Download Acidic and Basic Character of Carboxylic Acids

19-4
Acidic and Basic Character of Carboxylic Acids
Carboxylic acids are relatively strong acids.
Carboxylic acids have much lower pKa values than do alcohols.
The lowered pKa values are due to the electron-withdrawing effect of the
positively polarized carbonyl carbon and the resonance stabilization of the
carboxylate group.
Two of the three resonance forms of the carboxylate ion are equivalent, leading to
a symmetrical ion with equal carbon-oxygen bond lengths (1.26 Å), midway
between a carbon-oxygen double bond (1.20 Å) and a carbon-oxygen single bond
(1.34 Å).
Electron-withdrawing substituents increase the acidity of carboxylic
acids.
The inductive effect of electron-withdrawing groups close to the carboxy group
causes an increase in acidity.
Three electron-withdrawing groups on the -carbon sometimes results in acidity
near that of some inorganic acids.
The dioic acids have two pKa values.
In ethanedioic and propanedioic acids, the first pKa is lowered by the electronwithdrawing effect of the second.
In higher dioic acids, both pKa values are close to monocarboxylic acids.
Carboxylate salts of carboxylic acids can be prepared by treatment of the acid with
a base, such as NaOH, Na2CO3 or NaHCO3. These salts are much more water
soluble than the corresponding acids.
Carboxylate salts are named by specifying the metal and then replacing “ic acid”
with “ate”.
Carboxylic acids may be protonated on the carbonyl oxygen.
The carbonyl oxygen of a carboxylic acid may be protonated by strong acids to give
alkyloxonium ions.
The carbonyl oxygen is more basic than the –OH group of alcohols due to
resonance stabilization of the alkyloxonium ion.
Note that the protonation reaction is not particularly strong.
19-5
Carboxylic Acid Synthesis in Industry
Formic acid and acetic acid are manufactured on a large scale industrially.
Other important industrial carboxylic acids include the two dicarboxylic acids:
•Hexanedioic acid
•1,4-benzenedicarboxylic acid
Nylon
Plastics
19-6
Methods for Introducing the Carboxy Functional
Group
Oxidation of primary alcohols and of aldehydes furnishes carboxylic
acids.
Primary alcohols oxidize first to aldehydes, which then may further oxidize to
carboxylic acids.
Oxidants include CrO2, KMnO4 and HNO3.
Nitric acid is often chosen as the oxidant because it is one of the cheapest strong oxidants.
Organometallic reagents react with carbon dioxide to give carboxylic
acids.
Carbonation, or reaction of an organometallic reagent with CO2 (dry ice),
produces a carboxylate salt, which yields a carboxylic acid upon protonation in
aqueous acid.
A two step synthesis allows the conversion of an alkyl halide into the
corresponding carboxylic acid having one more carbon.
Nitriles hydrolyze to carboxylic acids.
A second method for preparing a carboxylic acid with an additional carbon is through the
synthesis and hydrolysis of a nitrile, RCN.
Nitrile hydrolysis is preferable to Grignard carbonation when the substrate
contains other functional groups capable of reacting with the Grignard reagent
(hydroxy, carbonyl, nitro).
19-7
Substitution at the Carboxy Carbon: the
Addition-Elimination Mechanism
The carbonyl carbon is attacked by nucleophiles.
Carboxylic acids and their derivatives of the form RCOL (L = leaving group) can be
attacked by nucleophiles.
Unlike the reactions with aldehydes and ketones, the attacking nucleophile
displaces the leaving group resulting in an addition-elimination reaction.
An addition-elimination reaction proceeds through a tetrahedral intermediate.
Addition-elimination is catalyzed by acid or base.
Acid catalysis of an addition-elimination reaction proceeds by initial protonation
of the carbonyl group and subsequent protonation of the leaving group.
Base catalysis proceeds by deprotonating the nucleophile.