Test for Carboxyl Group

Carboxylic acids are highly versatile organic compounds known for their excellent physical and chemical properties. Their chemical structure consists of a carbonyl functional group and a hydroxyl group, enabling easy interaction with polar compounds and involvement in many significant chemical reactions. Carboxylic acids are recognized as the most important functional group containing C=O.

Aim:

To identify the presence of carboxylic functional group in a given organic compound.

Theory:

Carboxylic acids exhibit a propensity to donate protons, functioning as acids. This characteristic proves beneficial in pinpointing the presence of the -COOH group.

The carboxylic functional group can be identified through any of the following tests:

1. Litmus test
2.  Sodium bicarbonate test (or Sodium hydrogen carbonate test)
3. Ester test
4. Fluorescein test

(a) Litmus Test:

The carboxylic acid turns blue litmus red due to the acidity of the hydroxyl group present in -COOH, which is more acidic than in alcohol.

The chemical reaction can be represented as:

R-COOH + H₂O → R-COO^– + H₃O⁺

Note: If the blue litmus paper changes color to red, then a carboxylic group is present. Phenol also gives this test.

(b) Sodium Bicarbonate Test:

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

The chemical reaction can be represented as:

RCOOH + NaHCO₃ → RCOONa + H₂O + CO₂↑ (brisk effervescence)

Note: This test is used to distinguish between carboxylic acid from phenol. Phenol does not give this test.

(c) Ester Test:

Carboxylic acid reacts with alcohol in the presence of concentrated sulphuric acid and forms a pleasant smelling ester. This reaction is known as esterification.

The chemical reaction is given below.

RCOOH + R-OH + H₂SO₄ → RCOO-R (Ester) + H₂O

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

(d) Fluorescein Test:

This test is given by dicarboxylic acid. Dicarboxylic acid on heating gives acid anhydride. When this anhydride is treated with resorcinol in the presence of concentrated sulphuric acid a fluorescent
dye is formed and so this reaction is called fluorescein test.

The chemical reaction is given below.

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

Materials Required:

1. Blue litmus paper
2. Sodium bicarbonate (or) sodium hydrogen carbonate
3. Ethyl alcohol
4. Concentrated sulphuric acid
5. Resorcinol
6. Acid anhydride
7. Given organic compound
8. Test tubes
9. Test tube holders
10. Beaker
11. Glass rod
12. Stirrer

Procedure:

(a) Litmus Test:

1. Add a drop of given organic compound on blue litmus paper.
2. Observe the colour change in blue litmus paper.
3. If the colour of blue litmus changes to red the presence of carboxylic acid.

Note: Blue litmus solution is also used in the place of blue litmus paper.

(b) Sodium Bicarbonate Test:

1. Prepare a saturated solution of sodium bicarbonate by dissolving sodium bicarbonate in 1ml of water.
2. Add the given organic compound to the saturated solution of sodium bicarbonate solution.
3. Shake the solution well.
4. If there is an evolution of brisk effervescence then it indicates the presence of carboxylic acid.

Note: Use acid free alcohol for the test.

(c) Ester Test:

1. Mix the given compound with ethyl alcohol and concentrated sulphuric acid.
2. Heat the mixture in a dry test tube in a water bath.
3. Pour the reaction mixture into a beaker carefully containing water.
4. Neutralise the excess sulphuric acid.
5. If a sweet smelling substance is sensed then it indicates the presence of acid.

(d) Fluorescein Test:

1. Take the given organic compound to be tested in a test tube.
2. Mix with 100mg of resorcinol and 0.5ml of concentrated sulphhuric acid.
3. Heat the test tube gently on a bunsen burner.
4. Pour the mixture into a beaker containing dilute sodium hydroxide solution.
5. Appearance of green colour fluorescent solution indicates the presence of carboxylic acid group.

Note: The resultant solution should be alkaline

Observations:

Results and Discussion:

The given organic compound is ___________ .

Precautions:

1. In sodium bicarbonate test use acid free alcohol so that it should not interfere with the functional group of the given organic compound.
2. The addition of sodium bicarbonate should be carried out slowly so that the effervescence is visible clearly.
3. Dicarboxylic acids like oxalic acid, terephthalic acid and isophthalic acid do not give fluorescein test.