Isolation, Identification & Analysis of Phytoconstituents
This practical-oriented unit teaches you HOW to extract, isolate, purify, and identify specific phytoconstituents from crude plant material. You will study the step-by-step isolation procedures for key compounds from each major class: Terpenoids (Menthol, Citral, Artemisin), Glycosides (Glycyrhetinic acid, Rutin), Alkaloids (Atropine, Quinine, Reserpine, Caffeine), and Resins (Podophyllotoxin, Curcumin). Each procedure utilizes principles of solvent extraction, acid-base partitioning, chromatography, and crystallization.
Syllabus & Topics
- 1General Principles of Isolation: (1) Selection of plant material (correct species, part, season). (2) Drying and powdering. (3) Extraction (maceration, percolation, Soxhlet, solvent extraction). (4) Fractionation (liquid-liquid partition between immiscible solvents). (5) Purification (column chromatography, crystallization, distillation). (6) Identification (melting point, TLC, spectroscopy).
- 2Terpenoids – Menthol Isolation: From Mentha arvensis oil (mint oil). Steam distillation of fresh herb → crude peppermint oil. Fractional distillation under reduced pressure separates Menthol (bp 212°C) from Menthone. Alternatively, cooling the oil to -22°C causes Menthol to crystallize out. Purified by recrystallization from petroleum ether. Identified by mp 42-44°C, optical rotation, and characteristic cooling sensation.
- 3Terpenoids – Citral Isolation: From Lemongrass oil (Cymbopogon flexuosus). Steam distillation → crude lemongrass oil (70-80% Citral). Citral extracted by bisulfite method: oil shaken with saturated NaHSO₃ → forms water-soluble Citral-bisulfite adduct → washed to remove non-aldehyde components → decomposed with Na₂CO₃ → free Citral recovered by ether extraction. Identified by preparation of semicarbazone (mp 171°C).
- 4Terpenoids – Artemisin (Artemisinin) Isolation: From Artemisia annua leaves. Dried leaves extracted with petroleum ether or hexane (non-polar solvent – artemisinin is a sesquiterpene lactone with endoperoxide bridge). Concentrated extract subjected to column chromatography (silica gel, hexane:ethyl acetate gradient). Pure white crystals, mp 156-157°C. Identified by IR (endoperoxide absorption at ~830 cm⁻¹).
- 5Glycosides – Glycyrhetinic Acid Isolation: From Glycyrrhiza glabra roots. Aqueous extract of liquorice root → add H₂SO₄ (acid hydrolysis of Glycyrrhizin glycoside) → Glycyrhetinic acid (aglycone) precipitates as insoluble solid → filtered → washed → crystallized from ethanol. mp 296°C. Triterpenoid structure confirmed by IR and NMR.
- 6Glycosides – Rutin Isolation: From Ruta graveolens or Buckwheat (Fagopyrum esculentum). Plant material extracted with boiling water or dilute methanol → concentrated → cooled → Rutin crystallizes out (yellow needles). Purified by recrystallization from hot methanol-water. Identified by acid hydrolysis → Quercetin (aglycone, yellow, mp 316°C) + Rutinose (sugar). Positive Shinoda test (Mg + HCl → pink/magenta color for flavonoids).
- 7Alkaloids – General Extraction Principle: Most alkaloids are basic (contain nitrogen). Powdered drug moistened with alkali (NH₄OH/Na₂CO₃) → liberates free base. Extracted with organic solvent (CHCl₃, CH₂Cl₂). Organic layer shaken with dilute acid → alkaloid salt passes to aqueous layer. Re-basified with NH₄OH → free base re-extracted with CHCl₃. This acid-base partitioning selectively isolates alkaloids from neutral/acidic compounds.
- 8Alkaloids – Atropine Isolation: From Atropa belladonna or Datura stramonium. Powdered leaves moistened with NH₄OH → extracted with CHCl₃. Chloroform extract partitioned with dilute H₂SO₄ → aqueous acidic layer basified with NH₄OH → Atropine free base extracted with CHCl₃ → evaporated → crystallized from acetone. Identified by Vitali-Morin test (fuming HNO₃ → yellow → KOH in acetone → violet color, specific for tropane alkaloids).
- 9Alkaloids – Quinine Isolation: From Cinchona bark. Bark extracted with dilute HCl → filtered → basified with NaOH → quinoline alkaloids precipitate → extracted with CHCl₃. Quinine separated from Quinidine by fractional crystallization from ethanol (different solubilities). Identified by fluorescence (blue fluorescence in dilute H₂SO₄ under UV), Thalleioquin test (Br₂ water + NH₃ → emerald green).
- 10Alkaloids – Reserpine Isolation: From Rauwolfia serpentina roots. Root powder extracted with methanol → concentrated → partitioned between CHCl₃ and dilute acid → acid layer basified → CHCl₃ extraction → column chromatography (alumina) separates Reserpine from other alkaloids (Ajmaline, Ajmalicine). Crystallized from methanol. Identified by UV absorption (268 nm, 295 nm), vanillin-HCl test (pink color).
- 11Alkaloids – Caffeine Isolation: From tea leaves (Camellia sinensis) – simplest alkaloid isolation. Tea powder boiled with water + Na₂CO₃ (liberates caffeine from tannin complex) → filtered → extracted with CHCl₃ (caffeine partitions into organic layer) → CHCl₃ evaporated → crude caffeine → sublimation at 178°C gives pure white needles. Identified by Murexide test (caffeine + HCl + KClO₃ → heated → NH₃ vapors → purple residue).
- 12Resins – Podophyllotoxin Isolation: From Podophyllum peltatum/hexandrum rhizomes. Dried rhizome extracted with ethanol → concentrated → diluted with water → resin precipitates (podophyllin). Resin dissolved in CHCl₃ → chromatographed on silica gel column → Podophyllotoxin eluted with CHCl₃:MeOH. Crystallized from ethyl acetate. mp 183-184°C. Precursor for anticancer drugs Etoposide and Teniposide.
- 13Resins – Curcumin Isolation: From Curcuma longa (turmeric) rhizomes. Powdered turmeric extracted with acetone or ethanol (Soxhlet) → concentrated → residue dissolved in ethyl acetate → washed with water to remove sugars, proteins → ethyl acetate layer chromatographed on silica gel → Curcumin eluted with CHCl₃:MeOH (95:5). Orange-yellow crystals, mp 183°C. Identified by absorption at 420 nm (UV-Vis), positive Borate test (red color with H₃BO₃).
Learning Objectives
Exam Prep Questions
Q1. What Is Acid–Base Partitioning and Why Is It Important for Alkaloid Isolation?
Acid-base partitioning exploits the basic nitrogen present in Alkaloids. In acidic solution (such as dilute HCl or H₂SO₄), alkaloids form water-soluble salts and move into the aqueous layer. When the solution is made alkaline using bases like NH₄OH or NaOH, the alkaloids convert back to their free base form, which becomes soluble in organic solvents such as CHCl₃. By repeatedly transferring between aqueous and organic layers, alkaloids can be selectively separated from neutral and acidic plant components. This principle forms the basis of most alkaloid isolation procedures.
Q2. How Is Caffeine Isolated from Tea Leaves?
Caffeine can be isolated from Tea leaves through solvent extraction. Tea powder is boiled with water and sodium carbonate (Na₂CO₃), which breaks the caffeine–tannin complex and releases free caffeine into solution. The mixture is filtered to remove solid plant material. The filtrate is then extracted with chloroform (CHCl₃), where caffeine preferentially dissolves. Evaporation of the solvent gives crude caffeine, which is purified by sublimation to obtain white crystalline needles. Identity is confirmed using the Murexide test.
Q3. What Is the Shinoda Test?
The Shinoda test (Magnesium–HCl reduction test) is used for the identification of Flavonoids. In this test, magnesium turnings and concentrated HCl are added to an ethanolic plant extract. Different flavonoids produce characteristic colors: flavones give yellow-orange, flavonols produce red-magenta, flavanonols give red-violet, and flavanones produce crimson coloration. The color develops due to reduction of the flavonoid structure forming an anthocyanidin-like colored cation.
Q4. Why Is Sublimation Used for Caffeine Purification?
Sublimation is used because caffeine vaporizes directly from solid to gas at around 178 °C without passing through the liquid state. During heating, caffeine molecules sublime and leave behind non-volatile impurities. The vapor then condenses on a cooler surface above the sample as pure white crystalline needles, providing a simple and efficient purification method.
Q5. What Is the Bisulfite Method for Citral Isolation?
Citral is an aldehyde present in Lemongrass oil. When the oil is treated with saturated sodium bisulfite (NaHSO₃), citral reacts to form a water-soluble bisulfite addition compound while other non-aldehyde terpenoids remain in the oil layer. After separation, the bisulfite complex is decomposed using sodium carbonate (Na₂CO₃), regenerating free citral. The liberated citral is then extracted with ether to obtain purified citral.