Lassaigne's Test for Detection of Elements

• Organic compounds usually contain carbon and hydrogen, but oxygen, nitrogen, halogens, and sulphur can also be present.

Detection of Extra Elements

• Lassaigne's Test is used to confirm the presence of elements like nitrogen, chlorine, bromine, iodine, and sulphur.
• In Lassaigne's Test, salt is fused with sodium metal to create a water-soluble salt called sodium fusion extract (or Lassaigne's extract).
• To prepare sodium fusion extract, a small amount of organic compound is heated with dry sodium in a fusion tube.
• Then, the red hot tube is crushed in distilled water, and the filtered solution is Lassaigne's extract.
• Chemical reactions of various elements with Na are:
  • Na + C + N → NaCN
  • 2Na + S → Na₂S
  • Na + X → NaX (Where X is Cl, Br or I)
• If both nitrogen and sulphur are present in the compound, sodium thiocyanate forms: Na + C + N + S → NaSCN
• Sodium thiocyanate is formed only if there is a small amount of sodium metal.

Detection of Nitrogen

• Add sodium fusion extract to a test tube with freshly prepared saturated ferrous sulphate solution.
• Then acidify the solution with concentrated sulphuric acid to yield hexacyanidoferrate (II).
• Heating with sulfuric acid oxidizes some iron (II) ions to iron (III), which reacts with sodium hexacyanidoferrate (II) to produce iron (III) hexacyanidoferrate (II).
• A Prussian blue precipitate indicates the presence of nitrogen.
  • 6CN⁻ + Fe²⁺ → [Fe(CN)₆]⁴⁻ (hexacyanidoferrate (II)).
  • 4Fe³⁺ + 3[Fe(CN)₆]⁴⁻ + XH₂O → Fe₄[Fe(CN)₆]₃·XH₂O (Prussian blue or Iron (III) hexacyanidoferrate (II)).

Detection of Sulphur (when nitrogen is not present)

• Add sodium fusion extract with sodium nitroprusside, a violet color indicates sulphur is present, according to:
  • S²⁻ + [Fe(CN)₅NO]²⁻ → [Fe(CN)₅NOS]⁴⁻
• Adding acetic acid and lead acetate to the extract, where a black precipitate of lead sulphide confirms sulphur, according to:
  • Pb²⁺ + S²⁻ → PbS (black precipitate).

Test for Nitrogen and Sulphur Together

• When both nitrogen and sulphur are present, sodium thiocyanate is formed.
• The sodium fusion extract is acidified with dilute HCl, and 2-3 drops of FeCl3 are added
• A blood red colour confirms both nitrogen and sulphur in the organic compound.
  • Fe³⁺ + SCN⁻ → [Fe(SCN)]²⁺

Detection of Halogens

• To test for halogens, remove nitrogen and sulphur as HCN and H₂S by adding a few drops of concentrated HNO₃.
  • NaCN + HNO₃ → NaNO₃ + HCN
  • Na₂S + 2HNO₃ → 2NaNO₃ + H₂S
• Adding AgNO₃ to the solution and a white precipitate shows chlorine, which dissolves in ammonium hydroxide.
  • Ag⁺ + Cl⁻ → AgCl (white precipitate)
  • AgCl + 2NH₃ → [Ag(NH₃)₂]Cl (soluble)
• If a yellow precipitate partially soluble in ammonium hydroxide solution forms, bromine is confirmed.
  • Ag⁺ + Br⁻ → AgBr (Yellow precipitate)
• If iodine ion forms a dark yellow ppt that is insoluble in ammonia solution.
  • Ag⁺ + I⁻ → AgI (Dark yellow precipitate)

Layer Test for Bromine and Iodine

• Add sodium fusion extract with CCl₄ and chlorine water.
• An orange layer confirms bromine.
• A violet layer confirms iodine.

Test for Alcoholic [R-OH] Group

• Ceric Ammonium Nitrate Test:
• Alcoholic compounds react with ceric ammonium nitrate to give a red colour.
  • (NH₄)₂[Ce(NO₃)₆] + 3ROH → [Ce(NO₃)₄(ROH)₃] + 2NH₄NO₃

Lucas Test

• Lucas reagent (ZnCl₂ and conc. HCl) converts alcohols into alkyl chlorides, done by:
  • 1° alcohol: No reaction at room temperature.
  • 2° alcohol: Turbidity appears within minutes.
  • 3° alcohol: Turbidity appears immediately.

Iodoform Test

• Given by alcohols and carbonyl compounds with α-methyl groups.
• Heating the compound with I₂ + Na₂CO₃/I₂ + NaOH/NaOI/NaIO₃ gives yellow iodoform.
  • CH₃CH₂OH → CHI₃ + HCOONa
• A yellow precipitate of iodoform confirms the test.

Test for Phenolic [Ar – OH] Group

• Ferric Chloride Test: phenols form a violet complex with freshly prepared FeCl₃ solution.
  • 6C₆H₅OH + FeCl₃ → [Fe(C₆H₅O)₆]³⁻ + 3HCl
• Resorcinol, o-, m-, and p-cresol give violet or blue color
• Catechol gives green color that darkens quickly.
• 1- and 2-Naphthol do not give characteristic colors.
• Always use freshly prepared, neutral and very dilute solution of ferric chloride for best results.

Phthalein Dye Test

• Phenols condense with phthalic anhydride in the presence of concentrated H₂SO₄.
• Phenol condenses to give phenolphthalein, which gives a dark pink color with NaOH solution.
• This is called phthalein dye test.

Test for Aldehydic and Ketonic Groups (-CHO and -C=O)

• 2,4-Dinitrophenylhydrazine test (2,4-DNP test): aldehydes and ketones react with 2,4-Dinitrophenylhydrazine.
• The product is yellow/red for aldehydes/ketones.

Tests to Distinguish Aldehydes and Ketones

• Schiff's, Fehling's, and Tollen's tests are exclusive to aldehydes.
• Carbonyl compounds that test positive to 2,4-DNP, Fehling's, Schiff's, and Tollen's contain an aldehydic group
• Ketones will test negative in the above tests.

Fehling's Reagent

• A mix of Fehling solution A (aqueous copper sulphate) and Fehling solution B (alkaline sodium potassium tartarate).
• Fehling reagent (mixture) is unstable so mix the two solutions right before reaction. It has copper with tartarate.
• Cupric ions oxidize aldehydes into carboxylic acid and orange-red Cu₂O
• Aromatic aldehydes do not give positive Fehling results.
  • RCHO + 2Cu²⁺ (complexed) + 5OH⁻ → RCOO⁻ + Cu₂O + 3H₂O
• Benedict solution is more stable a modified Fehling solution using copper sulphate and sodium citrate.

Schiff’s Test

• Schiff's reagent is decolorized p-rosaniline hydrochloride with sodium sulphite or SO₂ gas.
• Pink color on adding Schiff's reagent indicates an aldehyde group

Tollens’ Test

• Add sodium hydroxide solution to silver nitrate solution to get silver oxide precipitate.
• The precipitate is dissolved in ammonium hydroxide.
  • An aqueous or alcoholic solution added to Tollens' reagent is heated over water.
  • Silver metal layer (silver mirror) indicates an aldehyde.
  • RCHO + 2[Ag(NH₃)₂]⁺ + 2OH⁻ → 2Ag + 3NH₃ + H₂O + RCOONH₄

Test for Carboxyl Group [-COOH]

• Sodium Hydrogencarbonate Test
  • Blue Litmus turns red
  • The carbon dioxide gas is released with effervescence when they are added to an aqueous solution, according to:
    • RCOOH + NaHCO₃ → RCOONa + H₂O + CO₂ Note: This NaHCO₃ test distinguishes carboxylic acids from phenols

Ether Test

• Organic compound has a carboxylic acid group when added to alcohols in acidic medium
• Then a compound with fruity sweet smell is formed.
  • RCOOH + R'OH → RCOOR' + H₂O

Test for Amino Group (-NH₂)

• Carbylamine Test:
• Primary amine heated with chloroform and alkali forms foul-smelling isocyanide or carbylamine:
• RNH₂ + CHCl₃ + 3KOH → R-NC + 3KCl + 3H₂O
• Secondary and Tertiary amines do not give positive carbylamine tests.
• Solubility Test: Amines react with acids to form water-soluble salts.
• Azo Dye Test
• Identifies primary aromatic amines using two steps:
• Diazotization: Aromatic amine + NaNO₂ and HCl produces diazonium chloride at 0°C -5°C.

Coupling Reaction

• Adding β-naphthol to diazonium salt causes coupling reaction to form sparingly soluble scarlet dye.
• The formation of a scarlet red dye confirms the presence of aromatic primary amine.

Potash Alum

• Crystallisation of an equimolar mixture of potassium sulphate and aluminium sulphate results in the formation of potash alum.
  • K₂SO₄ + Al₂(SO₄)₃ + 24H₂O → K₂SO₄·Al₂(SO₄)₃·24H₂O or 2KAl(SO₄)₂·12H₂O

Mohr's Salt

• Crystallisation of an equimolar mixture of ferrous sulphate and ammonium sulphate results in the formation of Mohr's salt.
  • FeSO₄ + (NH₄)₂SO₄ + 6H₂O → FeSO₄(NH₄)₂SO₄·6H₂O

Preparation of Double Salt: Potassium Aluminium Sulphate (Potash Alum)

• Dissolve 6.6 g of aluminium sulphate with 0.4 mL of dilute sulphuric acid in 10 mL of distilled water, and heat to 40°C. Add 2.4 g of powdered potassium sulphate and heat, then cool the mixture. Separate white crystals of potash alum through cooling. Wash crystals with cold water and alcohol, then dry.

Preparation of Double Salt: Ferrous Ammonium Sulphate

• Take ferrous sulphate and ammonium sulphate in 2:1 ratio and dissolve in minimum distilled water.
• Add 0.5-1 mL of dilute sulphuric acid and concentrate by heating. Light green crystals of ferrous ammonium sulphate form on cooling. Wash with cold water and alcohol, then dry.

Preparation of Acetanilide

• Acetanilide is prepared from aniline by replacement of one H-atom from -NH₂ group with acetyl group (-C-CH₃) using glacial acetic acid.
• Acetylation of aniline is done using acetic anhydride or acetyl chloride.
• Procedure:
  • Take 5 mL of Aniline in 100 mL round bottom flask and mix with 5 mL acetic anhydride and 5 mL glacial acetic acid [acetylating mixture]
  • Fit air condenser and add pumice, then refluxmixture on sand bath for 10-15 minutes.
  • Steps
    • Cooling
    • Filtering

Preparation of p-nitroacetanilide

• Acetanilide is prepared using acetylation of aniline.
• Formation of p-Nitroacetanilide follows electrophilic aromatic substitution reaction with nitronium ion (NO₂). - NH-C-CH₃ of acetanilide donates electrons to the benzene ring via +M effect, directing to ortho and para positions.
• Procedure:
  • Dissolve 2 g acetanilide in 2 mL glacial acetic acid in 100 mL beaker.
    • Add 4 mL concentrated H₂SO₄, and cool the mixture in ice at 0 - 5°C.

Preparation of Aniline Yellow

• Diazonium salt and aniline form p-aminoazobenzene (yellow dye). This follows electrophilic substitution.
• Procedure: -Dissolve powdered diazoaminobenzene in 7 mL aniline in a 100 mL conical flask. -Steps include: - Mixture of aniline hydrochloride. - Temperature of mixture lowered by standing at room temperature.

Preparation of Iodoform

• Iodoform (CHI₃) is a pale yellow crystalline solid with a unique odor, used as a mild antiseptic and disinfectant.
• Iodoform can be prepared by treating organic compound containing CH₃CH(OH)- or CH₃CO- with iodine with sodium hydroxide. with either ethanol or propanone.

Acids and Bases

• Acids taste sour/ are proton donors
• Bases taste bitter and slippery, and are proton acceptors.
• Arrhenius theory: acids generate H+ ions and bases generate OH- ions in solution.
• Brønsted-Lowry theory defines acids and bases as proton donors and acceptors.
• Lewis definition describes acids and bases as electron-pair acceptors and donors.
• pH of Acids and Bases
  • The pH scale measures acidity or basicity and ranges from 0-14, zero being acidic and 14 basic.
  • Litmus paper can also indicate acidity, acid turns red and alkaline turns blue.
• acid-base indicators are sensitive to pH change.

Methyl Orange

• Methyl orange is a weak base that is yellow in colour in the unionised form.
• Working pH range of indicator 3.1 to 4.5