Acidic and Basic Character of Carboxylic Acids19-4

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 electron-withdrawing 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.

Carboxylic Acid Synthesis in Industry19-5

Formic acid and acetic acid are manufactured on a large scale industrially.

Other important industrial carboxylic acids include the two dicarboxylic acids:

• Hexanedioic acid Nylon

• 1,4-benzenedicarboxylic acid Plastics

Methods for Introducing the Carboxy Functional Group

19-6

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, RCN.

Nitrile hydrolysis is preferable to Grignard carbonation when the substrate contains other functional groups capable of reacting with the Grignard reagent (hydroxy, carbonyl, nitro).

Substitution at the Carboxy Carbon: the Addition-Elimination Mechanism

19-7

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.