Carboxylic Acids and Its Derivatives

1. Solubility in 5% NaHCO3

Carboxylic acids can be easily dissolved in a basic solution such as sodium bicarbonate (NaHCO3). This is a reaction-solubility test since NaHCO3 reacts with acetic acid and benzoic acid producing water-soluble salts, RCO2- Na+, thus giving off a positive result to the reaction.

Reaction of acetic acid with NaHCO3

Reaction of benzoic acid with NaHCO3

Carboxylates, ions which are negatively charged, are transformed as carboxylic acids that lose their hydrogen ion. Since they are ions, they become more soluble to the solution. The reaction also produces carbon dioxide where formation of bubbles is visible making it a stand-alone test for carboxylic acids.

2. Reaction with FeCl3

Acetic acid Formation of red-colored solution

Slightly acidic, pH 6

Benzoic acid Formation of flesh-colored precipitate

Slightly basic, pH 8

Acid derivatives react with nucleophiles to form a tetrahedral intermediate which, through loss of a leaving group, gives the product. In this process, acyl transfer occurs from the acid derivative to the nucleophile. The order of reactivity of acid derivatives is acyl halides> acid anhydrides> esters> amides. Acetic acid is an acyl halide that reacted with FeCl3 indicated by the formation of red-colored solution and being slightly acidic.

Reaction of acetic acid with ferric chloride

In the reaction above, the color of the complex interfere with the produced HCl. Higher acidity can interfere in the final results therefore should be made neutral before performing the test with ferric chloride solution. In the experiment, NaOH was used to neutralize the acid yielding sodium acetate and water.

Neutralization of acetic acid using NaOH

Reaction of sodium acetate with ferric chloride

Carboxylic acid, like benzoic acid gives color with FeCl3 due to formation of salts. To form salts, carboxylic acids are neutralized by bases in a manner similar to that of inorganic acids. Neutralization of carboxylic acid produces a carboxylic acid salt and water. In the experiment, dilute ammonia was used to neutralize benzoic acid. The formation of flesh-colored precipitate and a slightly basic solution was noted in the reaction with FeCl3.

Reaction of benzoic acid with ferric chloride

Most carboxylic acids are only partially dissociated in aqueous solution. Consequently, when the salt of a carboxylic acid is dissolved in water, it is partially hydrolyze to the undissociated acid and hydroxide ion (Hart, 1983). This hydrolysis can be readily demonstrated by testing the solution of a carboxylic acid salt with an indicator such as litmus paper.

3. Formation of Esters

Acetic acid + EtOH + H2SO4 Vinegar-like

Benzoyl chloride + EtOH + NaOH Sweet and minty smell

Esters can be formed by the reaction of alcohols with acids or their derivatives (Hart, 1983). One of the most important synthetic methods for preparing esters is by direct condensation of a carboxylic acid and an alcohol in the presence of a strong acid catalyst, known as the Fischer esterification ( McKinstry and Sapochak, 2000).

Reaction of acetic acid with ethyl alcohol

The addition of a small amount of strong acid is necessary to activate the carbonyl group of the carboxylic acid toward attack by the alcohol. In the absence of an acid catalyst, the reaction is very slow (McKinstry and Sapochak, 2000). Acetic acid reacted with ethyl alcohol and in the presence of an acid-catalyst, sulfuric acid, thus the formation of ethyl acetate indicated by the vinegar-like odor.

This reaction is also reversible. Each step of the reaction mechanism is in equilibrium. According to Le Chatlier's Principle, if equilibrium is disturbed, the components of the equilibrium will adjust to offset the disturbance. Therefore, a reversible reaction can be driven toward the desired product by either removing one of the products as it is formed, or by using an excess of one of the reagents. (McKinstry and Sapochak, 2000)

Reaction of benzoyl chloride with ethanol

Esters may also be formed from acyl halide through the Schotten-Baumann reaction. The Schotten-Baumann reaction is a method to synthesize amides from amines and acid chlorides. This reaction is also used to indicate the reaction between an acid chloride and an alcohol to form an ester. The reaction was first described in 1883 by German chemists Carl Schotten and Eugen Baumann. (Smith and March, 2001)

If you add an acyl chloride to an alcohol, you get a vigorous (even violent) reaction at room temperature producing an ester and clouds of steamy acidic fumes of hydrogen chloride (Clark, 2004). Benzoyl chloride has a -COCl group attached directly to a benzene ring. In the experiment benzoyl chloride was reacted with ethanol. The former is much less reactive than simple acyl chlorides like ethanoyl chloride therefore the reaction was not vigorous. Sodium hydroxide was added with vigorous shaking to increase the reactivity of the two compounds yielding ethyl benzoate. This was indicated by a sweet and minty smell.

4. Reducing Property of Some Acids

Formic acid + H2SO4 + KMnO4 Decolorization stops after 5 drops of KMnO4

Oxalic acid + H2SO4 + KMnO4 Decolorization continues after 30 drops of KMnO4

Formic acid acts as an oxidizing agent in reduction of some organic materials. Those involving sulfur compounds like, H2SO4 are among the most significant. Normal oxidizing agents like dichromate and permanganate are of course reduced also by formic acid. Formic acid shares an aldehyde-like reducing property. In the presence of certain acids, including sulfuric and hydrofluoric acids, a variant of the Koch reaction takes place instead, and formic acid adds to the alkene to produce a bigger carboxylic acid. Formic acid reduces when