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Tests for Functional Groups Present in Organic Compounds

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:

Experiment	Procedure	Observation	Inference
Bromine Test	Dissolve a bit of the given organic compound in about 2 millilitres of distilled water in a test tube and add a few drops of Br water to it and shake well.	Disappearance of the orange-red colour bromine.	Compounds are unsaturated.
Bromine Test		No disappearance of the orange-red colour.	Compounds are saturated.
Baeyer's Test (Alkaline KMnO4 Test)	Dissolve a little of the given chemical compound in about 2 millilitres of DI in a test tube and add a few drops of an alkaline solution of KMnO4 to that and shake well.	Disappearance of the pink colour of KMnO4.	Compounds are unsaturated.
Baeyer's Test (Alkaline KMnO4 Test)		No disappearance of the pink colour of KMnO4.	Compounds are saturated.

Tests for Alcoholic Group (O-H):

Experiment	Procedure	Observation	Inference
Sodium metal Test	Take about 1 millilitre of the given liquid in a dry test tube, add about 1 gram of anhydrous calcium sulfate and shake well to remove water. Filter the liquid into another clean, dry tubing and add a little piece of Na metal.	Brisk effervescence is produced.	Brisk effervescence is because of the evolution of the H gas, indicating the presence of the alcoholic group.
Ester Test	Take about one ml of the given liquid in a clean dry test tube, add about one ml glacial acetic acid, and 2-3 drops of conc. sulphuric acid. For approx. 10 minutes heat the mixture in a water bath. Take 20 ml cold water in a beaker and add the mixture to it. Smell the mix.	A fruity smell is created.	the Presence of an alcoholic group.
Ceric ammonium nitrate Test	Take about 1 millilitre of the given liquid in a clean, dry test tube and add a few drops of ceric ammonium nitrate reagent and shake it well.	Pink or red colour is developed.	Presence of alcoholic group.

Tests for Primary, Secondary, and tertiary Alcohols:

Experiment	Procedure	Observation	Inference
Lucas Test	Take about one ml of dry alcohol in a clean dry test tube and add 8-10 drops of Lucas chemical agent. Shake the mixture well.	If cloudiness appears instantly.	Presence of tertiary alcohol.
		If cloudiness appears within 5 minutes.	Presence of secondary alcohol.
		If cloudiness appears only upon heating.	Presence of primary alcohol.

Tests for Phenolic Group:

Experiment	Procedure	Observation	Inference
Litmus Test	Add a drop of the given liquid sample (or a crystal if solid) on a blue litmus paper.	Blue litmus test turns to red.	Presence of phenolic group.
Ferric chloride Test	Take about one ml of neutral ferric chloride solution in a clean test tube and add 2-3 drops of the given liquid (or the crystal is solid).	A violet colour is seen.	Presence of phenolic group.
Liebermann’s Test	Take a small amount of sodium nitrite in a clean, dry test tube and add about one ml of the given liquid (or crystal if solid). Heat the test tube very gently for half a minute and allow it to cool down. Then add about one ml of conc. H2SO4, and to mix the contents, shake the tube properly.	A deep blue or green colouration is seen.	Presence of phenolic group.

Tests for Amine Group:

Experiment	Procedure	Observation	Inference
Solubility Test	Take a little amount of the given compound in a clean test tube and add 2-3 ml of dil. HCl and shake the test tube well.	Compounds are soluble in dil. HCl.	Compound may be an amine.
Litmus Test	Place a drop of the given liquid (or a pinch of solid) on a wet red litmus paper.	Red litmus turns to blue.	Compound may be an amine.
Azo-Dye Test	Take 3 test tubes A, B, and C. In tube A, dissolve a small amount of the compound in 2 millilitres of HCl. In tube B, prepare a solution of sodium nitrite. In tube C, dissolve a small amount of β-naphthol in dilute NaOH. Place all the 3 test tubes in AN ice bath. sodium nitrite is added to the solution in tube A and the resulting mixture is transferred to test tube C.	The formation of a red or orange dye.	The presence of aromatic primary amine is confirmed.

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.

Observation	Inference
Brisk effervescence	Alcohol is present
Appearance of red colour	Ketone is present
Green blue colour precipitates appear	Aldehyde is present

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 is the test for carboxylic groups?

Formic acid and acetic acid are liquids. Oxalic acid, phthalic acid, benzoic acid, tartaric acid, and salicylic acid are colourless crystalline solids. Acyclic acids are soluble in water, whereas aromatic acids are less soluble in water. The following tests often know carboxylic acids: litmus test, sodium bicarbonate test, and Ester test. The carboxylic acids turn blue litmus red. Carboxylic acids react with Na hydrogen carbonate to provide CO2 gas that is known by the effervescence produced. This test is used to differentiate carboxylic acids from phenols.

2. What are the tests for aldehyde and ketone groups?

2, 4-Dinitrophenylhydrazine Test (2, 4—DNP Test) and Sodium bisulfite Test are used to confirm ketone or aldehyde group in an organic compound.

3. What are the points to be noted while performing Schiff's and Tollen’s tests?

The Schiff's chemical agent shouldn't be warmed. Schiff’s chemical agent should not be treated with alkalies. Otherwise, the pink colour develops even in the absence of aldehydes. With benzaldehyde, the pink colour develops slowly, whereas, in tollen’s test, different types of compounds provide a positive silver mirror test however they do not give a 2,4-dinitrophenylhydrazine test. Formic acid, hydroxy acid, and many carbohydrates like glucose give a silver mirror test.