[Chemistry Class Notes] on Test for Carboxyl Group Pdf for Exam

Carboxylic acids are compounds containing the carboxyl functional group in their molecules. The carboxyl group is made up of carbonyl and hydroxyl groups and therefore, the name carboxyl is derived from carbo (from carbonyl) and oxyl from the hydroxyl group. The carboxylic acids may be aliphatic or aromatic depending upon whether the -COOH group is attached to the aliphatic alkyl chain or aryl group respectively.

The functional groups that consist of a Carbonyl Group (C=O) along with a hydroxyl group (O-H) which is attached to the same carbon atom are known as carboxyl groups. The formula for Carboxyl Groups is-

-C(=O)OH

Acids with the presence of one carboxyl group are termed carboxylic acids and since they are proton-donors, they are also known as Bronsted-Lowry acids. Acids with the presence of two carboxyl groups are known as dicarboxylic acids and the ones with the presence of three carboxyl groups are known as tricarboxylic acids.

Salts and esters of carboxylic acids are known as carboxylates.

Though the IUPAC nomenclature of carboxylic acids is ‘oic acids’ in the suffix, ‘ic acids’ is used more commonly.

Qualitative Test for Carboxylic Acid

The following tests are performed for the identification of carboxylic acid.

1. Action with Blue Litmus

All carboxylic acids turn blue litmus red.

Procedure-

• Place the droplet of the liquid, solid or crystal on a moist blue litmus paper and observe the colour change.

• If the red colour changes to blue, it indicates the presence of a carboxylic group.

2. Action with Carbonates and Bicarbonates

Carboxylic acids decompose carbonates and bicarbonates evolving carbon dioxide with brisk effervescence. 

Carboxyl groups react with sodium hydrogen carbonate releasing carbon dioxide gas which can be identified by the effervescence produced. To distinguish carboxylic acids from phenols, this test can be used.

[RCOOH + NAHCO_3 rightarrow RCOONa + Co_2 uparrow + H_20]

Procedure-

• Take one ml of organic liquid in a test tube and add a pinch of sodium bicarbonate [(NaHCO_{3} )] to it.

• If carboxylic acid is present in the organic compound, a brisk effervescence is observed.

3. Carboxylic Acid NaHCO3 Mechanism

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The reaction of carboxylic acids with aqueous sodium carbonate solution leads to the evolution of carbon dioxide producing brisk effervescence. However, most phenols do not produce effervescence with an aqueous solution of sodium bicarbonate. Therefore, this reaction may be used to distinguish between carboxylic acids with sodium bicarbonate or sodium carbonate, the carbon dioxide evolved comes from Na₂CO₃ or NaHCO₃ and does not form a carboxyl group.

4. Formation of Ester

When carboxylic acids are heated with alcohols in the presence of concentrated sulphuric acid and hydrochloric acid, esters are formed. The reaction is reversible and is called esterification.

Carboxylic acids react with alcohol when there is a presence of sulphuric acid to form an ester that has a fruity smell.

Procedure-

• To 0.1 g of the organic compound add 1 ml ethyl alcohol and one or two drops of concentrated sulphuric acid in a test tube. After heating the mixture in a water bath for about five minutes, pour it into a beaker that has water. If a fruity smell is observed, it indicates the presence of the carboxyl group in the organic compound. 

[RCOOH + C_2H_5OH rightarrow COOC_2H_5 + H_2O]

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5. Fluorescein Test

This test is used for the identification of the dicarboxylic group. When the dicarboxylic compound is heated, it produces acid anhydride. This anhydride reacts with resorcinol in the presence of Conc. H₂SO₄ and produces a fluorescent dye. 

Procedure-

• Take a small amount of organic compound and heat it with resorcinol and one or two drops of concentrated sulphuric acid in a clean and dry test tube. 

• After a few minutes, the solution turns dark-brown and a liquid is formed.

• Add a few drops of this solution to a dilute NaOH solution.

• If the solution turns red with green fluorescence, it indicates the presence of dicarboxylic acid.

6. Reaction with FeCl₃

Some carboxylic acids give precipitates when they react with iron trichloride. For example, acetic acid gives puff coloured precipitate.

Did You know?

• Methanoic acid is used in leather tanning

• Methanoic acid is used as an antiseptic.

• Benzoic acid and some of its salts are used as urinary antiseptics.

• Carboxylic acid esters are used in perfumery.