In organic chemistry, identifying different functional groups helps us understand what a compound is and how it behaves. One such important group is carboxyl group (COOH), which is found in many organic acids like acetic acid and citric acid. Testing for the carboxyl group is a basic but important part of lab work, especially in school or entrance exam preparation. It helps students learn how to recognize acids through simple experiments and understand their chemical nature.

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What is Carboxyl Group?

A Carboxyl Group is a special part of a molecule made up of a carbon atom double bonded to an oxygen atom and also bonded to a hydroxyl group(OH). It’s written as COOH. This group is what makes a compound behave like an acid. You can find carboxyl groups in many everyday substances like vinegar (acetic acid) and citrus fruits (citric acid). They play a big role in both chemistry and biology, as they affect how molecules react and interact.

Testing for the carboxyl group helps us know if a substance is an acid. It’s a common step in lab experiments, especially in school or college. These tests are useful because many important compounds like organic acids contain the COOH group. By doing simple tests, we can confirm if compounds like organic acids contain the COOH group. By doing simple tests, we can confirm if a compound has this group and better understand its properties. This is also helpful in preparing for practical exams and entrance tests.

Common Test to Identify Carboxyl Groups

There are a few easy and reliable tests to check if a substance has a carboxyl group. These tests are usually done using simple chemicals like sodium bicarbonate, litmus paper, or alcohol with acid. Each test gives a clear sign like fizzing, a color change, or a fruity smell that helps confirm the presence of the COOH group. These methods are often used in school labs and are helpful for learning and exam preparation.

Objective:

The goal of this experiment is to detect the carboxylic functional group in a given organic compound.

Background:

Carboxylic acids tend to donate protons, behaving as acids. This property is crucial in the identification of a -COOH group .

The carboxylic functional group can be detected using any of the following tests:

• Litmus test
• Sodium bicarbonate test (or) Sodium hydrogen carbonate test
• Ester test
• Fluorescein test

(a) Litmus Test:

Carboxylic acids turn blue litmus paper red, as the hydroxyl group in -COOH is more acidic than in alcohol.

The chemical reaction is as follows:

\mathrm{R}-\mathrm{COOH}+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{R}-\mathrm{COO}-+\mathrm{H}_3 \mathrm{O}

Note: If the blue litmus paper turns red, a carboxylic group is present. Phenol can also produce this reaction.

(b) Sodium Bicarbonate Test:

When carboxylic acid reacts with sodium bicarbonate solution, carbon dioxide is released causing brisk effervescence , and sodium acetate is formed.

The chemical reaction is as follows:

\mathrm{RCOOH}+\mathrm{NaHCO}_3 \rightarrow \mathrm{RCOONa}+\mathrm{H}_2 \mathrm{O}+\mathrm{CO}_2 \uparrow \text { (brisk effervescence) }

Note: This test is used to distinguish carboxylic acid from phenol. Phenol does not react in this way.

(c) Ester Test:

Carboxylic acid reacts with alcohol in the presence of concentrated sulphuric acid to form a sweet-smelling ester. This reaction is known as esterification.

The chemical reaction is as follows:

\mathrm{RCOOH}+\mathrm{R}-\mathrm{OH}+\mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{RCOO}-\mathrm{R}(\text { Ester })+\mathrm{H}_2 \mathrm{O}

Note: The formation of a sweet-smelling compound indicates the presence of a carboxylic group in the given organic compound.

(d) Fluorescein Test:

This test is given by dicarboxylic acid. When heated, dicarboxylic acid forms an acid anhydride . This anhydride reacts with resorcinol in the presence of concentrated sulphuric acid to form a fluorescent dye, hence the name fluorescein test.

The chemical reaction is as follows:

Note: This test should only be performed if the compound gives positive results in the litmus test and sodium bicarbonate test.

Required Materials:

• Blue litmus paper
• Sodium bicarbonate (or) sodium hydrogen carbonate
• Ethyl alcohol
• Concentrated sulphuric acid
• Resorcinol
• Acid anhydride
• Given organic compound
• Test tubes
• Test tube holders
• Beaker
• Glass rod
• Stirrer

Method:

(a) Litmus Test:

• Apply a drop of the given organic compound to blue litmus paper.
• Observe any change in colour of the blue litmus paper.
• If the blue litmus turns red, this indicates the presence of a carboxylic acid.

Note: Blue litmus solution can also be used instead of blue litmus paper.

(b) Sodium Bicarbonate Test:

• Prepare a saturated solution of sodium bicarbonate by dissolving it in 1ml of water.
• Add the given organic compound to the saturated sodium bicarbonate solution.
• Shake the solution well.
• If brisk effervescence is observed, this indicates the presence of a carboxylic acid.

Note: Use alcohol free of acid for this test.

(c) Ester Test:

• Mix the given compound with ethyl alcohol and concentrated sulphuric acid.
• Heat the mixture in a dry test tube in a water bath.
• Carefully pour the reaction mixture into a beaker containing water.
• Neutralise any excess sulphuric acid.
• If a sweet-smelling substance is detected, this indicates the presence of an acid.

(d) Fluorescein Test:

• Add the given organic compound to be tested to a test tube.
• Mix with 100mg of resorcinol and 0.5ml of concentrated sulphuric acid.
• Gently heat the test tube over a Bunsen burner.
• Pour the mixture into a beaker containing dilute sodium hydroxide solution.
• The appearance of a green fluorescent solution indicates the presence of a carboxylic acid group.

Note: The resulting solution should be alkaline.

Precautions:

• In the sodium bicarbonate test, use acid-free alcohol to avoid interference with the functional group of the given organic compound.
• Add sodium bicarbonate slowly to clearly observe the effervescence.
• Dicarboxylic acids such as oxalic acid, terephthalic acid, and isophthalic acid do not give a fluorescein test.

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