Purification — crystallisation, sublimation, distillation, differential extraction & chromatography (principles & applications)

Why Purify Organic Compounds?

Organic compounds obtained from natural sources or synthesised in a laboratory are rarely pure. They are typically contaminated with by-products, unreacted starting materials, or other substances from the source. The process of removing these unwanted impurities to obtain a pure substance is called purification.

A pure compound has a definite set of physical properties, such as a sharp melting point and boiling point. Impurities alter these properties, for instance, by lowering the melting point and broadening its range. Therefore, purification is an essential first step before any analysis or characterisation of a compound can be performed. The choice of purification method depends on the nature of the substance (solid/liquid) and the type of impurities present.

1. Crystallisation

This is the most common method used to purify solid organic compounds. It is based on a simple principle: the difference in solubility of the compound and its impurities in a suitable solvent.

The Principle of Crystallisation

The core idea is to find a solvent in which the compound to be purified is sparingly soluble at room temperature but significantly more soluble at a higher temperature (usually the solvent's boiling point). The impurities, ideally, should either be completely insoluble in the hot solvent (so they can be filtered off) or highly soluble even in the cold solvent (so they remain in the solution when the desired compound crystallises out).

The process involves dissolving the impure solid in the minimum amount of hot solvent to create a saturated solution. Upon cooling this solution, the solubility of the desired compound decreases, and it separates out from the solution in the form of pure crystals, leaving the more soluble impurities behind in the solution (the mother liquor).

{VISUAL: diagram: A labelled diagram showing the setup for crystallisation. It should include a beaker with the saturated solution, which is then filtered hot using a funnel with fluted filter paper to remove insoluble impurities. The filtrate is then shown in another beaker being cooled in an ice bath to form crystals. Finally, the crystals are filtered using a Büchner funnel and suction filtration.}

Steps in Crystallisation

1. Choice of Solvent: This is the most critical step. A good solvent should:

   • Dissolve the compound when hot but not when cold.
   • Not react chemically with the compound.
   • Either not dissolve the impurities at all, or dissolve them so well that they remain in solution upon cooling.
   • Be volatile enough to be easily removed from the purified crystals.
   • Commonly used solvents include water, ethanol, methanol, acetone, ethyl acetate, and hexane. Sometimes a mixture of solvents is used.

2. Preparation of the Solution: The impure solid is placed in a flask, and the solvent is added in small portions. The mixture is heated until all of the solid dissolves, creating a saturated or near-saturated solution.

3. Hot Filtration: If any insoluble impurities are present (like dust or sand), the hot saturated solution is quickly filtered through a fluted filter paper into a clean beaker. This must be done quickly to prevent the solution from cooling and crystallising prematurely in the funnel.

4. Cooling and Crystallisation: The hot, clear filtrate is allowed to cool slowly and undisturbed. As the solution cools, it becomes supersaturated, and the pure solid begins to crystallise out. Slow cooling promotes the formation of large, well-defined crystals. Rapid cooling (e.g., in an ice bath) leads to smaller crystals.

5. Separation of Crystals: The crystals are separated from the mother liquor by filtration, usually using a Büchner funnel under suction to remove as much solvent as possible.

6. Washing and Drying: The crystals on the filter paper are washed with a small amount of cold, pure solvent to remove any adhering mother liquor. They are then dried in an oven or a desiccator.

Fractional Crystallisation

This technique is used to separate a mixture of two or more solids that have different solubilities in the same solvent. When a hot solution of such a mixture is cooled, the substance which is less soluble crystallises out first, while the more soluble one remains in the solution. By repeated crystallisation steps, the components can be separated.

{CALLOUT: type=exam | text=The key to fractional crystallisation is the difference in solubility. The compound with the lower solubility value at a given temperature will precipitate first upon cooling. This is a common concept tested in JEE.}

2. Sublimation

Sublimation is a process where a substance transitions directly from the solid to the gas phase, without passing through the liquid phase. The reverse process, gas to solid, is called deposition. This technique is used to separate a sublimable volatile compound from non-sublimable impurities.

The impure substance is heated in a dish. The sublimable compound turns into vapour, leaving the non-volatile impurities behind. The vapours are then cooled on a cold surface (like a funnel cooled with water or plugged with wet cotton), where they deposit back into pure solid crystals.

{VISUAL: diagram: A simple labelled diagram of a sublimation apparatus. It shows a china dish containing the impure solid being heated on a sand bath. An inverted funnel is placed over the dish, with its stem loosely plugged with cotton wool. Pure crystals of the sublimed substance are shown deposited on the cold inner walls of the funnel.}

This method is only applicable to a specific class of compounds.

{KEY: type=points | title=Common Sublimable Compounds | text=

• Naphthalene
• Camphor
• Iodine
• Benzoic acid
• Anthracene
• Ammonium chloride (NH₄Cl) }

3. Distillation

Distillation is a widely used technique for purifying liquids and separating mixtures of liquids with different boiling points. The principle involves heating a liquid to create vapour, then cooling the vapour to condense it back into a liquid, which is collected in a separate container.

Simple Distillation

This method is used to separate:

1. A mixture of two liquids with a large difference in their boiling points (typically > 25 °C or 25 K).
2. A volatile liquid from a non-volatile solid solute (e.g., purifying water from salt).

The liquid with the lower boiling point (the more volatile component) vaporises first. The vapours pass through a condenser, which cools them and turns them back into a pure liquid (the distillate). The less volatile component remains in the original flask.

Example: A mixture of chloroform (boiling point = 334 K) and aniline (boiling point = 457 K) can be easily separated by simple distillation. Chloroform will vaporise, condense, and be collected first.

{VISUAL: diagram: A labelled setup for simple distillation. It must show a round-bottomed flask (distilling flask) with the liquid mixture, a thermometer with its bulb positioned just below the side arm opening, a Liebig condenser with water inlet at the bottom and outlet at the top, and a receiving flask (receiver) to collect the distillate.}

Fractional Distillation

What if the boiling points of the liquids in a mixture are very close? Simple distillation won't work effectively. In this case, we use fractional distillation.

The apparatus is similar to simple distillation but with the crucial addition of a fractionating column placed between the distilling flask and the condenser. This column is packed with obstacles like glass beads, rings, or has a complex internal structure. These packings provide a large surface area for repeated cycles of vaporisation and condensation.

How it works: As the mixed vapours rise through the column, they cool, condense, and re-vaporise multiple times. In each cycle, the vapour becomes progressively richer in the more volatile component. By the time the vapours reach the top of the column, they consist almost entirely of the pure, more volatile liquid. These vapours then enter the condenser and are collected as the first fraction.

Applications: Fractional distillation is used extensively in industry, most famously in the petroleum industry to separate crude oil into various useful fractions like petrol, diesel, kerosene, and lubricating oil. It is also used to separate a mixture of acetone (b.p. 329 K) and methanol (b.p. 338 K).

{VISUAL: diagram: A labelled diagram of a fractional distillation apparatus. It should look similar to the simple distillation setup but with a fractionating column (e.g., a Vigreux column or a packed column) inserted between the flask and the condenser.}

Distillation Under Reduced Pressure (Vacuum Distillation)

Many organic compounds have very high boiling points. If we try to boil them at atmospheric pressure, they might decompose before reaching their boiling point. To purify such liquids, we use vacuum distillation.

The Principle: A liquid boils at the temperature at which its vapour pressure becomes equal to the external pressure. By reducing the pressure above the liquid surface using a vacuum pump, we can lower the temperature at which it boils.

This allows the liquid to be distilled at a temperature well below its normal boiling point, thus preventing decomposition.

Example: Glycerol has a normal boiling point of 563 K (290 °C), at which it decomposes. By reducing the pressure to about 12 mm Hg, its boiling point is lowered to 453 K (180 °C), allowing it to be distilled without decomposition.

Steam Distillation

This special type of distillation is used to purify substances that are:

1. Steam volatile (i.e., have an appreciable vapour pressure at the boiling point of water).
2. Immiscible with water.
3. Have a high boiling point.
4. The impurities present should be non-volatile.

The Principle: When a mixture of two immiscible liquids is heated, it boils when the sum of their individual vapour pressures equals the atmospheric pressure. P_total = P_A + P_B where P_A and P_B are the partial vapour pressures of the two components.

For a mixture of an organic compound and water, it will boil when: P_atm = p_organic + p_water

Since p_water is significant, p_organic required to reach P_atm is much less than the atmospheric pressure itself. This means the organic liquid vaporises and distills at a temperature lower than its own boiling point and also lower than the boiling point of water (100 °C).

Example: Aniline has a boiling point of 184 °C. However, when distilled with steam, it boils at 98 °C. This method is used to extract essential oils from plants, like clove oil and eucalyptus oil.

{VISUAL: diagram: A diagram of the steam distillation apparatus. It shows a steam generator flask, a main round-bottomed flask containing the impure organic compound and some water (with a steam inlet tube reaching the bottom and a vapour outlet), a condenser, and a receiver where two layers (organic and aqueous) are collected.}

The composition of the vapour phase is determined by the partial pressures and molar masses of the components. According to Dalton's law of partial pressures: (n_s / n_w) = (p_s / p_w)

Where n_s and n_w are moles of substance and water, and p_s and p_w are their partial pressures. Since n = W/M (mass/molar mass), we can write: (W_s / M_s) / (W_w / M_w) = p_s / p_w

Rearranging this gives the crucial formula for steam distillation calculations:

{FORMULA: expr=W_s / W_w = (p_s × M_s) / (p_w × M_w) | symbols=W_s: Mass of substance distilled, W_w: Mass of water distilled, p_s: Vapour pressure of substance, p_w: Vapour pressure of water, M_s: Molar mass of substance, M_w: Molar mass of water}

{{SOLVE: {"problem":"An organic compound was subjected to steam distillation. The distillation was carried out at 99.4 °C at a total pressure of 754.4 torr. The vapour pressure of water at this temperature is 744.4 torr. Calculate the ratio of the mass of the organic compound to the mass of water collected in the distillate. The molar mass of the compound is 150 g/mol.","type":"numerical","subject":"chemistry","intro":"Let's tackle this typical JEE problem on steam distillation using the formula we just learned. Let's head to the whiteboard.","outro":"And that's how you solve it. The key is to first find the partial pressure of the organic compound. Now, let's get back to the lesson.","steps":[{"explanation":"First, we need to find the partial pressure of the organic substance (p_s). We know the total pressure (P_total) and the partial pressure of water (p_w). The total pressure is the sum of the partial pressures.","write":"P_total = p_s + p_w"},{"explanation":"Now, let's plug in the given values and solve for p_s. The total pressure is 754.4 torr and water's vapour pressure is 744.4 torr.","write":"754.4 = p_s + 744.4","tough":false},{"explanation":"Subtracting 744.4 from both sides gives us the partial pressure of the organic compound.","write":"p_s = 754.4 - 744.4 = 10.0 torr"},{"explanation":"Now we use the main steam distillation formula to find the ratio of masses (W_s / W_w).","write":"W_s / W_w = (p_s × M_s) / (p_w × M_w)","tough":true,"alt_explanation":"The formula connects the mass ratio to the pressure ratio and molar mass ratio. Remember to match substance with substance and water with water."},{"explanation":"Let's substitute all the known values: p_s = 10.0 torr, p_w = 744.4 torr, M_s = 150 g/mol, and M_w (molar mass of water) = 18 g/mol.","write":"W_s / W_w = (10.0 × 150) / (744.4 × 18)","tough":false},{"explanation":"Now, we just need to calculate the final value. 10 times 150 is 1500. 744.4 times 18 is approximately 13400.","write":"W_s / W_w = 1500 / 13399.2"},{"explanation":"Finally, calculating the ratio gives us the answer.","write":"W_s / W_w ≈ 0.112"}]}}}

4. Differential Extraction

This method is used to extract an organic compound from its aqueous solution using an organic solvent. The technique relies on the principle that the organic compound is more soluble in the organic solvent than it is in water. The organic solvent and water must be immiscible (they should not mix, like oil and water).

The process is carried out in a separating funnel. The aqueous solution containing the organic compound is placed in the funnel, and a suitable organic solvent (like ether, chloroform, or carbon tetrachloride) is added. The funnel is stoppered and shaken vigorously to allow the compound to move from the aqueous layer to the organic layer.

After shaking, the funnel is allowed to stand, and the two immiscible layers separate out. The lower layer is drained off, and the organic layer containing the desired compound is collected. This process can be repeated several times with fresh portions of the organic solvent to extract the maximum amount of the compound.

{KEY: type=definition | title=Partition Coefficient (K) | text=The ratio of the concentrations of a solute in two immiscible solvents at equilibrium is constant and is called the partition or distribution coefficient, K. <br> K = C₁ / C₂ = (Concentration in organic solvent) / (Concentration in aqueous solvent)}

It is always more efficient to perform several small extractions rather than one single large extraction. For example, using two 50 mL portions of ether one after the other is more effective than using one 100 mL portion at once.

5. Chromatography

Chromatography is a modern, powerful, and versatile purification technique used to separate mixtures, identify compounds, and determine their purity. The name comes from Greek words chroma (colour) and graphein (to write) because it was first used to separate coloured plant pigments.

The Principle: Chromatography is based on the differential movement of the components of a mixture over a stationary phase under the influence of a mobile phase. The component that interacts more strongly with the stationary phase moves slower, while the component that is more soluble in the mobile phase moves faster. This difference in movement causes the components to separate.

• Stationary Phase: A solid or liquid supported on a solid. It stays fixed in place. Examples: Silica gel, alumina, filter paper.
• Mobile Phase: A liquid or gas that flows over the stationary phase, carrying the mixture with it. Examples: Hexane, ethyl acetate, ethanol.

Adsorption Chromatography

This is based on the principle of differential adsorption. The components of the mixture are adsorbed on the surface of the stationary phase (the adsorbent) to different extents.

• Adsorbent (Stationary Phase): Commonly used adsorbents are silica gel (SiO₂) and alumina (Al₂O₃).
• Eluent (Mobile Phase): A solvent or a mixture of solvents like hexane, benzene, or chloroform.

A compound that is more strongly adsorbed on the stationary phase will move more slowly down the column, while a weakly adsorbed compound will be carried along faster by the mobile phase.

(a) Column Chromatography

This is a preparative technique used to separate individual components of a mixture. A glass tube (the column) is packed with the adsorbent. The mixture to be separated is placed on top of the column. The eluent is then passed through the column. The components separate into bands, which move down the column at different rates and can be collected in separate flasks as they exit.

(b) Thin-Layer Chromatography (TLC)

TLC is an analytical technique used for rapid separation and qualitative analysis. A thin layer of adsorbent (like silica gel) is coated onto a glass plate or plastic sheet. A spot of the mixture is applied at the bottom (the baseline). The plate is then placed in a chamber with a shallow pool of the mobile phase. The solvent moves up the plate by capillary action, carrying the mixture with it and separating the components.

The separated spots can be visualised under UV light or by spraying with a staining reagent (like iodine or potassium permanganate solution).

{VISUAL: diagram: A diagram illustrating Thin-Layer Chromatography (TLC). It shows a TLC plate with a baseline drawn near the bottom. A spot of the initial mixture is on the baseline. The plate is inside a beaker (developing chamber) with a small amount of solvent (mobile phase) at the bottom, below the baseline. A second diagram shows the plate after development, with the solvent front near the top and the initial spot separated into two or three spots at different heights. The distances travelled by a spot and the solvent front are labelled to show the calculation of the R_f value.}

A key parameter in TLC is the Retardation Factor (R_f).

{FORMULA: expr=R_f = (Distance travelled by the substance from baseline) / (Distance travelled by the solvent front from baseline) | symbols=R_f: Retardation Factor (dimensionless)}

The R_f value is a characteristic of a compound for a given stationary and mobile phase. It can be used to identify compounds by comparing their R_f values with those of known standards. R_f values always lie between 0 and 1.

Partition Chromatography

This type of chromatography is based on the continuous differential partitioning of components of a mixture between the stationary and mobile phases.

Paper Chromatography

This is a specific type of partition chromatography. A special chromatography paper (like Whatman paper) is used as the stationary phase. This paper contains water trapped in its cellulose fibres, which acts as the liquid stationary phase. The mobile phase is a suitable solvent or mixture of solvents.

The process is very similar to TLC. A spot of the mixture is applied to the paper, and the solvent moves up (or down), separating the components based on how they partition between the stationary water phase and the mobile solvent phase. The R_f values are calculated and used in the same way as in TLC.

{FLASHCARD: q="Which purification technique is best for separating a mixture of heat-sensitive, high-boiling point amino acids?" | a="Chromatography (specifically, techniques like paper chromatography or ion-exchange chromatography) is the most suitable method. Distillation would cause decomposition, and crystallisation is difficult for complex mixtures like amino acids."}