Revision: Class 11 >> Purification and Characteristics of Organic Compounds NEET (UG) Purification and Characteristics of Organic Compounds

Stationary phase is a solid or a liquid supported on a solid which remains fixed in a place and on which different solutes are adsorbed to a different extent.

• Organic compounds obtained from natural sources or prepared in the laboratory are usually impure.
• Purification is required to obtain compounds in a pure form for the study of physical and chemical properties.
• Impurities may be solids, liquids, or gases mixed with the compound.
• Different purification methods are used based on physical properties like solubility, boiling point, and volatility.
• The choice of method depends on the nature of the compound and impurity.

• Introduced by Tswett in 1906.
• Based on the difference in rates at which different components of a mixture move through a porous medium (stationary phase) under the influence of a solvent or gas (mobile or moving phase).
• The technique separates a mixture by distributing its components between one stationary phase and one mobile phase.
• Mobile phase determines the Rᶠ or R factor — dependent on the solubility of components in the given eluent.

Based on the fact that different compounds are adsorbed to different degrees on an adsorbent.

When a mobile phase (solvent) moves over the stationary phase, the components of the mixture move by different distances depending on their adsorption tendencies.

(i) Column Chromatography

• Adsorbent (alumina Al₂O₃, silica SiO₂, Fuller's earth) is packed as a column in a long, burette-like tube → acts as the stationary phase.
• Mixture dissolved in solvent → poured on top of column.
• The most strongly adsorbed component remains at the top.
• Solvent (eluent) is passed → components elute at different rates.

(ii) Thin Layer Chromatography (TLC)

• Separation of a mixture of substances over a thin layer of adsorbent coated on a glass plate.
• A glass plate is placed in a closed jar containing eluant.
• As the eluent rises, components move to different heights depending on their degree of adsorption → separation occurs.

Based on differences in the tendencies of substances to distribute or partition between the stationary phase and mobile phase.

Paper Chromatography

• A type of partition chromatography applied as a drop on paper.
• Paper is suspended in a suitable solvent or mixture → solvent rises by capillary action.
• Paper selectively retains different components according to their partition in the two phases.
• Colourless compounds are visualised using a ninhydrin spray (which forms coloured products).

Key terms:

• Stationary phase: The phase that is stationary with respect to the components of the mixture. In column chromatography, the stationary phase adsorbs components at different heights.
• Mobile phase (eluent): The phase that moves along with the components of the mixture.

Rᶠ Factor (Retardation Factor):

\[R_f=\frac{\text{Migration distance of substance}}{\text{Migration distance of solvent front}}\]

• Used to identify the product separated using paper or thin-layer chromatography.
• Rᶠ value is constant for a given compound under given conditions.

Estimation of Carbon and Hydrogen (Liebig's Combustion Method):

Principle: A known mass w gram of organic compound is heated strongly with an excess of dry CuO in a current of dry air or oxygen.

• Carbon oxidised to CO₂ (absorbed by KOH solution)
• Hydrogen oxidised to H₂O (absorbed by anhydrous CaCl₂)

Reactions:

• \[C+2CuO\xrightarrow{\text{strongly heat}}CO_2+2Cu\]
• \[2H+CuO\xrightarrow{\text{strongly heat}}H_2O+Cu\]

Formulas:

\[\%C=\frac{12\times\text{wt of }CO_2}{44\times\text{wt of organic compound}}\times100=\frac{12\times x\times100}{44\times w}=\frac{27.27\times x}{w}\]

\[\%H=\frac{2\times\mathrm{wt~of}H_2O}{18\times\text{wt of organic compound}}\times100=\frac{2\times y\times100}{18\times w}=\frac{11.11\times y}{w}\]

Where x = mass of CO₂ absorbed, y = mass of H₂O absorbed, and w = mass of organic compound.

Dumas Method:

• A known mass w gram of the organic compound is heated with cupric oxide in an atmosphere of carbon dioxide.
• Carbon and hydrogen are oxidised to CO₂ and H₂O.
• Sulphur oxidised to SO₂.
• Nitrogen is set free — any oxide of N is reduced to free N₂ by heated copper gauze.
• Gaseous mixture (CO₂, H₂O, N₂) collected over KOH solution → all gases except N₂ are absorbed.
• Volume of N₂ measured (V mL at STP).

\[\%N=\frac{28\times V\times100}{22400\times w}=\frac{0.125\times V}{w}\]

Nitrogen is estimated as gaseous nitrogen using Dumas' method.

Kjeldahl's Method:

Faster than Dumas' method.

Used only for organic compounds that are converted quantitatively to ammonium sulphate on heating strongly with concentrated sulphuric acid.

Cannot be used for compounds containing:

• Nitro (–NO₂) and diazo (–N=N⁻) groups
• Nitrogen in the ring (e.g., pyridine, quinoline)

Step 1 — Digestion: Organic compound digested with conc. H₂SO₄ in the presence of a small quantity of potassium sulphate and copper sulphate (in Kjeldahl's flask).

\[\text{Organic compound}+\mathrm{H}_{2}\mathrm{SO}_{4}\xrightarrow{CuSO_{4}+K_{2}SO_{4}}(\mathrm{NH}_{4})_{2}\mathrm{SO}_{4}\]

Step 2 — Distillation: (NH₄)₂SO₄ decomposed with excess NaOH → NH₃ liberated → absorbed in standard acid (say H₂SO₄) of known normality:

• V_1​ mL of N normality H₂SO₄ taken
• V₂​ mL of N normality NaOH used for back titration
• Volume of H₂SO₄ left used = V₂​ mL of N normality

\[\%N=\frac{1.4\times\text{Normality of acid}\times\text{Volume of acid used to neutralise }NH_3}{\text{Mass of the substance (w)}}\]

Carius Method:

Principle: A known mass of organic compound is heated with fuming nitric acid (HNO₃) in the presence of silver nitrate (AgNO₃) in a Carius tube → carbon and hydrogen oxidised to CO₂ and H₂O → halogen present reacts with AgNO₃ to form the corresponding silver halide (AgX).

Silver halide is filtered, washed, dried, and weighed.

Formulas:

\[\%X=\frac{\text{At wt of halogen}\times\text{wt of AgX formed}\times100}{\text{Mol. wt of silver halide}\times\text{wt of organic compound}}\]

\[\%Cl=\frac{35.5\times\text{wt of AgCl}\times100}{143.5\times w}=\frac{24.74\times\text{wt of AgCl}}{w}\]

\[\%Br=\frac{80}{188}\times\frac{\text{Mass of AgBr}\times100}{\text{Mass of compound}}=\frac{42.55\times\text{wt of AgBr}}{w}\]

\[\%I=\frac{127}{235}\times\frac{\text{Mass of AgI}\times100}{\text{Mass of compound}}=\frac{54\times\text{wt of AgI}}{w}\]

Messengers Method, Carius Method, and Fusion Method are the three methods for sulphur estimation.

Carius Method for Sulphur:

• A known mass ww gram of organic compound is heated with fuming HNO₃ or sodium peroxide (Na₂O₂) in a Carius tube → sulphur is oxidised to H₂SO₄.
• In the presence of BaCl₂ solution, sulphur is precipitated as BaSO₄ → filtered, dried, and weighed.

\[\%S=\frac{32\times\text{wt of BaSO}_4\times100}{233\times\text{wt of organic compound}}=\frac{13.73\times\text{wt of BaSO}_4\mathrm{ppt}}{w}\]

• 233 g BaSO₄ contains 32 g sulphur.

Carius (Second Method) for Phosphorus

• A known mass of organic compound is heated with fuming HNO₃ in a Carius tube → phosphorus is converted to H₃PO₄.
• Magnesium mixture (MgCl₂ + NH₄Cl) is added → gives precipitate of magnesium ammonium phosphate (MgNH₄PO₄) → on heating gives magnesium pyrophosphate (Mg₂P₂O₇) → weighed.

\[\%P=\frac{\text{Atomic mass of P}}{\text{Molecular mass of Mg}_2P_2O_7}\times\frac{\text{Mass of Mg}_2P_2O_7\times100}{\text{Mass of compound}}\]

\[=\frac{62}{222}\times\frac{\text{Mass of Mg}_2\mathrm{P}_2\mathrm{O}_2\quad100}{\text{Mass of compound}}\]

\[=\frac{27.93\times\text{wt of Mg}_2\mathrm{P}_2\mathrm{O}_7\mathrm{ppt}}{w}\]

Molecular mass of Mg₂P₂O₇ = 222 g; 2 atoms of P = 62 g.