Functional Group Tests to Classify Organic Compounds

Our Objective Our objective is to identify the functional groups present in an organic compound through; Tests for Unsaturatuion Tests for A...

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

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 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.

Functional Group Tests of Organic Compounds
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

Organic compounds containing > C = C < and / or — C ≡ C – bonds are called unsaturated compounds. These compounds undergo addition reaction with bromine water or the solution of bromine in carbon tetrachloride, chloroform or glacial acetic acid. Addition of bromine to an alkene results in the formation of vicinal dibromide. The reddish orange colour of the solution of bromine in carbon tetrachloride disappears on reaction with an alkene. The reaction is as follows :

Bromine Test

2. Baeyer’s Test (Alkaline KMnO4 Test)

Alkenes decolourise the neutral/alkaline KMnO4 solution and vicinal glycols are formed (Bayer’s test). Reaction takes place as follows :

Baeyers Test

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.An alcohol is generally classified by the number of hydroxyl groups in its molecule.An alcohol that has 

  • one hydroxyl group is called monohydric; monohydric alcohols
  • two hydroxyl group is called dihydric; dihydric alcohols
  • three hydroxyl group is called trihydric; trihydric alcohols
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 H2 gas that can be observed in the form of effervescence.
Sodium metal test of alcohols

2. Ester test

Alcohols react with carboxylic acids in presence of concentrated sulphuric acid to form fruity smelling compounds called esters and water. The reaction between alcohol and carboxylic acid is called esterification reaction and is catalysed by an acid such as concentrated sulphuric acid(it also absorbs the water).
esterification reaction

3. Ceric ammonium nitrate test

Alcoholic compounds on reaction with ceric ammonium nitrate give a red colouration due to the formation of a complex.
Ceric ammonium nitrate test

4. Acetyl chloride test

5. Iodoform test

Distinction between primary, secondary and tertiary alcohols can be done on the basis of iodoform test and Lucas test. Ethanol and secondary alcohols which contain CH3—CH(OH)R group (iodoform reaction) give positive iodoform test.
Iodoform test

6. Lucas Test 

Lucas reagent contains zinc chloride and concentrated hydrochloric acid. This reagent reacts with primary, secondary and tertiary alcohols at different rates. 
  • Tertiary alcohols react almost instantaneously, 
  • secondary alcohols react in about 1-5 minutes and 
  • primary alcohols react very slowly. The reaction may take 10 minutes to several days.
Lucas Test of alcohol

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 weakly acidic, therefore they are soluble in NaOH solution but at the same time they are not sufficiently acidic to be soluble is sodium hydrogencarbonate solution.Other examples of phenols are:
Other examples of phenols
The phenolic group can be detected by the following tests: 

1. Litmus test

Phenol is a weak acid called carbolic acid, it gives red colour with litmus paper. The dissociation of phenol in water is represented as follows:
dissociation of phenol in water

2. Ferric chloride test

phenol gives a complex of violet colour as follows :
Resorcinol, o–, m– and p–cresol give violet or blue colouration, catechol gives green colour which rapidly darkens. 1 and 2–Naphthol do not give characteristics colours.

3. Liebermann’s test/Liebermann's nitroso reaction

When phenol is reacted with NaNO2 and concentrated H2SO4, it provides a deep green or blue colour which changes to red on dilution with water. The red colour is due to the formation of indophenol.Generated substance in presence of NaOH / KOH restores original green or blue colour. This reaction is termed as Liebermann's nitroso reaction.The blue or green colour is due to the formation of indophenols anion.
Liebermann’s test/Liebermann's nitroso reaction

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.

Carbonyl groups in aldehydes and ketones are identified by the following tests:

1.      2,4-dinitrophenyl hydrazine test (2,4-DNP test)

2,4-Dinitrophenylhydrazine can be used for the qualitative identification of carbonyl functionality such as ketone or aldehyde group . A successful test is indicated by the formation of a precipitate yellow, orange, or red known as dinitrophenylhydrazone.
2,4-dinitrophenyl hydrazine test (2,4-DNP test)

2,4-dinitrophenyl hydrazine test (2,4-DNP test)

2. Sodium bisulphite test

Most carbonyl functional group such as many 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.
Sodium bisulphite test of carbonyl group

2. Iodoform test

Iodoform test reagent is I2 + NaOH. Aldehydes/Ketones that have ketomethyl group CH3-C=O react with  I2 + NaOH reagent to form yellow iodoform, accumulated onto the inner surface of the reaction vessel.


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.Distinction can be made between these two types of carbonyl compounds on the basis of difference in their reactivity. Following tests are performed for the identification of aldehydic and ketonic groups:

Tests given by aldehydes only

Following tests namely Schiff’s test, Fehling’s test and Tollen’s test are given by aldehydes only

1. Schiff’s Test

Aldehydes give pink or magenta colour with Schiff’s reagent.(Note: With benzaldehyde the pink colour developes slowly.)
Schiff’s Test

2. 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. 
  • A is an aqueous solution of copper sulphate
  • 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.
Aldehydes reduces the Cu(II) ions in the fehling’s solution to red precipitate of cuprous oxide(copper (I) oxide).
Fehlings Test
Note: Benzaldehyde may or may not give this test as the reaction is very slowly.

3. Tollen’s Test

Tollen’s reagent is ammoniacal silver nitrate. Aldehydes react with Tollen’s reagent producing silver mirror on the inner surface of the vessel.
Tollen’s Test


Tests given by ketone only

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.
Sodium nitroprusside Test

Tests for Carboxylic group

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Assignments:

1. The best reagent/test to distinguish between the following compounds

  1. Borsche’s reagent
  2. Tollen’s reagent
  3. Sodium nitroprusside solution
  4. Ferric chloride solution

2. The best reagent/test to distinguish between the following compounds

  1. Biuret reaction
  2. Hydroxamic acid test
  3. Hinsberg reaction
  4. Azo dye formation

3. The best reagent/test to distinguish between the following compounds

  1. Azo dye test
  2. Alkaline hydrolysis with NaOH
  3. Biuret test
  4. Bicarbonate solution

4. The chemical composition of the reagents used is given to you.  Give the names by which they are known in the chemistry lab.

  1. Diamminesilver (I) cation
  2. 2,4-Dinitrophenylhydrazine
  3. Fuchsin-sulphurous acid

5.The most appropriate reagent to effect the following conversion is?



Reference

Books:

  • Frederick George Mann, Bernard Charles Saunders, Practical organic chemistry. 1960
  • Solomons, Fryhle. Organic Chemistry, 8th Edition Wiley-India, 2006.
  • The systematic identification of organic compounds by Ralph L Shriner 8th edition

Webliography

Searches related

Functional Group Tests to Classify Organic Chemicals

Procedures for Identification Tests for Organic Chemicals

How do you identify the functional group of an organic compound?

Tests for the functional groups (Theory) : Class 12 : Chemistry

Detection of Functional Groups (Theory) : Organic Chemistry

Tests for the Functional Groups Present in Organic Compounds

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Advanced Chemistry: Functional Group Tests to Classify Organic Compounds
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