Practical Organic Chemistry

Practical organic chemistry encompasses element detection, functional group identification, important preparations, and analytical chemistry. Though a lower-weight topic, direct-recall questions appear frequently and are easy marks.

Lassaigne's Test (Sodium Fusion Extract): An organic compound is fused with sodium metal, converting C, H, N, S, and halogens into water-soluble ionic forms (NaCN, Na₂S, NaX). The cooled fusion mass is extracted with distilled water. For nitrogen detection: NaCN in the extract reacts with FeSO₄ and NaOH (boiled, cooled, acidified with H₂SO₄) to give Prussian blue precipitate (Fe₄[Fe(CN)₆]₃). For sulfur: Na₂S + sodium nitroprusside → purple/violet color; alternatively, Na₂S + lead acetate → black PbS precipitate. When both N and S are present: NaSCN forms instead, and FeCl₃ gives blood-red color (Fe(SCN)₃), NOT Prussian blue — a critical distinction. For halogens: NaX + HNO₃ + AgNO₃ → AgCl (white, soluble in NH₃), AgBr (pale yellow, partially soluble in NH₃), AgI (yellow, insoluble in NH₃). If N or S is present alongside halogens, the extract must first be boiled with HNO₃ to decompose NaCN/Na₂S (which would interfere with the AgNO₃ test).

Functional Group Detection: Phenolic -OH gives violet/blue/green color with FeCl₃; bromine water gives 2,4,6-tribromophenol (white precipitate). Aldehyde -CHO is detected by Tollens' test (silver mirror), Fehling's test (red Cu₂O), and Schiff's test (pink color with decolorized fuchsin). 2,4-DNP (orange precipitate) detects any carbonyl group (both aldehyde and ketone). Iodoform test (I₂/NaOH → yellow CHI₃) identifies methyl ketones. Carboxyl -COOH gives effervescence with NaHCO₃ (CO₂ released). Primary amine -NH₂ is detected by the carbylamine test (foul-smelling isocyanide with CHCl₃/KOH) and the azo dye test (diazotize + $\beta$-naphthol → orange-red dye).

Important Preparations: Acetanilide () is prepared by acylation of aniline with acetic anhydride or acetyl chloride. p-Nitroacetanilide is made by nitrating acetanilide (acetamido group is o/p directing, para is major due to steric preference). Iodoform () is prepared from acetone or ethanol + I₂/NaOH → CHI₃ (yellow crystals).

KMnO₄/Oxalic Acid Titration: $5C2O4^{2}$- + 2MnO₄^- + 16H+ → 10CO₂ + $2Mn^{2}$+ + 8H₂O. This titration is self-indicating — KMnO₄ (purple) decolorizes as it reacts with oxalic acid, and the endpoint is the first persistent pink color when all oxalate is consumed. Temperature must be maintained at 60-70 deg C for a measurable reaction rate (the reaction is too slow at room temperature). No separate indicator is needed.

Qualitative Salt Analysis: Cation group analysis uses systematic precipitation: Group I ($Pb^{2}$+ → PbCl₂ white), Group II ($Cu^{2}$+ → CuS black), Group III ($Al^{3}$+ → Al(OH)₃ white; $Fe^{3}$+ → Fe(OH)₃ brown), Group IV ($Zn^{2}$+ → ZnS white), Group V ($Ca^{2}$+ → CaCO₃ white), Group VI ($Mg^{2}$+ → Mg(OH)₂; NH₄+ detected by Nessler's reagent → brown precipitate). Key anion tests: $CO3^{2}$- (dilute acid → CO₂ effervescence, lime water milky), $S^{2}$- (dilute acid → H₂S rotten egg smell), $SO4^{2}$- (BaCl₂ → white BaSO₄ insoluble in HCl), NO₃^- (brown ring test with FeSO₄ + conc. H₂SO₄), Cl^-/Br^-/I^- (AgNO₃ precipitates with different colors and NH₃ solubility).

The key testable concept is the Lassaigne's test modification when both N and S are present (blood-red FeCl₃ test, not Prussian blue) and that KMnO₄/oxalic acid titration is self-indicating with no external indicator needed.