Carboxylic acid is an organic compound that consists minimum of one carboxyl group (-COOH). This acid is a versatile organic compound. Carboxylic acids have excellent chemical properties as well as physical properties. The chemical structure consists of one carbonyl functional group and hydroxyl group.
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Key Terms: Carboxylic Acid, Litmus Paper, Sodium Bicarbonate, Esterification, Phenols, Organic Compound, Carboxyl group, Anions, Aromatic carboxylic acids
What is Carboxylic Acid?
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Carboxylic acid is a compound that comprises at least one carboxyl group. They are extremely acidic compounds as they are easily deprotonated forming carboxylate anions. The common formula of aliphatic carboxylic acid class is R-COOH, where the letter ‘R’ defines the rest of the molecules like acetic acid. Alternately, aromatic carboxylic acids possess the general formula Ar-COOH, where ‘Ar’ has minimum one aromatic ring as for aromatic carboxylic acids.
Carboxyl Functional Group
Carboxylic acids are classified into mono, di, and tricarboxylic acids based on the number of -COOH functional groups present in the structure. There are different organic compounds of various functional groups that reside in carboxylic acids. These include amino acids, fatty acids, keto acids, etc.
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| Uses of Carboxylic Acids | Acids and Bases | Phenolic Acid |
| Tollen’s Test | Nucleophilic Addition Reaction | Etard Reaction |
| Carboxylic Acid | Classification of Organic Compounds | Malonic Acid |
Tests for Carboxyl Group
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Different tests are carried out in order to identify the carboxylic functional group. These include litmus test, sodium bicarbonate test, ester test, and fluorescein test. Carboxylic acids tend to provide protons and act as acids. This characteristic is useful in the determination of a -COOH group.
Sodium Bicarbonate Test
When carboxylic acid reacts with sodium bicarbonate solution, carbon dioxide is formed with a brisk effervescence as well as sodium acetate is formed. This experiment is conducted to differentiate carboxylic acid from phenol. The chemical reaction for the same is as follows:
RCOOH + NaHCO3 → RCOONa + H2O + CO2↑ (Brisk effervescence)
Litmus Test
The hydroxyl group available in -COOH is far more acidic than in alcohol. As a result, the carboxylic acid turns blue litmus into red.
R-COOH + H2O → R-COO– + H3O+
If blue litmus paper turns into red, then a carboxyl group is present. Phenol also gives this test.
Change in Colour Shown by Litmus Paper
Ester Test
A reaction of carboxylic acid takes place with alcohol in the presence of concentrated sulfuric acid creating a pleasant smelling ester. This reaction is also called esterification.
RCOOH + R-OH + H2SO4 → RCOO-R (Ester) + H2O
The formation of a sweet smelling compound defines the presence of a carboxylic group in the given organic compound.
Fluorescein Test
This reaction is given by dicarboxylic acid. When the acid is reacted with thermal energy, acid anhydride forms. This anhydride reacts with resorcinol in the presence of concentrated sulfuric acid. As a result, a fluorescent dye is formed. Hence, the reaction is known as fluorescein test. This reaction is performed only if the compound provides positive results in sodium bicarbonate and litmus tests.
Required Materials for Tests
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The materials required to conduct the above mentioned experiments are as follows:
- Sodium Hydrogen Carbonate or Sodium Bicarbonate
- Blue Litmus Paper
- Ethyl Alcohol
- Resorcinol
- Concentrated Sulfuric Acid
- Test Tubes
- Acid Anhydride
- Organic Compound
- Stirrer
- Glass Rod
- Beaker
- Test Tube Holders
Procedure for Tests
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The procedures of the above mentioned tests are given below:
Litmus Test
- Put a drop of the given compound on the blue litmus paper.
- Detect the color shift in the blue litmus paper.
- Confirm the presence of carboxylic acid if the color of blue litmus changes to red.
- Instead of blue litmus paper, blue litmus solution can also be used for the test.
Sodium Bicarbonate Test
- Make a saturated solution of sodium bicarbonate by dissolving sodium bicarbonate in 1 ml of water.
- Put the given compound on the saturated sodium bicarbonate solution and shake it well.
- The presence of carboxylic acid can be confirmed if there is a brisk effervescence.
- Also, the acid free alcohol can be used for the test.
Fluorescein Test
- Put the given compound in a test tube and examine.
- Take 0.5ml of concentrated sulfuric acid and 100mg of resorcinol and mix them together.
- Put the test tube on a bunsen burner and heat it gently.
- Pour the mixture of compound into a beaker along with the dilute sodium hydroxide solution.
- The presence of carboxylic acid can be confirmed if the green fluorescent solution appears. Also, the resultant solution must be alkaline.
Ester Test
- Mix the compound with concentrated sulfuric acid and ethyl alcohol.
- Put the mixture in a dry test tube in a water bath and heat it.
- Take a beaker containing water and pour the reaction mixture into it.
- Neutralize the sulfuric acid.
- The presence of acid can be confirmed if a sweet smelling substance is sensed.
Things to Remember
- Carboxylic acids are organic compounds containing at least one carboxyl (-COOH) group.
- Carboxylic acids are categorized as per the number of carboxylic acids in their structure into mono, di and tricarboxylic acids.
- The different tests carried out to find the carboxylic functional group are litmus test, sodium bicarbonate test, ester test, and fluorescein test.
- Sodium bicarbonate test is used to differentiate carboxylic acid from phenol.
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Sample Questions
Ques: Define carboxylic acid? (2 marks)
Ans: These are the organic compounds comprising carboxyl functional groups in their molecules. The carboxylic acids could be aromatic or aliphatic based on whether the -COOH group is attached to the aryl group or aliphatic alkyl chain.
Ques: Write down the tests which could differentiate alcohol and carboxylic acid? (3 marks)
Ans: Tests used to differentiate alcohol and carboxylic acids are -
- Based on liberation of carbon dioxide, acetic acid gives effervescence with NaHCO but ethanol does not.
- Ethanol provides a yellow precipitate with the help of an alkaline solution of iodine. At the same time, acetic acid does not give this test.
Ques: Name some of the carboxylic acids that a person encounters in daily life? (3 marks)
Ans: Some of the carboxylic acids that we encounter in our daily lives are as follows:
- Salicylic acids - They are mostly used in skin care products to flake off dead skin cells.
- Lactic acid - This acid is formed in the muscles while doing anaerobic exercises.
- Citric acid - It is used in industrial applications due to its high acid.
- Acetyl-salicylic acids - They are extracted from willow barks and mostly used in aspirin.
Ques: How are carboxylic acids used in the pharmaceutical industry? (3 marks)
Ans: These acids have a significant role in the pharmaceutical sectors because of the following reasons:
- They help in modulating solubility and permeation of cells.
- Those prodrug compounds and bio precursors that are not biologically active can be transformed into active ones under certain conditions (like drugs from antithrombotic, antihypertensive and antiviral classes).
- Pharmacophore gives certain interactions with enzymes by blocking and triggering their biological response.
Ques: Give some of the applications of carboxylic acids? (3 marks)
Ans: Some applications of carboxylic acids are -
- The acids and derivatives are used in the production of polymers, biopolymers, coatings, pharmaceutical drugs, adhesives, etc.
- They are used as food solvents, flavorings, antimicrobials etc.
- They are also used in the production of perfumes.
Ques: Explain the properties of carboxylic acids? (3 marks)
Ans: The properties of carboxylic acids are as follows -
- The acids are hydrogen bond donors as well as acceptors. As a result, they are polar and participate in hydrogen bonding.
- The surface areas of carboxylic acids are great, hence the boiling point is higher.
- Carboxylic acids are also known as bronsted lowry acids and they are proton (H+) donors.
- The acids are considered as weak acids. The esters have a pleasant odour.
Ques: How to trace carboxylic acids? (2 marks)
Ans: They are also known by their trivial names like acetic acid, formic acid, etc. They have the suffix ‘ic acids’ in the names. Butyric acid is also known as butanoic acid according to IUPAC guidelines.
Question: How to differentiate phenol from aniline? (2 marks)
Ans: Aniline is soluble in dilute hydrochloric solution but phenol is not. Similarly, phenol is soluble in aqueous sodium hydroxide solution but aniline is not soluble.
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