Purification And Characterisation Of Organic Compounds

Organic chemistry begins with a practical problem: a compound freshly made in the lab is almost never pure, and even after purification we still don't know what it is made of. This topic — purification, detection of elements, and estimation of their percentages — supplies both the reasoning questions and the very predictable numerical problems (percentage of C, H, N, S, halogen; empirical formula) that appear in NEET.

Criteria of Purity and the Choice of Purification Method

A pure organic solid melts at a single sharp temperature (usually within a 0.5–1 °C range), and a pure liquid boils at a fixed temperature under a fixed pressure. Impurities lower and broaden the melting range of a solid and generally raise or spread out the boiling point of a liquid. So the simplest purity check is: measure the melting or boiling point and compare with the recorded value. A modern supplementary check is chromatography — a pure substance gives a single spot on a TLC plate or a single peak in a chromatogram.

Which method you use depends on the physical properties of the compound and of the impurity — chiefly solubility, volatility, and the difference between their boiling points. The important techniques are:

• Simple crystallisation — the compound must be sparingly soluble in a solvent when cold but freely soluble when hot, while the impurity behaves oppositely (either stays dissolved in the cold mother liquor or is insoluble even in the hot solvent and is filtered off). The hot saturated solution is filtered and cooled slowly; crystals separate and are filtered, washed and dried. Coloured impurities are removed by boiling with a little activated charcoal.
• Fractional crystallisation — when two solutes have only slightly different solubilities, repeated crystallisation separates them, the less soluble one appearing in earlier crops.
• Sublimation — for solids that pass directly into vapour on heating (camphor, naphthalene, benzoic acid, anthracene) while the impurity is non-volatile.
• Simple distillation — for liquids that vaporise without decomposing, mixed with a non-volatile impurity, or for two liquids whose boiling points differ appreciably (chloroform 61 °C from aniline 184 °C).
• Fractional distillation — boiling points close together (differing by only a few degrees); a fractionating column provides many successive vaporisation–condensation cycles. This is how petroleum fractions and acetone–methanol mixtures are separated.
• Distillation under reduced pressure — for liquids that decompose at or below their normal boiling point. Lowering the external pressure lowers the boiling point; glycerol, which chars at 290 °C, is distilled this way, and sugarcane juice is concentrated in vacuum evaporators.
• Steam distillation — for substances that are volatile in steam, immiscible with water, and accompanied by non-volatile impurities. The compound distils over at a temperature below 100 °C because the mixture boils when the sum of the partial pressures of the compound and of water equals atmospheric pressure. Used for aniline and essential oils.
• Differential (solvent) extraction — a compound dissolved in water is shaken in a separating funnel with an immiscible organic solvent (ether, chloroform) in which it is more soluble; it partitions into the organic layer, which is separated and evaporated. Several small extractions are more efficient than one large one.
• Azeotropes — remember that constant-boiling mixtures (95.6% ethanol–water) cannot be separated by fractional distillation at all.

Chromatography

Chromatography separates the components of a mixture by their differing distribution between a stationary phase and a mobile phase. It is the most powerful of these methods because it works on milligram quantities and on very similar substances.

Two broad classes:

1. Adsorption chromatography — the stationary phase is a solid adsorbent (silica gel, alumina). Components are adsorbed to different extents and are therefore carried different distances by the moving solvent. This includes column chromatography, where the mixture is loaded on top of a packed column and eluted with solvent (the most weakly adsorbed component comes out first), and thin layer chromatography (TLC), where a thin layer of adsorbent on a glass plate is used and the solvent rises by capillary action.
2. Partition chromatography — the stationary phase is a liquid held on a support; separation depends on differing solubility (partition) between two liquid phases. Paper chromatography is the classic example: the water trapped in the cellulose of the chromatography paper is the stationary phase.

In TLC and paper chromatography each component is characterised by its retardation factor:

$$R_f = \frac{\text{distance travelled by the component}}{\text{distance travelled by the solvent front}}$$

$R_f$ is always less than 1 and is constant for a given compound, adsorbent and solvent. Colourless spots are detected under UV light, by iodine vapour, or by spraying a suitable reagent.

Qualitative Analysis: Detecting the Elements Present

Carbon and hydrogen are detected by heating the compound with dry copper(II) oxide: carbon is oxidised to CO₂ (turns lime water milky) and hydrogen to water (turns anhydrous CuSO₄ blue).

Nitrogen, sulphur, halogens and phosphorus are detected by the Lassaigne's test (sodium fusion test). The compound is fused with a small piece of sodium metal; the covalently bound elements are converted into ionic sodium salts (NaCN, Na₂S, NaX, Na₃PO₄), the melt is extracted with distilled water, and the filtrate — the "sodium extract" or Lassaigne's extract — is tested:

• Nitrogen: the extract is boiled with FeSO₄ solution and then acidified with concentrated H₂SO₄. A Prussian blue colour, Fe₄[Fe(CN)₆]₃, confirms nitrogen. (Cyanide from NaCN gives ferrocyanide, which is oxidised by Fe³⁺ to the blue complex.)
• Sulphur: with sodium nitroprusside the extract gives a deep violet colour; with lead acetate after acidification with acetic acid, a black precipitate of PbS.
• Nitrogen and sulphur together: if both are present, sodium thiocyanate NaSCN may form; with Fe³⁺ it gives a blood-red colour, [Fe(SCN)]²⁺, and no Prussian blue. Using excess sodium destroys NaSCN, giving the usual separate tests.
• Halogens: acidify the extract with dilute HNO₃ and add AgNO₃. A white precipitate soluble in ammonia = chlorine; pale yellow, sparingly soluble = bromine; yellow, insoluble in ammonia = iodine. If N or S is present the extract must be boiled with dilute HNO₃ first, to expel HCN and H₂S which would otherwise precipitate with silver.
• Phosphorus: heat the compound with sodium peroxide (or use the extract) and boil with concentrated HNO₃ and ammonium molybdate. A yellow precipitate of ammonium phosphomolybdate confirms phosphorus.

Quantitative Analysis: Estimating the Percentages

Each element is converted quantitatively into a weighable or measurable product; the arithmetic is always the same idea — find the mass of the element inside the product, and express it as a percentage of the mass of compound taken. Let w be the mass of the organic compound taken.

1. Carbon and hydrogen (Liebig's combustion method). A known mass is burnt in a stream of oxygen over CuO. CO₂ is absorbed in KOH solution and H₂O in anhydrous CaCl₂ (or Mg(ClO₄)₂), and the increases in mass are noted.

$$%,C=\frac{12}{44}\times\frac{m_{CO_2}}{w}\times100 \qquad\qquad %,H=\frac{2}{18}\times\frac{m_{H_2O}}{w}\times100$$

2. Nitrogen — Dumas method. The compound is heated with CuO in an atmosphere of CO₂; nitrogen is liberated as N₂ and collected over KOH solution (which absorbs CO₂) in a graduated tube. If V is the volume of N₂ reduced to STP,

$$%,N=\frac{28}{22400}\times\frac{V(\text{mL at STP})}{w}\times100$$

Reduce the volume to STP using $\dfrac{P_1V_1}{T_1}=\dfrac{P_2V_2}{T_2}$, remembering to subtract the aqueous tension from the measured pressure.

3. Nitrogen — Kjeldahl's method. The compound is heated with concentrated H₂SO₄ (with K₂SO₄ and a little CuSO₄ catalyst); nitrogen is converted to (NH₄)₂SO₄. The mixture is made alkaline with NaOH and the ammonia distilled into a known excess of standard acid; the unreacted acid is back-titrated with standard alkali. If V mL of acid of molarity M (basicity n) is actually neutralised by the ammonia,

$$%,N=\frac{1.4\times M\times n\times V}{w}$$

Kjeldahl's method is simple and is used for food, fertiliser and soil analysis, but it fails for nitro and azo compounds, for nitrogen in a ring (pyridine), and for diazo compounds, because their nitrogen is not converted to ammonium sulphate.

4. Halogens — Carius method. Heated with fuming HNO₃ and AgNO₃ in a sealed Carius tube; the halogen is weighed as silver halide.

$$%,X=\frac{\text{at. mass of }X}{\text{formula mass of }AgX}\times\frac{m_{AgX}}{w}\times100$$

(For chlorine the factor is 35.5/143.5, for bromine 80/188, for iodine 127/235.)

5. Sulphur — Carius method. Heated with fuming HNO₃ in a sealed tube, sulphur is oxidised and precipitated as BaSO₄ with BaCl₂ solution.

$$%,S=\frac{32}{233}\times\frac{m_{BaSO_4}}{w}\times100$$

6. Phosphorus — Carius method. The compound is oxidised similarly, and phosphorus is precipitated either as ammonium phosphomolybdate or, after further treatment, as magnesium ammonium phosphate (Mg₂P₂O₇ on ignition).

$$%,P=\frac{2\times31}{222}\times\frac{m_{Mg_2P_2O_7}}{w}\times100$$

7. Oxygen is rarely estimated directly; conventionally its percentage is found by difference — 100 minus the sum of the percentages of every other element detected.

From Percentage Composition to a Molecular Formula

Once every element's percentage is known, the standard four-step calculation recovers first the empirical formula and then the molecular formula, and NEET numericals test this chain constantly:

1. Divide each element's percentage by its atomic mass to get relative moles.
2. Divide every value obtained by the smallest one among them, to get a simple mole ratio.
3. If any ratio is not close to a whole number, multiply all the ratios by a small common factor (2, 3, ...) until they are — these whole numbers are the empirical formula subscripts.
4. Find the empirical formula mass, divide the experimentally known molar mass by it to get n, and multiply every subscript in the empirical formula by n to obtain the molecular formula.

The molar mass itself is obtained independently — by colligative-property methods for non-volatile solutes, Victor Meyer's method for volatile compounds, or mass spectrometry — and it is this independently measured mass that fixes n; percentage composition alone can never distinguish a compound from any whole-number multiple of its empirical formula (CH₂O could be methanal, or equally well acetic acid, or glucose).

Common Mistakes and Exam Traps

• Forgetting to subtract aqueous tension when reducing gas volume to STP in the Dumas method. The nitrogen is always collected over an aqueous solution of KOH, so the measured pressure includes water vapour pressure, which must be subtracted before applying the combined gas law — omitting this step is a very common numerical slip.
• Applying Kjeldahl's method to nitro, azo or diazo compounds, or to ring nitrogen. The method relies on nitrogen being converted to ammonium sulphate, which fails for compounds where the nitrogen is not in a simply reducible amine-like form (nitro/azo compounds) or is part of an aromatic ring (pyridine); using it there under-reports nitrogen badly, and NEET tests awareness of exactly this limitation.
• Confusing molecular formula with empirical formula when percentages alone are given. Percentage composition fixes only the simplest whole-number ratio; without an independently measured molar mass, a question that gives only percentages can be answered with an empirical formula, not a molecular one, and picking a molecular-formula-looking option is a common trap.
• Mixing up which purification method suits which physical property. Sublimation needs volatility of the solid itself (not just a low-boiling impurity); steam distillation needs the compound to be steam-volatile and water-immiscible; choosing steam distillation for a compound that is miscible with water, for instance, would not work, and exam questions often test this property-to-method matching directly.