Tests for the functional groups

Our Objective

Our objective is to identify the functional groups present in an organic compound through;

  • Tests for Unsaturatuion
  • Tests for Alcoholic Group
  • Tests for Phenolic Group
  • Tests for Aldehydic and Ketonic Groups
  • Tests for Carboxylic Groups
  • Tests for Amino group

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)
  • Unsaturated Hydrocarbons (Alkenes & Alkynes)

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.

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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.

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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.

 

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