Class 12 Chemistry Notes Chapter 8 (Tests For Functional Groups in Organic Compounds) – Lab Manual (English) Book
Detailed Notes with MCQs of a crucial aspect of practical organic chemistry – identifying the functional groups present in an organic compound. This chapter, 'Tests For Functional Groups in Organic Compounds', is vital not just for your practical exams but also forms the basis for many questions in competitive government exams where chemistry is a component. Understanding these tests helps in deducing the structure and properties of unknown organic substances.
Chapter 8: Tests For Functional Groups in Organic Compounds - Detailed Notes
1. Introduction
Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. Identifying these groups is the first step in determining the structure of an unknown organic compound. We use simple, qualitative chemical tests based on specific reactions that produce observable changes (like colour change, precipitation, gas evolution).
2. Tests for Unsaturation (Presence of C=C or C≡C)
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(a) Baeyer's Test (Alkaline KMnO₄ Test):
- Reagent: 1% Alkaline Potassium Permanganate (KMnO₄) solution (Baeyer's Reagent). It's purple in colour.
- Procedure: Dissolve a small amount of the organic substance in water or acetone. Add Baeyer's reagent dropwise and shake.
- Observation: The purple colour of KMnO₄ disappears (decolourises), and a brown precipitate of Manganese Dioxide (MnO₂) may form.
- Inference: Presence of unsaturation (double or triple bond).
- Reaction (General):
C=C + H₂O + [O] (from KMnO₄) → C(OH)-C(OH) (Glycol) + MnO₂ (Brown ppt)
C≡C + [O] → Carboxylic acids / Ketones + MnO₂ - Note: Compounds easily oxidised (like aldehydes, some alcohols, phenols) can also give this test positive.
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(b) Bromine Water Test:
- Reagent: Bromine dissolved in water (Br₂/H₂O) or Carbon Tetrachloride (Br₂/CCl₄). It's reddish-brown in colour.
- Procedure: Dissolve the organic substance in water or CCl₄. Add bromine water/solution dropwise and shake.
- Observation: The reddish-brown colour of bromine disappears (decolourises).
- Inference: Presence of unsaturation.
- Reaction (General - Addition):
C=C + Br₂ → C(Br)-C(Br) (Vicinal Dibromide)
C≡C + 2Br₂ → C(Br₂)-C(Br₂) (Tetrabromoalkane) - Note: Phenols and anilines react via substitution and also decolourise bromine water, often forming a precipitate.
3. Tests for Alcoholic Group (-OH)
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(a) Sodium Metal Test:
- Reagent: Clean, dry piece of Sodium metal.
- Procedure: Take the dry organic liquid/solution in a dry test tube. Add a small, dry piece of sodium metal.
- Observation: Brisk effervescence (bubbles) of Hydrogen gas.
- Inference: Presence of an alcoholic group (or any active hydrogen like carboxylic acid, phenol).
- Reaction (General):
2R-OH + 2Na → 2R-ONa (Sodium Alkoxide) + H₂↑
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(b) Esterification Test:
- Reagent: Glacial Acetic Acid (CH₃COOH) and conc. Sulphuric Acid (H₂SO₄).
- Procedure: Heat a small amount of the organic compound with acetic acid and a few drops of conc. H₂SO₄. Pour the mixture into a beaker containing sodium bicarbonate solution.
- Observation: A fruity smell (due to ester formation).
- Inference: Presence of an alcoholic group.
- Reaction (General):
R-OH + R'-COOH (H⁺) ⇌ R'-COOR (Ester) + H₂O
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(c) Ceric Ammonium Nitrate Test:
- Reagent: Ceric Ammonium Nitrate solution. It's typically yellow.
- Procedure: Add a few drops of the reagent to a small amount of the organic compound dissolved in water or dioxane.
- Observation: Appearance of a pink or red colour.
- Inference: Presence of an alcoholic group.
- Note: Phenols give a brown or black colour/precipitate.
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(d) Lucas Test (Distinguishing Primary (1°), Secondary (2°), and Tertiary (3°) Alcohols):
- Reagent: Anhydrous Zinc Chloride (ZnCl₂) dissolved in concentrated Hydrochloric Acid (HCl) (Lucas Reagent).
- Procedure: Add Lucas reagent to the alcohol at room temperature and shake. Observe the appearance of turbidity (cloudiness).
- Observations & Inference:
- Tertiary (3°) Alcohols: Turbidity appears immediately. (Reaction via SN1, stable 3° carbocation forms quickly).
- Secondary (2°) Alcohols: Turbidity appears within 5-10 minutes. (Reaction via SN1, less stable 2° carbocation forms slower).
- Primary (1°) Alcohols: Turbidity appears only upon heating, or not at all at room temperature. (Reaction via SN2 is slow with HCl).
- Reaction (General):
R-OH + HCl (ZnCl₂) → R-Cl (Alkyl Chloride - insoluble, causes turbidity) + H₂O
4. Tests for Phenolic Group (-OH attached directly to Benzene Ring)
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(a) Litmus Test:
- Procedure: Test the aqueous or alcoholic solution of the compound with blue litmus paper.
- Observation: Blue litmus turns red.
- Inference: Presence of an acidic group (Phenols are weakly acidic).
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(b) Ferric Chloride Test:
- Reagent: Neutral Ferric Chloride (FeCl₃) solution. (Prepare by adding dilute NaOH to FeCl₃ solution dropwise until a faint permanent brown precipitate appears, then filter).
- Procedure: Add a few drops of neutral FeCl₃ solution to the aqueous or alcoholic solution of the compound.
- Observation: Appearance of violet, blue, green, or red-brown colour.
- Inference: Presence of a phenolic group.
- Note: Salicylic acid also gives this test positive due to the phenolic -OH. Most simple alcohols do not give this test.
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(c) Libermann's Nitroso Reaction: (Specific for phenols with a free para position)
- Procedure: Heat the phenol with conc. H₂SO₄ and Sodium Nitrite (NaNO₂). A deep green or blue solution is formed. Dilute with water; the colour changes to red. Add excess NaOH or KOH solution; the original green or blue colour reappears.
- Inference: Presence of a phenolic group.
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(d) Phthalein Dye Test:
- Procedure: Heat the compound with Phthalic Anhydride and a few drops of conc. H₂SO₄. Cool the mixture and pour it into a dilute NaOH solution.
- Observation: Appearance of a pink, blue, or green colour (depending on the phenol). Phenolphthalein itself is formed from phenol, giving a pink colour in alkali.
- Inference: Presence of a phenolic group.
5. Tests for Aldehydic (-CHO) and Ketonic (>C=O) Groups (Carbonyl Group)
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(a) 2,4-Dinitrophenylhydrazine Test (2,4-DNP Test or Brady's Test): (General test for Carbonyl)
- Reagent: 2,4-Dinitrophenylhydrazine solution (Brady's Reagent).
- Procedure: Add Brady's reagent to the organic compound (dissolved in alcohol if needed).
- Observation: Formation of a yellow, orange, or red precipitate (2,4-dinitrophenylhydrazone derivative).
- Inference: Presence of a carbonyl group (aldehyde or ketone).
- Reaction (General):
>C=O + H₂N-NH-C₆H₃(NO₂)₂ → >C=N-NH-C₆H₃(NO₂)₂ (Precipitate) + H₂O
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(b) Tollen's Test (Silver Mirror Test): (Specific for Aldehydes)
- Reagent: Tollen's Reagent - Ammoniacal Silver Nitrate solution. [Prepare fresh: Add dilute NaOH to AgNO₃ solution. A brown ppt of Ag₂O forms. Add dilute Ammonia solution dropwise just until the precipitate dissolves].
- Procedure: Warm the organic compound gently with freshly prepared Tollen's reagent in a clean test tube.
- Observation: Formation of a silver mirror on the inner walls of the test tube, or a black precipitate of silver.
- Inference: Presence of an aldehydic group. (Aldehydes reduce Ag⁺ to Ag).
- Reaction (General):
R-CHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → R-COO⁻ + 2Ag↓ (Silver Mirror) + 4NH₃ + 2H₂O - Note: Ketones do not give this test (except α-hydroxy ketones).
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(c) Fehling's Test: (Specific for Aliphatic Aldehydes)
- Reagent: Fehling's Solution A (Copper Sulphate solution) and Fehling's Solution B (Alkaline solution of Sodium Potassium Tartrate - Rochelle Salt). Mix equal volumes just before use. The resulting solution is deep blue (due to Cu²⁺ complex).
- Procedure: Warm the organic compound with freshly prepared Fehling's solution.
- Observation: Formation of a reddish-brown precipitate of Cuprous Oxide (Cu₂O). The blue colour fades.
- Inference: Presence of an aliphatic aldehydic group.
- Reaction (General):
R-CHO + 2Cu²⁺(complex) + 5OH⁻ → R-COO⁻ + Cu₂O↓ (Red-brown ppt) + 3H₂O - Note: Aromatic aldehydes (like Benzaldehyde) generally do not give this test. Ketones do not give this test.
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(d) Schiff's Test: (Specific for Aldehydes)
- Reagent: Schiff's Reagent (Rosaniline hydrochloride solution decolourised by Sulphur Dioxide). It is colourless.
- Procedure: Add a few drops of Schiff's reagent to the aqueous solution of the organic compound and shake.
- Observation: Restoration of pink, red, or magenta colour.
- Inference: Presence of an aldehydic group.
- Note: Ketones do not give this test. Heating should be avoided as SO₂ might escape, restoring colour even without an aldehyde.
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(e) Iodoform Test (Sodium Hypoiodite Test): (Test for Methyl Ketones or compounds oxidisable to methyl ketones)
- Reagent: Iodine (I₂) solution and Sodium Hydroxide (NaOH) solution (or Sodium Hypoiodite, NaOI).
- Procedure: Warm the organic compound with I₂ solution and NaOH solution.
- Observation: Formation of a yellow precipitate of Iodoform (CHI₃) with a characteristic antiseptic smell.
- Inference: Presence of a methyl ketone (CH₃-C=O group) or an alcohol group like CH₃CH(OH)- (which gets oxidised to a methyl ketone under reaction conditions, e.g., Ethanol, Isopropanol).
- Reaction (General for Methyl Ketone):
R-CO-CH₃ + 3I₂ + 4NaOH → R-COONa + CHI₃↓ (Yellow ppt) + 3NaI + 3H₂O
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(f) Sodium Nitroprusside Test: (Test for Ketones)
- Reagent: Sodium Nitroprusside solution and NaOH solution.
- Procedure: Add a few drops of sodium nitroprusside solution to the compound, then add NaOH solution dropwise.
- Observation: Appearance of a red or reddish-violet colour.
- Inference: Presence of a ketone (specifically one with an α-methyl or α-methylene group). Aldehydes usually don't give this test.
6. Tests for Carboxylic Acid Group (-COOH)
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(a) Litmus Test:
- Procedure: Test the aqueous solution/suspension of the compound with blue litmus paper.
- Observation: Blue litmus turns red.
- Inference: Presence of an acidic group (Carboxylic acids are acidic).
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(b) Sodium Bicarbonate Test (Sodium Hydrogen Carbonate Test):
- Reagent: 5% Sodium Bicarbonate (NaHCO₃) solution.
- Procedure: Add NaHCO₃ solution to a small amount of the organic compound.
- Observation: Brisk effervescence (bubbles) of Carbon Dioxide (CO₂) gas.
- Inference: Presence of a carboxylic acid group.
- Reaction (General):
R-COOH + NaHCO₃ → R-COONa + H₂O + CO₂↑ - Note: Strong electron-withdrawing groups can make some phenols acidic enough to give this test (e.g., Picric acid), but most simple phenols do not.
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(c) Esterification Test:
- Reagent: An alcohol (e.g., Ethanol) and conc. Sulphuric Acid (H₂SO₄).
- Procedure: Heat the carboxylic acid with an alcohol and a few drops of conc. H₂SO₄. Pour into sodium bicarbonate solution.
- Observation: Fruity smell of ester.
- Inference: Presence of a carboxylic acid group.
- Reaction (General):
R-COOH + R'-OH (H⁺) ⇌ R-COOR' (Ester) + H₂O
7. Tests for Amino Group (-NH₂) (Amines)
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(a) Solubility Test:
- Procedure: Test the solubility of the compound in dilute Hydrochloric Acid (HCl).
- Observation: The compound dissolves in dilute HCl.
- Inference: Presence of a basic amino group (forms soluble salt).
- Reaction (General):
R-NH₂ + HCl → R-NH₃⁺Cl⁻ (Soluble Salt)
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(b) Litmus Test:
- Procedure: Test the aqueous solution of the compound with red litmus paper.
- Observation: Red litmus turns blue (for aliphatic amines). Aromatic amines are generally too weak to affect litmus significantly.
- Inference: Presence of a basic group (Aliphatic amines are basic).
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(c) Carbylamine Test (Isocyanide Test): (Specific for Primary Amines - Aliphatic or Aromatic)
- Reagent: Chloroform (CHCl₃) and Alcoholic Potassium Hydroxide (KOH).
- Procedure: Warm the organic compound with alcoholic KOH and a few drops of chloroform. (Caution: Isocyanides are highly toxic and have extremely unpleasant odours. Perform in a fume hood).
- Observation: Formation of an extremely foul-smelling substance (Isocyanide or Carbylamine).
- Inference: Presence of a primary amino group (1° amine).
- Reaction (General):
R-NH₂ + CHCl₃ + 3KOH (alc.) → R-N≡C (Isocyanide - foul smell) + 3KCl + 3H₂O - Note: Secondary (2°) and Tertiary (3°) amines do not give this test.
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(d) Azo Dye Test: (Specific for Primary Aromatic Amines, e.g., Aniline)
- Procedure:
- Diazotisation: Dissolve the amine in dilute HCl and cool in an ice bath (0-5 °C). Add cold aqueous Sodium Nitrite (NaNO₂) solution dropwise. This forms a diazonium salt solution.
- Coupling: Prepare an alkaline solution of β-naphthol (2-naphthol) in NaOH solution, also cooled in ice. Add the cold diazonium salt solution slowly to the β-naphthol solution.
- Observation: Formation of a brightly coloured (red, orange, or yellow) precipitate or dye (Azo dye).
- Inference: Presence of a primary aromatic amino group.
- Reactions (General for Aniline):
C₆H₅NH₂ + NaNO₂ + 2HCl (0-5°C) → C₆H₅N₂⁺Cl⁻ (Benzene Diazonium Chloride) + NaCl + 2H₂O
C₆H₅N₂⁺Cl⁻ + β-Naphthol (in NaOH) → Bright Orange/Red Azo Dye - Note: Aliphatic primary amines form unstable diazonium salts that decompose, usually without forming a dye.
- Procedure:
Multiple Choice Questions (MCQs)
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Which reagent is used in Baeyer's test for unsaturation?
a) Bromine water
b) Alkaline KMnO₄ solution
c) Tollen's reagent
d) Lucas reagent
Answer: b) -
A silver mirror formation with Tollen's reagent indicates the presence of:
a) Ketonic group
b) Alcoholic group
c) Aldehydic group
d) Carboxylic acid group
Answer: c) -
Lucas test is used to distinguish between:
a) Aldehydes and Ketones
b) Primary, Secondary, and Tertiary Alcohols
c) Aliphatic and Aromatic Amines
d) Alkenes and Alkynes
Answer: b) -
Which functional group gives a characteristic fruity smell when heated with alcohol and concentrated H₂SO₄?
a) Phenol
b) Carboxylic acid
c) Aldehyde
d) Amine
Answer: b) -
The formation of a yellow precipitate with an antiseptic smell on warming an organic compound with Iodine and NaOH solution indicates the presence of:
a) -CHO group
b) -COOH group
c) CH₃-C=O group or CH₃CH(OH)- group
d) Phenolic -OH group
Answer: c) -
Neutral Ferric Chloride solution gives a characteristic colour (violet/blue/green) with:
a) Alcohols
b) Aldehydes
c) Phenols
d) Carboxylic acids
Answer: c) -
Brisk effervescence with Sodium Bicarbonate solution is a characteristic test for:
a) Alcohols
b) Phenols (most simple ones)
c) Carboxylic acids
d) Amines
Answer: c) -
The Carbylamine test, involving heating with chloroform and alcoholic KOH, is given by:
a) Secondary amines
b) Tertiary amines
c) Primary amines
d) All amines
Answer: c) -
Fehling's test (formation of red-brown precipitate) is typically positive for:
a) Aromatic aldehydes
b) Ketones
c) Aliphatic aldehydes
d) Tertiary alcohols
Answer: c) -
Brady's reagent (2,4-DNP) gives a yellow/orange/red precipitate indicating the presence of:
a) An unsaturated bond
b) A carbonyl group (aldehyde or ketone)
c) An amino group
d) An alcoholic group
Answer: b)
Study these tests thoroughly, focusing on the reagents, specific conditions (like temperature), observations, and the functional group they identify. Pay attention to the exceptions and limitations of each test. Good luck with your preparation!