formation.
?
?
AM1 calculations reveals both charges are
0.59, i.e. the charge is equally shared by the
two oxygens.
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Substitutent Effects on Acidity
weakest
strongest
pKa 4.74 2.86
1.26 0.64
Why this trend? Cl stabilizes the carboxylate
anion by withdrawing
electrons by induction, thus favoring its
formation.
?
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Consider these aromatic acids. Rank their
acidities.
pKa 3.41 3.98
4.19 4.36
Synthesis of Carboxylic Acids

• Oxidation of 10 alcohols
• 2. Oxidative cleavage of alkenes using KMnO4

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3. Side Chain Oxidation
?
?
Cleavage is between the ? and ? carbons
4. Carboxylation of Grignard reagents.
?
?
?- ? ?- ?- ? ?-
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5. Hydrolysis of Nitrile
Mechanism
?
?
?
?
?
-H
?
NH3
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Reactions of Acids 1. Formation of Acid chloride
This reaction is important because acid chlorides
are easily converted to a variety of other
products (next chapter). The mechanism is
similar to the reaction with alcohols to alkyl
chlorides (Chapter 17).
2. Esterification with Alcohols
This reaction is reversible. Water or ester is
removed from the reaction mixture to favor the
forward reaction (le Chatliers principle).
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Mechanism
?
?
?
?
3. Esterification with diazomethane to prepare a
methyl ester.
?
?
?
?
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4. Amide Synthesis
..
?
?
..
5. Reduction with LiAlH4 (LAH)
10 alcohol
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Mechanism
The first step is removal of the acidic proton by
hydride, a strong base.
?
?
?
?
Second step is hydride attack on the carbonyl
group, followed by aldehyde formation.
?
?
?
?
?
?
?
?
?
?
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6. Formation of Ketone
Mechanism
?
?
?
?
?
?
Hydrate - unstable
H2O
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C4H10O
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C4H8O
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C7H8O2
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C8H8O2