Test for Carboxyl Group

Carboxylic acids are versatile organic compounds. It has excellent physical and chemical properties. The carboxylic acid chemical structure contains a carbonyl functional group and hydroxyl group. It interact easily with polar compounds and contributes to many important chemical reactions. The carboxylic acids are the most important functional group that present C=O.

Aim:

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

Theory:

Carboxylic acids have a tendency to donate protons and act as acids. It is this property which is helpful in the identification of a -COOH group.

Any of the following test can be carried out to detect the carboxylic functional group.

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

(a) Litmus Test:

The carboxylic acid turns blue litmus red. Because the hydroxyl group present in -COOH is far acidic than in alcohol.

The chemical reaction is given below.

R-COOH + H2O → R-COO + H3O+

Note: If the colour of the blue litmus paper changes to red then 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 along with sodium acetate is formed.

The chemical reaction is given below.

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

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

(c) Ester Test:

Carboxylic acid reacts with alcohol in the presence of concentrated sulfuric acid 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 sulfuric acid a fluorescent
dye is formed and so this reaction is called fluorescein test.

The chemical reaction is given below.

Fluorescein Test

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 sulfuric 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 on 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 sulfuric 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 sulfuric 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 sulfuric 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:

Litmus test

Carboxylic group turns blue litmus red.

Sodium bicarbonate test

Brisk effervescence indicates the presence of carboxylic acid

Ester test

Formation of a sweet smelling compound indicates the presence of carboxylic group.

Fluorescein test

Appearance of green colour fluorescent solution indicates the presence of acid group.

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.

Viva Questions:

1. How to distinguish phenol from aniline?

Ans: Phenol is soluble in aqueous sodium hydroxide solution whereas aniline is not soluble. In other way aniline is soluble in dilute hydrochloric solution but phenol is not soluble.

2. How will you distinguish between phenol and carboxylic acid?

Ans: With aqueous sodium bicarbonate and carboxylate salt and carbon dioxide, carboxylic acid reacts. This test is not given by phenol.

3. What causes brisk effervescence in sodium bicarbonate test?

Ans: The evolution of carbon dioxide during the reaction causes brisk effervescence in sodium bicarbonate test.

4. What is carboxylic acid?

Ans: A carboxylic acid is an organic compound containing a group of carboxyles. A carboxylic acid’s general formula is R-COOH, with R referring to the remainder of the molecule.

5. Why is the name carboxyl is addressed to carboxylic acid?

Ans: Due to the carbonyl group (C = O) and hydroxyl group, the carboxyl (COOH) group is so-called.

 

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