Test for Carboxyl Group

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What is Carboxylic Acid?

Carboxylic acids are the 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 hydroxyl group. The carboxylic acids may be aliphatic or aromatic depending upon whether the -COOH group is attached to aliphatic alkyl chain or aryl group respectively.


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.


2. Action with Carbonates and Bicarbonates

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


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.


3. Formation of Ester

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


4.

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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. 

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6. Reaction with FeCl3

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

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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.

FAQ (Frequently Asked Questions)

Question: What is Carboxylic Acid?

Answer: Carboxylic acids are the compounds containing the carboxyl functional group in their molecules. The carboxylic acids may be aliphatic or aromatic depending upon whether the -COOH group is attached to aliphatic alkyl chain or aryl group respectively.

Question: Write the Tests that Can Show the Difference Between Alcohol and Carboxylic Acid.

Answer: 

(i) Acetic acid gives effervescence with NaHCO₃ due to libration of carbon dioxide.

Ethanol does not give effervescence with NaHCO₃.

(ii) Ethanol gives a yellow precipitate with an alkaline solution of iodine while acetic acid does not give this test.