Chemistry Experiment: Tests for Functional Groups Present in an Organic Compound
We can define functional groups as specific groupings of certain atoms inside molecules that have their own characteristic properties. Common examples of functional groups are alcohols, alkenes, alkynes, amines, carboxylic acids, aldehydes, ketones, esters, and ethers, among others. Molecules of organic compounds, except hydrocarbons, are often divided into 2 parts: a reactive part, also called a functional group, and another part with carbon atoms, an alkyl group. As an example, the functional group for an aldehyde is -CHO. The ketone functional group is -RCOR, while the carboxylic acid functional group is -COOH.
Table of Content
Aim
Articles Required
Theory
Procedure
Result
Observation
Precautions
Viva Questions
Summary
Aim
Functional group tests for functional groups present in an organic compound.
Articles Required
Test tube
Glass rod
Burner
Tripod Stand
Theory
Some of the important functional groups which are present in organic compounds are:
Hydrocarbons: Compounds containing only carbon and H are referred to as hydrocarbons. Different organic compounds are derived from hydrocarbons by replacing 1 or additional H atoms with other atoms or a group of atoms (functional group) like —OH, —CHO, —COOH, etc. The hydrocarbons are classified into saturated and unsaturated hydrocarbons.
Alcohols: Compounds in which the hydroxyl (—OH) is connected to an acyclic carbon chain or in the side chain of an organic compound are referred to as alcohols. Alcohols are further classified as primary, secondary, and tertiary because the —OH group is hooked up to primary, secondary, and tertiary carbon atoms.
Aldehydes and Ketones: Aldehydes and ketones contain the carbonyl group. In aldehydes, the carbonyl group is hooked to an H atom and to an open-chain or aromatic radical. Formaldehyde is an exception in which the carbonyl group is hooked up to 2 H atoms. The tests used to confirm aldehydes and ketones:
2, 4-dinitrophenylhydrazine test.
sodium bisulfite test.
Dissolve Na nitroprusside in distilled water in a very clean test tube. Add 1 ml of the given chemical compound to be tested. Add NaOH solution dropwise. If there is a red colour then the presence of ketone is confirmed.
Take the given chemical compound in a very clean tube. Add 1 ml of the chromic acid chemical agent to the given compound. The appearance of a green or blue precipitate indicates aldehydes' presence.
Amines: Amines could also be considered substitution products of ammonia. When one hydrogen atom of ammonia is replaced with an alkyl chemical group or aryl group, the resulting amine is termed primary amine (R—NH2).
Procedure
Tests for Unsaturation:
Tests for Alcoholic Group (O-H):
Tests for Primary, Secondary, and tertiary Alcohols:
Tests for Phenolic Group:
Tests for Amine Group:
Observation
The functional groups present in the organic compound are ketones, alcohol, and aldehydes.
Tests performed confirm the presence of the following functional groups in the organic compound. Brisk effervescence is produced.
Result
The functional groups present in the organic compound are ketones, alcohol, and aldehydes.
Precautions
Following precautions must be taken while carrying out the experiment:
The equipment and apparatus should be cleaned and dried properly.
Sodium metal should be rigorously treated because it responds violently to water.
Lab Manual Questions
1. Why is it necessary to identify the functional groups present in an organic compound?
Ans. Functional groups are necessary for chemistry because they're the portion of a molecule capable of characteristic reactions. They, therefore, verify the properties and chemistry of many organic compounds.
2. What is the aim of a functional group?
Ans. Functional groups are usually used to “functionalise” a compound, affording it different physical and chemical properties than it would have in its original form.
3. Is phenol a saturated or unsaturated compound?
Ans. Phenol isn't an unsaturated compound and is additionally decolourised By water. This is due to the substitution reaction forming bromine-substituted phenol.
4. Why are most organic compounds colourless?
Ans. Most simple organic compounds, having few multiple bonds and few functional groups, do not absorb visible radiation and appear colourless or white. More complex molecules, having many multiple bonds that are conjugated, seem to be coloured.
5. Which functional group is always polar?
Ans. Hydroxyl is always polar. Carbonyl determines the 2 groups of sugars. Carboxyl has acidic properties, and a carboxyl is written –COOH.
Viva Questions
Q1.What is Baeyer’s test for unsaturation?
Ans. When Baeyer’s reagent (alkaline potassium permanganate) is added to unsaturated compound, its colour gets discharged, indicating the presence of C = C or C = C in the compound.
Q2. Do alkynes turn blue litmus paper red?
Ans. No.
Q3.Name two tests that distinguish aldehydes from ketones.
Ans. Tollen’s test and Fehling’s test.
Q4. Which is the qualitative test for amines and the qualitative test for alcohol.
Ans. Azo dye test for amines and litmus test for alcohols. Phenol turns blue litmus paper red. Amine turns red and litmus blue.
Q5. What is the functional group present in amide and aniline?
Ans. The functional group present in amide contains a carbonyl group and an N atom, while in aniline, it contains a benzene ring with an N atom attached to it, which means amine is its functional group.
Q6. What is the use of Schiff’s reagent?
Ans. Schiff’s chemical agent is used to detect the aldehyde group.
Q7. How can phenol and aniline be distinguished chemically?
Ans. Phenol is soluble in liquid NaOH solution, whereas aniline is not.
Phenol is not soluble in dilute HCl, while aniline is soluble in dilute HCl.
Q8. Which test is used to identify primary, secondary, and tertiary amines?
Ans. Hinsberg test is used to identify the primary, secondary and tertiary amines.
The nitrous acid test is a qualitative test used to confirm primary, secondary, and tertiary amines.
Summary
The functional groups in an organic compound determine the properties of a compound. Different compounds having the same functional group have similar properties and are classified as a family of compounds. Compounds having different functional groups have different properties and belong to different families of compounds. The functional groups present in an organic compound are often determined with the help of these tests. The tests for the functional groups of organic compounds viva questions with answers are mentioned in the above experiment.
FAQs on Tests for Functional Groups Present in Organic Compounds
1. What are two important chemical tests frequently asked in CBSE exams to distinguish between an aldehyde and a ketone?
For the CBSE 2025-26 examinations, two key distinguishing tests are essential:
- Tollen’s Test (Silver Mirror Test): Aldehydes react with Tollen's reagent (ammoniacal silver nitrate solution) to form a bright silver mirror on the inner side of the test tube. Ketones do not give this test.
- Fehling’s Test: Aldehydes, when heated with Fehling's solution, give a red-brown precipitate of Copper(I) oxide. Aliphatic ketones do not respond to this test. Aromatic aldehydes also do not respond to this test.
2. How can you chemically distinguish between a phenol and a carboxylic acid? This is an expected question for 3 marks.
To distinguish between a phenol and a carboxylic acid, the most reliable test is the Sodium Bicarbonate Test.
- When sodium bicarbonate (NaHCO₃) solution is added to a carboxylic acid, it produces brisk effervescence due to the release of carbon dioxide (CO₂) gas.
- Phenols are generally not acidic enough to react with NaHCO₃, so no effervescence is observed.
Another distinguishing test is the neutral Ferric Chloride (FeCl₃) test, where phenols typically give a characteristic violet, green, or blue colouration, while carboxylic acids do not.
3. What is the Lucas test used for, and how do the results vary for primary, secondary, and tertiary alcohols?
The Lucas test is a crucial method used to distinguish between primary, secondary, and tertiary alcohols. It is based on the difference in reactivity of the alcohols with the Lucas reagent (a solution of anhydrous zinc chloride in concentrated hydrochloric acid). The expected observations are:
- Tertiary Alcohols: React almost immediately to form a cloudy solution or turbidity due to the formation of an alkyl halide.
- Secondary Alcohols: React more slowly, typically forming turbidity within 5 to 10 minutes.
- Primary Alcohols: Do not show any visible reaction or turbidity at room temperature. Heating may be required for a reaction to occur.
4. Explain the principle and observation for the Hinsberg test, an important method for identifying amine types.
The Hinsberg test is a significant chemical test used to distinguish between primary, secondary, and tertiary amines. The principle relies on their reaction with benzenesulphonyl chloride (Hinsberg's reagent).
- Primary (1°) Amines: React to form an N-alkylbenzenesulphonamide, which has an acidic hydrogen atom on the nitrogen. This makes it soluble in alkali (like KOH or NaOH).
- Secondary (2°) Amines: React to form an N,N-dialkylbenzenesulphonamide, which has no acidic hydrogen. Thus, the precipitate formed is insoluble in alkali.
- Tertiary (3°) Amines: Do not react with Hinsberg's reagent at all, so the solution remains clear.
5. Why are aldehydes easily oxidised by mild agents like Tollen's reagent, while ketones are not? Explain the structural reason.
The difference in oxidisability between aldehydes and ketones is due to their molecular structure. Aldehydes have a hydrogen atom directly attached to the carbonyl carbon (-CHO group). This C-H bond is relatively weak and can be easily broken and oxidised by mild oxidising agents. Ketones, on the other hand, have two alkyl or aryl groups attached to the carbonyl carbon (C=O). They lack this specific hydrogen atom. Oxidising a ketone requires breaking a much stronger carbon-carbon (C-C) bond, which necessitates very harsh oxidising conditions, not provided by mild agents like Tollen's or Fehling's reagents.
6. What crucial precautions are important while performing Tollen's test to ensure an accurate result in a lab-based exam question?
For an accurate and clear silver mirror in Tollen's test, certain precautions are vital. Neglecting them can lead to a false negative result.
- The test tube must be scrupulously clean and free from any grease. The silver mirror deposits on clean glass; impurities can cause the silver to precipitate as a black powder instead.
- Tollen's reagent must always be freshly prepared just before the test and should never be stored, as it can form an explosive precipitate (silver fulminate) upon standing.
- The solution should be warmed gently in a water bath, not heated strongly or directly over a flame, to prevent the decomposition of the reagent.
7. A student is given two unlabelled organic compounds, A and B. Compound A gives a violet colour with neutral FeCl₃, while Compound B gives brisk effervescence with NaHCO₃. Identify the functional groups in A and B.
This is a classic identification problem frequently seen in board exams. Based on the given observations:
- Compound A: The formation of a violet colour with neutral ferric chloride (FeCl₃) is the characteristic test for the phenolic (-OH) group. Therefore, Compound A is a phenol.
- Compound B: The production of brisk effervescence with sodium bicarbonate (NaHCO₃) is the definitive test for the carboxylic acid (-COOH) group. The effervescence is due to the liberation of CO₂ gas. Therefore, Compound B is a carboxylic acid.
8. What is Baeyer's test, and for which important functional property is it used as a confirmatory test?
Baeyer's test is a chemical test used to detect the presence of unsaturation (i.e., double or triple bonds) in an organic compound. The reagent used is a cold, dilute, alkaline solution of potassium permanganate (KMnO₄), which is deep purple in colour. When this reagent is added to an unsaturated compound, the permanganate is reduced, and its purple colour is discharged, often forming a brown precipitate of manganese dioxide (MnO₂). This decolourisation confirms the presence of a C=C or C≡C bond.
