Organic Compound Qualitative Tests

• Organic chemistry focuses on compounds containing carbon and hydrogen, sometimes with N, S, O, and halogens (X).
• Tests to identify the presence of these elements are called qualitative tests.

Qualitative Tests for Carbon and Hydrogen

• A dry sample of the organic compound is heated with an oxidizing agent (e.g., pure oxygen or CuO) and a suitable catalyst.
• Carbon oxidizes to CO₂(g), hydrogen oxidizes to H₂O(g): C, H + O₂(g) → CO₂(g) + H₂O(g).
• CO₂(g) is identified by bubbling it through lime water, which turns milky.
• The reaction is CO₂(g) + Ca(OH)₂ → CaCO₃ + H₂O.
• H₂O(g) is detected using white anhydrous Copper(II) tetraoxosulphate (VI) (CuSO₄).
• CuSO₄ turns blue in the presence of H₂O: CuSO₄(s) + 5H₂O → CuSO₄.5H₂O.
• Alternatively, cobalt(II) chloride paper (CoCl₂) turns from blue to pink with H₂O: CoCl₂ + XH₂O → CoCl₂.XH₂O.

Qualitative Tests for Nitrogen, Sulfur, and Halogens (N, S, X)

• Two steps are involved: Lassaigne's Sodium Fusion Test (LSFT) and specific tests.

Lassaigne's Sodium Fusion Test (LSFT)

• The organic sample is heated with metallic sodium in a small glass test tube until red hot.
• The tube is then dropped into a clean mortar containing water, and the contents are ground and filtered.
• The filtrate contains ionic radicals of the elements present (H, C, N, S, X).
• The reactions are noted as: N + Na → NaCN, S + Na → Na₂S, X + Na → NaX.

Specific Test for Sulfur (S)

• Acidify a portion of the LSFT filtrate with dilute nitric acid or ethanoic acid.
• Add aqueous lead(II) nitrate [Pb(NO₃)₂] or lead(II) ethanoate [Pb(CH₃COO)₂] solution.
• A black precipitate (PbS) indicates sulfur.
• The reaction is Pb²⁺ + S²⁻ → PbS (black precipitate).
• Alternatively add sodium nitroprusside [Na₂[Fe(NO)(CN)₅]] to a portion of the filtrate.
• A purple or violet color indicates sulfur: S²⁻ + Na₂[Fe(NO)(CN)₅] → Na₄[Fe(CN)₅] + NO

Specific Test for Nitrogen (N)

• Make a portion of the LSFT filtrate alkaline by adding aqueous sodium hydroxide (NaOH).
• Add aqueous iron(II) sulfate (FeSO₄) solution.
• A dark green precipitate of iron(II) hydroxide [Fe(OH)₂] forms initially.
• Boil and cool the solution to expel any gases.
• Add iron(III) chloride (FeCl₃) in the presence of dilute sulfuric acid (H₂SO₄).
• A bluish green or Prussian blue precipitate indicates the presence of nitrogen.
• The reactions include: NaCN → Na⁺ + CN⁻, FeSO₄ → Fe²⁺ + SO₄²⁻, NaOH → Na⁺ + OH⁻, Fe²⁺ + 2OH⁻ → Fe(OH)₂,
• Additional reaction: Fe²⁺ + 6CN⁻ → [Fe(CN)₆]⁴⁻, FeCl₃ → Fe³⁺ +3Cl⁻,
• The final product created: 4Fe³⁺ + 3[Fe(CN)₆]⁴⁻ → Fe₄[Fe(CN)₆]₃ (bluish green/Prussian blue precipitate).
• If sulfur is present, add excess FeSO₄.
• The reaction if sulfur is present: Na₂S → 2Na⁺ + S²⁻, Fe²⁺ + S²⁻ → FeS (dark green).

Specific Tests for Chlorine, Bromine, or Iodine (Cl, Br, I)

• Acidify a portion of the LSFT filtrate with dilute nitric acid (HNO₃).
• Add excess aqueous silver nitrate (AgNO₃) solution.
• A precipitate of AgX forms.
• If X is Cl, a white precipitate (AgCl) forms.
• If X is Br, a cream precipitate (AgBr) forms.
• If X is I, a pale yellow precipitate (AgI) forms.
• AgCl is soluble in aqueous ammonia.
• AgBr is slightly soluble.
• AgI is insoluble in excess aqueous ammonia.
• The reactions include: NaX → Na⁺ + X⁻, AgNO₃ → Ag⁺ + NO₃⁻, Ag⁺ + X⁻ → AgX.
• The initial HNO₃ addition expels any cyanide (CN⁻) or sulfide (S²⁻) as HCN(g) or H₂S(g) to avoid interference.
• Add carbon tetrachloride (CCl₄) or benzene to a portion of the LSFT filtrate.
• Add chlorine water (Cl₂H₂O). The organic layer is separated from the aqueous layer.
• If the aqueous layer is colorless, chlorine is present.
• If the aqueous layer is brown or pale yellow, bromine is present.
• If the aqueous layer is violet or purple, iodine is present.

Applications to Compound Identification

• To determine C, H, N, and S in a compound like C₆H₅SO₂NH₂, use the qualitative tests.
• For C and H, describe combustion and detection of CO₂ and H₂O.
• For N and S, perform LSFT and then specific tests, including reactions and observations.

Example scenario for chlorine in organic compounds

• A student fused a chlorine-containing compound (ClC₆H₄NH₂) with sodium and dissolved the mixture in water, then added AgNO₃ without additional steps.
• The student's method isn't valid.
• Fusion converts elements to ionic forms (NaCN, NaCl). Nitric Acid (HNO3) is added to expel CN⁻ preventing the AgX precipitate.
• Corrected method is: add HNO3, then add AgNO3 is used to form AgCl.

Reactions

• C + O2 -> CO2
• H + O2 -> H2O
• CO2 + Ca(OH)2 -> CaCO3 + H2O
• CuSO4(s) + 5H2O -> CuSO4.5H2O
• CoCl2 + XH2O -> CoCl2.XH2O
• N + Na -> NaCN
• S + Na -> Na2S
• X + Na -> NaX
• Pb2+ + S2 -> PbS
• S + Na2[Fe(NO)(CN)5] -> Na4[Fe(CN)5] + NO
• NaCN -> Na+ + CN-
• FeSO4 -> Fe2+ + SO42-
• NaOH -> Na+ + OH-
• Fe2+ + 2OH -> Fe(OH)2
• Fe2+ + 6CN -> [Fe(CN)6]4
• FeCl3 -> Fe3+ + 3Cl
• 4Fe3+ + 3[Fe(CN)6] -> Fe4[Fe(CN)6]3
• Na2S -> 2Na+ + S2-
• Fe2+ + S2 -> FeS
• NaX -> Na+ + X-
• AgNO3 -> Ag+ + NO3-
• Ag+ + X -> AgX
• HNO3 -> H+ + NO3
• H+ + CN -> HCN
• AgNO3 -> Ag+ + NO3
• Ag+ + Cl -> AgCl