Our Objective
Our objective is to identify the functional groups present in an organic compound through;
The Theory
What is Organic Chemistry?
Organic chemistry is the branch of chemistry that deals with the structure, properties and reactions of compounds that contain carbon. The objects of study in organic chemistry include hydrocarbon, compounds containing carbon and hydrogen and the compositions based on carbon but containing other elements. Organic compound form the basis of earthly life and their range of application is enormous. They are the main constituents of drugs, petrochemicals, paints, food, plastics, explosive materials etc.
What are Hydrocarbons?
Organic compounds contain only carbon and hydrogen are called Hydrocarbons. Hydrocarbons can be classified into two:
Saturated Hydrocarbons (Alkanes)
Hydrocarbons that contain carbon-carbon single bonds are called saturated hydrocarbons. They are also called Paraffins or Aliphatic Hydrocarbons. They may have straight chain, branched or ring structure.
Unsaturated Hydrocarbons (Alkenes & Alkynes)
Hydrocarbons that contain carbon-carbon double bond or triple bond are called unsaturated hydrocarbons.
Alkenes: Aliphatic hydrocarbons that contain carbon-carbon double bond are called alkenes. One σ (sigma) bond and one π (pi) bond constitute a double bond.
Alkynes: Aliphatic hydrocarbons that contain carbon-carbon triple bond are called alkynes. A triple bond contains one σ (sigma) bond and two π (pi) bonds.
What does the 'Degree of Unsaturation' mean?
The number of π bonds present in a molecule of an organic compound is termed as the Degree of Unsaturation.
What does derivatives of hydrocarbons or families mean?
The compounds that are derived from hydrocarbons by replacing one or more hydrogen atoms by other atoms or groups of atoms are called derivatives of hydrocarbons or families.
What are functional Groups?
The atom or group of atoms that replaces hydrogen atoms from hydrocarbon are called functional groups. They may be –OH, -COOH, -CO, -CHO, -Cl, -COCl, -COOR etc. Functional groups are responsible for the characteristics of a molecule.
Let’s discuss some important functional groups and their identification tests.
Tests for Unsaturation
There are two tests for determining unsaturation in an organic compound.
1. Bromine Test
In this test, the orange-red colour of bromine solution disappears when it is added to an unsaturated organic compound (unsaturated hydrocarbon).
2. Baeyer’s Test (Alkaline KMnO4 Test)
In this test, pink colour of KMnO4 disappears, when alkaline KMnO4 is added to an unsaturated hydrocarbon. The disappearance of pink colour may take place with or without the formation of brown precipitate of MnO2.
Tests for Alcoholic Group
Alcohols are compounds in which the hydroxyl group (-OH) is linked to aliphatic carbon chain or in the side chain of an organic compound. Depending upon the number of hydroxyl group, alcohols are classified as mono (contain only one –OH group), di (contain two –OH groups) and trihydric (contains three –OH groups).
Alcohols are further classified as primary (1°), secondary (2°) and tertiary (3°) according to the –OH group is attached to the primary, secondary and tertiary carbon atoms respectively.
The alcoholic group can be detected by the following tests:
1. Sodium metal test
Alcohols react with active metals like sodium and liberate hydrogen gas that can be observed in the form of effervescence.
2. Ester test
Alcohols react with carboxylic acids to form fruity smelling compounds called esters. The reaction between alcohol and carboxylic acid is called esterification and is catalysed by an acid such as concentrated sulphuric acid.
3. Ceric ammonium nitrate test
Alcohols reacts with ceric ammonium nitrate to form a red coloured alkoxy cerium (IV) compound.
4. Acetyl chloride test
Alcohols react with acetyl chloride to form esters and gives out hydrogen chloride gas. The hydrogen chloride formed gives white fumes of ammonium chloride with ammonium hydroxide.
5. Iodoform test
This test is given by acetaldehyde, all methyl ketones and all alcohols containing CH3-CH-OH group. When alcohol is warmed with sodium hydroxide solution and iodine, a yellow precipitate of iodoform is formed.
Tests for Phenolic group
Phenols are compounds containing a hydroxyl group attached to an aromatic ring. The simplest phenol is C6H5OH that is solid in winter and liquid in summer. Phenols are generally colourless but are coloured when it comes in contact with air due to oxidation. Other examples of phenols are: o-cresol, m-cresol, p-cresol, quinol, catechol, resorcinol etc.
The phenolic group can be detected by the following tests:
1. Litmus test
Phenol is a weak acid, it gives red colour with litmus paper. The dissociation of phenol in water is represented as follows:
2. Ferric chloride test
Phenol reacts with ferric ions to form violet coloured complex.
3. Liebermann’s test
When phenol is treated with sodium nitrite in the presence of concentrated sulphuric acid, deep blue or green colour is produced. The blue or green colour changes to red or brown colour on treatment with water. The red colour is due to the formation of indophenol. The red colour again changes to blue or green by the addition of strong alkali. The blue or green colour is due to the formation of indophenols anion.
4. Phthalein Dye test
Phenol on heating with phthaleic anhydride in the presence of sulphuric acid produces phenolphthalein, which is colourless. Phenolphthalein gives pink colour on treating with alkali.
Tests for Aldehydic and Ketonic Groups
Aldehydes and Ketones are compounds containing carbonyl group. Carbonyl group consisting of a carbon atom bonded to oxygen atom by a double bond.
In Aldehydes the carbonyl carbon is attached to atleast one hydrogen atom and to a carbon containing group (aliphatic or aromatic radical). Formaldehyde is an exception, in which carbonyl group is attached to two hydrogen atoms.
But in ketones the carbonyl carbon is attached to two aliphatic or aromatic groups.
Carbonyl groups in aldehydes and ketones are identified by the following tests:
1. 2,4-dinitrophenyl hydrazine test (2,4-DNP test)
2,4-dinitrophenyl hydrazine can be used to qualitatively detect the carbonyl group of an eldehyde or ketone. A positive result is indicated by the formation of an yellow or orange-red precipitate of 2,4-dinitrophenyl hydrazone.
2. Sodium bisulphite test
Most aldehydes and ketones give bisulphate addition product with sodium bisulphate, which is white crystalline in nature.
Note: Acetone phenone and benzophenone do not give this test.
Differentiating tests for aldehydes
The major difference between aldehydes and ketones is that an aldehyde is readily oxidised to carboxylic acid whereas ketones cannot be oxidised easily. This difference forms the basis of the tests for distinguishing aldehydes and ketones.
The following are the tests for aldehydes but not for ketones:
1. Schiff’s Test
Aldehydes give pink or magenta colour with Schiff’s reagent.
Note: With benzaldehyde the pink colour developes slowly.
2. Tollen’s Test
Tollen’s reagent is ammoniacal silver nitrate. Aldehydes react with Tollen’s reagent to form elemental silver, accumulated onto the inner surface of the reaction vessel, producing silver mirror on the inner surface of the vessel.
3. Fehling’s Test
This is an important test to distinguish aldehydes from ketones. The reagents used in this test are Fehling’s solution A and Fehling’s solution B. Fehling’s solution A is an aqueous solution of copper sulphate and Fehling’s solution B is a clear solution of sodium potassium tartrate (Rochelle salt) and strong alkali (usually NaOH).
The final Fehling’s solution is obtained by mixing equal volmes of both Fehling’s solution A and Fehling’s solution B that has a deep blue colour. In Fehling’s solution, copper (II) ions form a complex with tartrate ions in alkali. Aldehydes reduces the Cu(II) ions in the fehling’s solution to red precipitate of cuprous oxide(copper (I) oxide).
Note: Benzaldehyde may or may not give this test as the reaction is very slowly.
Differentiating tests for Ketones
The following tests are given by ketones but not by aldehydes:
1. m-dinitrobenzene Test
Ketones react with m-dinitrobenzene to give a violet colouration.
2. Sodium nitroprusside Test
The anion of the keton formed by a alkali reacts with nitroprusside ion to form a red coloured complex.
Tests for Carboxylic group
Carboxylic acids are organic compounds containing carboxyl functional group. It is of two types aliphatic and aromatic. Aliphatic acids are soluble in water where as aromatic acids are sparingly soluble in water. Formic acid and acetic acid are the simplest aliphatic acid and benzoic acid is the simplest aromatic acid. Formic acid and acetic acid are liquids. Carboxylic acids such as benzoic acid, oxalic acid, phthalic acid, tartaric acid etc are colourless crystalline solids.
The following tests can be used to identify carboxylic acids:
1. Litmus Test
Carboxylic acid turns blue litmus red. The hydroxyl group in carboxylic is far more acidic than that in alcohol. The dissociation of carboxylic acid is represented as:
2. Sodium Hydrogen Carbonate Test
Carboxylic acids reacts with sodium hydrogen carbonate to produce carbon dioxide gas which can be seen in the form of a brisk effervescence.
3. Ester Test
Carboxylic acid reacts with alcohol in presence of conc. sulphuric acid to form ester that is identified by the presence of a fruity smell.
Tests for Amines
Amines are derivatives of ammonia in which one or more hydrogen atoms are replaced by alkyl or aryl groups.
When one of the three hydrogen atoms is replaced by alkyl or aryl group, primary amine is formed. It is generally represented as RNH2.
When two of the three hydrogen atoms are replaced by alkyl or aryl group, secondary anime is formed. It is generally represented as R2NH.
When all the three hydrogen atoms are replaced by alkyl or aryl substituents, tertiary amine is formed. It is generally represented as R3N.
1. Solubility Test
Amines are basic in nature and dissolves in mineral acids.
2. Litmus Test
Amines are basic in nature and turns red litmus blue.
3. Carbylamines Test
When primary amine is treated with alcoholic potassium hydroxide and chloroform, an offensive smelling isocyanide is formed.
4. Azo-Dye Test
This test is given by aromatic primary amines. Aromatic primary amines react with nitrous acid to form diazonium salts. These diazonium salts undergo coupling reaction with β-naphthol to form orange coloured azo dye.
Distinguishing tests for Primary, Secondary and Tertiary Amines
1. Nitrous acid Test
Primary aliphatic amines react with nitrous acid to produce nitrogen gas which is seen as bubbles.
Secondary amines react with nitrous acid to form a yellow oily nitrosoamine.
Tertiary amines react with nitrous acid to form soluble nitrite salts.
2. Hinsberg Test
The reactions of primary, secondary and tertiary amines are as follows.
Real Lab Procedure
Tests for Unsaturation
Experiment | Procedure | Observation | Inference |
Bromine Test | Dissolve a little of the given organic compound in about 2 ml of distilled water in a test tube and add few drops of bromine water to it and shake well. | Disappearance of orange-red colour of bromine. | Compound is unsaturated. |
No disappearance of orange-red colour. | Compound is saturated. | ||
Baeyer's Test (Alkaline KMnO4 Test) | Dissolve a little of the given organic compound in about 2 ml of distilled water in a test tube and add few drops of alkaline solution of KMnO4 to it and shake well. | Disappearance of pink colour of KMnO4. | Compound is unsaturated. |
No disappearance of pink colour of KMnO4. | Compound is saturated. |
Tests for Alcoholic Group
Experiment | Procedure | Observation | Inference |
Sodium metal Test | Take about 1 ml of the given liquid in a dry test tube, add about 1gram of anhydrous calcium sulphate and shake well to remove water. Filter the liquid into another clean dry test tube and add a small piece of sodium metal. | Brisk effervescence is produced. | Brisk effervescence is due to the evolution of hydrogen gas indicating the presence of alcoholic group. |
Ester Test | Take about 1 ml of the give liquid in a clean dry test tube, add about 1 ml glacial acetic acid and 2-3 drops of conc. sulphuric acid. Warm the mixture on a water bath for about 10 minutes. Pour it into about 20 ml of cold water taken in a beaker and smell. | A fruity smell is produced. | Presence of alcoholic group. |
Ceric ammonium nitrate Test | Take about 1 ml of the given liquid in a clean dry test tube and add few drops of ceric ammonium nitrate reagent and shake it well. | Pink or red colour is developed. | Presence of alcoholic group. |
Acetyl chloride Test | Take about 2 ml of the given liquid in a clean dry test tube, add 1 g of anhydrous calcium sulphate and shake well to remove water. Filter the liquid into another clean dry test tube and to the filtrate, add 2-3 drops of acetyl chloride. Shake the test tube well and bring a glass rod dipped in ammonium hydroxide solution near the mouth of the test tube. | White fumes are produced. | Presence of alcoholic group. |
Distinguishing Tests for Primary, Secondary and tertiary Alcohols
Experiment | Procedure | Observation | Inference |
Lucas Test | Take about 1 ml of dry alcohol in a clean dry test tube and add 8-10 drops of Lucas reagent. Shake the mixture well. | If cloudiness appears immediately. | Presence of tertiary alcohol. |
If cloudiness appears within five 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 | Place a drop of the given liquid (or a crystal if solid) on a moist blue litmus paper. | Blue litmus turns to red. | Presence of phenoilc group. |
Ferric chloride Test | Take about 1 ml of neutral ferric chloride solution in a clean test tube and add 2-3 drops of the given liquid (or crystal is solid). | Violet colour is seen. | Presence of phenolic group. |
Liebermann’s Test | Take a small quantity of sodium nitrite in a clean dry test tube and add about 1 ml of the given liquid (or crystal if solid). Heat the test tube very gently for half a minute and allow it to cool. Then add about 1 ml of conc. sulphuric acid and shake the test tube to mix the contents. | A deep blue or green colouration is seen. | Presence of phenolic group. |
Add distilled water carefully to the test tube. | Colour turns to red. | ||
Add excess of sodium hydroxide solution to the test tube. | Deep blue or green colour appears. | ||
Phthalein Dye Test | Take 1 ml of the given liquid in a clean dry test tube (or crystal if solid) and add 1-2 drops of conc. sulphuric acid. Heat the test tube for about 1 minute in an oil bath. Cool and pour the reaction mixture carefully into a beaker containing 15 ml of dil. sodium hydroxide solution. | Pink colour is seen. | Presence of phenolic group. |
Tests for Aldehydic and Ketonic Groups
Experiment | Procedure | Observation | Inference |
2,4-Dintrophenyl hydrazine Test | Take a small quantity of the given compound in a clean dry test tube, add rectified spirit until the compound just dissolves. Now add few drops of 2, 4-dinitrophenyl hydrazine solution. Cork the test tube, shake the mixture and allow it to stand for 5 minutes. | Yellow or orange precipitate is formed. | Presence of carbonyl group. |
Sodium bisulphite Test | Take about 2 ml of saturated solution of sodium bisulphite in a clean test tube and add a small quantity of the given compound. Cork the test tube, shake and leave it for 15-20 minutes. | White crystalline precipitate is formed. | Presence of carbonyl group. |
Differentiating Tests for Aldehydes
Experiment | Procedure | Observation | Inference |
Schiff’s Test | Dissolve a small quantity of the given compound in a clean test tube and add about 1 ml of Schiff’s reagent and shake it well. | Pink colour is seen. | Presence of aldehydic group is confirmed. |
Tollen’s Test (Silver mirror test) | Take about 1 ml of silver nitrate solution in a clean test tube and add 2-3 ml of dilute NaOH solution. A brown precipitate of silver oxide forms. Now add dilute ammonia solution dropwise until the brown precipitate of silver oxide just dissolves. To this add 3-4 drops of the given liquid (or 0.1 g if solid) and warm the test tube on a water bath for about 5 minutes. | Shining silver mirror is formed. | Presence of aldehydic group is confirmed. |
Fehling’s Test | Take 1 ml each of Fehling’s solution A and B in a test tube. Add 4-5 drops of the given organic liquid (or 0.2g if solid) and warm the test in a water bath for 4-5 minutes. | Red precipitate is formed. | Presence of aldehydic group is confirmed. |
Differentiating Tests for Ketones
Experiment | Procedure | Observation | Inference |
m-Dinitrobenzene Test | Take a small quantity of the given compound in a clean test tube and add about 0.1g of finely powdered m-dinitrobenzene. Now add about 1 ml of dilute sodium hydroxide solution and shake it well. | Violet colour is formed which slowly fades away. | Presence of ketonic group is confirmed. |
Sodium nitroprusside Test | Dissolve a small quantity of sodium nitroprusside in about 1 ml of distilled water in a clean test tube and then add a small quantity of the given compound. Shake the test tube well and add sodium hydroxide solution dropwise. | Red colouration. | Presence of ketonic group is confirmed. |
Tests for Carboxylic Group
Experiment | Procedure | Observation | Inference |
Litmus Test | Place a drop of the given liquid (or a crystal if solid) on a moist blue litmus paper. | Blue litmus turns to red. | Presence of carboxylic group. |
Sodium Hydrogen Carbonate Test | Take 1 ml of the given organic compound in a test tube and add a pinch of sodium bicarbonate to it. | Brisk effervescence is observed. | Presence of carboxylic group. |
Ester Test | Take a small quantity of the given organic compound in a clean test tube, add 1 ml of ethyl alcohol and 1-2 drops of conc. sulphuric acid into it. Heat the reaction mixture on a water bath for about 5 minutes. Pour the mixture into a beaker containing water. | You get a fruity smell. | Presence of carboxylic group. |
Tests for Amino Group
Experiment | Procedure | Observation | Inference |
Solubility Test | Take a small quantity of the given compound in a clean test tube and add 2-3 ml of dil.HCl and shake the test tube well. | Compound is soluble in dil.HCl. | Compound may be amine. |
Litmus Test | Place a drop of the given liquid (or a pinch of solid) on a moist red litmus paper. | Red litmus turns to blue. | Compound may be amine. |
Carbylamines Test | Take a small quantity of solid KOH in a clean dry test tube and add about 2 ml of ethanol. Warm the test tube until the pellets dissolve. To this add a few drops of chloroform and small amount of the given compound and warm gently. | An offensive smell is produced. | Presence of primary amine. |
Azo-Dye Test | Take three test tubes A, B and C. In test tube A, dissolve a small quantity of the compound in 2 ml of HCl. In test tube B, prepare an aqueous solution of sodium nitrite. In test tube C, dissolve a small quantity of β-naphthol in dilute sodium hydroxide. Place all the three test tubes in an ice bath. Now add sodium nitrite solution into test tube A and the resulting solution is added to test tube C. | Formation of a red or orange dye. | Presence of aromatic primary amine is confirmed. |
Distinguishing Tests for Primary, Secondary and Tertiary Amines
Experiment | Procedure | observation | Inference |
Nitrous acid Test | Make a solution of about 1 g of sodium nitrite in about 5 ml distilled water in a test tube and cool it in an ice bath. In a separate test tube, dissolve a small quantity of the given compound in about 1 ml of conc. sulphuric acid and cool this also in ice bath. Mix both the solutions. | Bubbles of nitrogen gas. | Presence of primary aliphatic amine. |
Formation of a yellow oily layer. | Presence of secondary amine. | ||
No visible change. | Presence of tertiary amine. | ||
Hinsberg Test | Take a small quantity of the given compound in a clean test tube, add about 2 ml of 25% NaOH, 2 ml of water and 1 ml of benzene sulphonyl chloride. Shake the mixture for about 10 minutes and then cool under tap water and note the formation of precipitate. Treat the precipitate with 2 ml of conc.HCl. | No precipitate (clear solution). On addition of conc. HCl, insoluble material is seen. | Presence of primary amine. |
Precipitate is formed in the test tube. It does not dissolve in conc. HCl. | Presence of secondary amine. | ||
Precipitate is formed in the test tube. It dissolves in conc. HCl. | Presence of tertiary amine. |
Simulator Procedure (as performed through the Online Labs)
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Unsaturation
Bromine Water Test
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Baeyer’s Test
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Functional group
Alcohols
Sodium Metal Test
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Ester Test
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Ceric Ammonium Nitrate Test
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Acetyl chloride test
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Iodoform Test
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Lucas Test
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Ferric Chloride Test
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Liebermann’s Test
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Phthalein Dye Test
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Aldehydes
Schiff’s Test
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Tollen’s Test
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Fehling’s Test
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Ketones
2,4–Dinitrophenylhydrazine Test
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Sodium Bisulphite Test
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m-dinitrobenzene Test
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Sodium Nitroprusside Test
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Carboxylic Acids
Litmus Test
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Sodium Hydrogen Carbonate Test
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Ester Test
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Amines
Solubility Test
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Litmus Test
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Carbylamines Test
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Azo Dye Test
Nitrous Acid Test
Primary Amines
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Secondary Amines
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Tertiary Amines
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Hinsberg Test
Primary Amine
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Secondary Amine
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Tertiary Amine
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26 videos|287 docs|64 tests
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1. What is the theory behind functional groups in organic chemistry? |
2. How are functional groups identified in organic compounds? |
3. What are some commonly used tests to detect specific functional groups? |
4. Can functional groups be modified or transformed in organic synthesis? |
5. How are functional groups important in pharmaceutical and drug development? |
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