Class 12 Chemistry Practicals Analysis Of Organic Compound For General Procedure

ANALYSIS OF ORGANIC COMPOUND FOR GENERAL PROCEDURE

Question 1. State the experimental procedure, observation, and inference for the preliminary odour test in the analysis of an organic compound. [1 Mark]

Answer:

Teacher's Note:
a) Odour gives a preliminary clue regarding the functional group present.
b) Phenolic and fruity odours point towards specific homologous series.

Question 2. Describe the litmus paper test used in the preliminary analysis of organic compounds. [1 Mark]

Answer:

Teacher's Note:
a) Moist litmus paper helps determine the acidic or basic nature of the organic compound.
b) Carboxylic acids and phenols show acidic character.

Question 3. Outline the sodium bicarbonate test for identifying carboxylic acids. [1 Mark]

Answer:

Teacher's Note:
a) Brisk effervescence is due to the evolution of carbon dioxide gas.
b) This test distinguishes carboxylic acids from phenols.

Question 4. Explain the action of Borsch's Reagent on an organic compound. [1 Mark]

Answer:

Teacher's Note:
a) Borsch's reagent is 2,4-dinitrophenylhydrazine (2,4-DNP).
b) Formation of a coloured precipitate confirms carbonyl compounds.

Question 5. Give the experimental details for the charring test. [1 Mark]

Answer:

Teacher's Note:
a) Concentrated sulfuric acid acts as a dehydrating agent.
b) Charring indicates the elimination of water from carbohydrate molecules.

Question 6. How is the ignition test performed to distinguish between aliphatic and aromatic compounds? [1 Mark]

Answer:

Teacher's Note:
a) High carbon percentage in aromatic rings causes incomplete combustion leading to soot.
b) Aliphatic compounds generally burn with a clean, non-sooty flame.

Question 7. Describe the test for unsaturation using bromine water. [1 Mark]

Answer:

Teacher's Note:
a) Decolourisation confirms the addition of bromine across double or triple bonds.
b) This is a standard test for alkenes and alkynes.

8. Test with KMnO4 solution: [2 Marks]
Take small amount of organic compound in test tube add water to dissolve it and few drops of very dil KMnO4 solution and shaken well

Answer:
1. The KMnO4 test is used to detect unsaturation or specific functional groups like aromatic amines and phenols.
2. Decolouration with a white precipitate indicates aromatic amines or phenol, whereas no decolouration shows a saturated compound.

Teacher's Note:
a) Potassium permanganate is a strong oxidizing agent that gets reduced by unsaturated bonds and oxidizable functional groups.
b) Always use very dilute KMnO4 solution to clearly observe the colour change without masking the precipitate.

Test for selected organic functional group
Test for aldehydes:
1. TEST FOR ALDEHYDE
Tollen's reagent test: [2 Marks]
Take 2 ml of Tollen's regent in a clean dry test tube. Add 3-4 drops of an organic compound to it, and warm the mixture on a water bath for about 2 minutes.

Answer:
1. Tollen's reagent is ammoniacal silver nitrate solution.
2. Formation of a shining silver mirror on the inner walls of the test tube confirms the presence of an aldehyde group.

Teacher's Note:
a) The test tube must be thoroughly cleaned with nitric acid and rinsed with distilled water to obtain a clear silver mirror.
b) Tollen's reagent should be freshly prepared to avoid the formation of explosive silver fulminate.

2. Fehling's test: [2 Marks]
Take 1 ml each of Fehling's solution A and B are taken in a test tube. Add 4-5 drops of an organic compound (or 0.5g of solid) to it, and warm the mixture on a water bath for about 5 minutes.

Answer:
1. Fehling's solution A is aqueous copper sulphate and Fehling's solution B is alkaline sodium potassium tartrate.
2. A red precipitate of cuprous oxide is formed when warmed with an aliphatic aldehyde.

Teacher's Note:
a) Aromatic aldehydes generally do not respond to Fehling's test.
b) Equal volumes of solutions A and B must be mixed before adding to the organic compound.

3. TEST FOR PHENOL
Neutral FeCl3 test: [2 Marks]
Take 1 ml of FeCl3 solution in a dry test tube. Add 2 - 3 drops or pinch of organic compound to it. If no colouration occurs add 3-4 drops of alcohol

Answer:
1. Phenols react with neutral ferric chloride to form colored coordination complexes.
2. The specific characteristic color observed distinguishes between simple phenols and naphthols.

Teacher's Note:
a) The ferric chloride solution used must be neutral; acidic solutions suppress complex formation.
b) Alcohol is added as a cosolvent if the organic compound is insoluble directly in water.

4. TEST FOR CARBOXYLIC ACID
Esterification reaction: [2 Marks]
Take 1 ml or a pinch of an organic compound in a clean test tube. Add 1 ml of ethyl alcohol and 4 - 5 drops of Conc. H2SO4 to it.Heat the reaction mixture strongly about 5 minutes. Then pour the mixture in to beaker containing dil.Na2CO3 solution and note the smell.

Answer:
1. Carboxylic acids react with alcohols in the presence of concentrated sulphuric acid to form sweet-smelling esters.
2. The sodium carbonate solution neutralizes unreacted acid and makes the fruity smell of the ester more prominent.

Teacher's Note:
a) Concentrated H2SO4 acts as both a catalyst and a dehydrating agent.
b) Heating should be done gently or on a water bath to avoid vigorous spurting.

5. TEST FOR KETONE
Legal's test: [2 Marks]
A small amount of the organic substance is taken in a test tube. Add 1 ml of sodium nitro prusside solution and 1 ml of sodium hydroxide solution is added in dropwise.

Answer:
1. Methyl ketones react with sodium nitroprusside in alkaline medium to give a red or blood-red coloration.
2. The appearance of red color confirms the presence of a ketonic group having an active methylene group.

Teacher's Note:
a) Sodium hydroxide must be added dropwise to maintain the appropriate alkalinity.
b) This test is specific for compounds containing a \(\text{-CH}_2\text{CO-}\) group.

6. TEST FOR AMINES: Dye test: Take A small amount of an organic substance in a clean test tube, add 2 ml of HCl to dissolve it. Add few crystals of NaNO₂, and cool the mixture in ice bath then add 2 ml of ice cold solution of \(\beta\)-naphthol in NaOH [2 Marks]

Answer:
1. Observation: Scarlet red dye is obtained.
2. Inference: Presence of aromatic primary amine.

Teacher's Note:
a) Primary aromatic amines undergo diazotization with nitrous acid followed by coupling with \(\beta\)-naphthol.
b) Ensure the reaction mixture is maintained at ice-cold temperatures to prevent the decomposition of the diazonium salt.

7. TEST FOR DIAMIDE: Biuret test: Take a small amount of an organic compound in a test tube. Heat strongly and then allow to cool. Dissolve the residue with 2 ml of water. To this solution add 1 ml of dil. CuSO₄ and few drops of 10% NaOH solution drop by drop [2 Marks]

Answer:
1. Observation: Violet colour is appeared.
2. Inference: Presence of a diamide.

Teacher's Note:
a) Urea and similar diamides on strong heating lose ammonia to form biuret.
b) Add copper sulfate carefully to avoid excess reagent masking the characteristic violet coordination complex.

8. TEST FOR CARBOHYDRATES: Molisch’s test: Take A small amount of an organic compound in a test tube. It is dissolved in 2 ml of water. Add 3-4 drops of \(\alpha\)-naphthol to it. Then add conc. H₂SO₄ in sides of the test tube [2 Marks]

Answer:
1. Observation: Violet or purple ring is formed at the junction of the two liquid.
2. Inference: Presence of carbohydrate.

Teacher's Note:
a) Concentrated sulfuric acid dehydrates carbohydrates to form furfural derivatives.
b) Pour sulfuric acid slowly down the side of the test tube to form a distinct layer without mixing.

9. OSAZONE TEST: Take A small amount of an organic compound in a test tube. Add 1 ml of phenylhydrazine solution and heat the mixture for about 5 minutes on boiling water bath [2 Marks]

Answer:
1. Observation: Yellow crystals are obtained.
2. Inference: Presence of carbohydrate.

Teacher's Note:
a) Reducing sugars react with excess phenylhydrazine to form characteristic osazone crystals.
b) Maintain a constant temperature using a boiling water bath for uniform crystal formation.

Report: Complete the given organic compound test report format [2 Marks]
The given organic compound contains ;
(i) Aromatic / Aliphatic
(ii) Unsaturated / Saturated
(iii) functional group
So, The given organic compound is ______ .

Answer:
1. The given organic compound contains specific structural features determined through systematic functional group analysis.
2. Fill in the blanks based on experimental results: (i) Aromatic or Aliphatic, (ii) Unsaturated or Saturated, (iii) specific functional group name, and conclude with the identified name of the compound.

Teacher's Note:
a) Summarizes the cumulative findings of preliminary and confirmatory tests.
b) Ensure all blanks match the observations recorded in the practical notebook.