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Chemistry · Organic Chemistry — Basic Principles & Techniques

Purification of Organic Compounds

An organic compound isolated from a natural source or made in the laboratory is almost never pure. Before its structure can be established or its properties studied, it must be freed of accompanying impurities. This subtopic, drawn from the techniques section of the Class 11 unit on the basic principles of organic chemistry, surveys the standard purification methods — sublimation, crystallisation, distillation in its several forms, differential extraction and chromatography — and the physical principle each one exploits. For NEET it is a reliably scoring area: questions ask you to match a separation problem to the correct technique.

Why Purification Matters

The behaviour of an organic compound — its reactions, its melting and boiling points, its spectra — can be interpreted reliably only if the sample under study is a single, pure substance. A purification technique works by exploiting a difference in some physical property between the desired compound and the impurities present. Solubility, volatility (boiling point), the tendency to pass directly from solid to vapour, and the relative affinity for two immiscible phases are the properties most commonly used.

There is no universal method. The technique chosen depends on the physical state of the compound, the nature of the impurities, and the quantity of material available. The criterion that a compound is pure is then checked against a fixed physical constant — a sharp melting point for a solid, a constant boiling point for a liquid, or a single spot in chromatography.

Source note (please read). The standard NCERT Class 11 file supplied for this unit was found to be encoding-corrupted (the body text is rendered as shifted mojibake and cannot be quoted reliably), and the available NIOS Chapter 23 covers nomenclature and qualitative/quantitative analysis rather than purification in depth. Accordingly, the descriptions below present well-established, standard NEET-syllabus content on purification, stated conservatively. No specific numeric data (boiling points, percentages, temperatures) has been invented; where a figure is genuinely diagnostic — for example that water and a steam-volatile compound co-distil below 100 °C — only the qualitative principle is asserted.

Sublimation

Certain solids, on being heated, pass directly from the solid state to the vapour state without melting, and the vapour reverts to the solid on cooling. This process is sublimation, and it is the basis of a purification method for solids that sublime when the impurities present are non-volatile.

The impure solid is heated; the volatile compound sublimes and its vapour is collected as a pure solid on a cool surface placed above the sample, while the non-volatile impurity is left behind. Substances purified in this way include camphor, naphthalene, anthracene and benzoic acid.

Concept check

Sublimation needs a sublimable solid

The method applies only to the relatively small set of solids that sublime. If the desired solid does not sublime, or if the impurity is itself volatile, sublimation will not separate the two; crystallisation is then the general-purpose alternative for solids.

Solid that sublimes + non-volatile impurity → sublimation.

Crystallisation

Crystallisation is the most widely used technique for purifying solid organic compounds. It rests on a difference in solubility of the compound and its impurities in a suitable solvent. A good crystallisation solvent dissolves the compound abundantly when hot but only sparingly when cold, while either leaving the impurities undissolved or keeping them in solution.

The impure solid is dissolved in the minimum volume of hot solvent to give a nearly saturated solution. Insoluble impurities are removed by hot filtration. On cooling, the solution becomes supersaturated and the pure compound separates as crystals, while soluble impurities, being present in small amount, stay behind in the mother liquor. The crystals are filtered, washed and dried.

When a single solvent is unsuitable, a combination of two miscible solvents — one in which the compound is very soluble and one in which it is nearly insoluble — may be used. Coloured impurities are removed by boiling the solution with a little activated charcoal before filtration. Repeated crystallisation improves purity further.

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Keep going

Once a compound is pure, you confirm which elements it contains and in what proportion. See Qualitative & Quantitative Analysis for detection and estimation of elements.

Distillation and Its Variants

Distillation separates substances on the basis of a difference in volatility, that is, in boiling point. A liquid is heated until it boils, the vapour is led into a condenser where it cools back to liquid, and the condensate (the distillate) is collected. The form of distillation chosen depends on how far apart the boiling points lie and on whether the compound is stable at its boiling point.

thermometer water jacket condenser distillate (receiver)
Figure 1. Simple distillation. The volatile liquid boils in the flask, its vapour is cooled in the water-jacketed condenser, and the pure distillate is collected in the receiver. The thermometer bulb is set at the side-arm to read the vapour temperature.

Simple distillation

Simple distillation is used to separate a volatile liquid from a non-volatile impurity, or to separate two liquids whose boiling points differ by a wide margin. The more volatile component vaporises first and is condensed and collected, leaving the less volatile material behind.

Fractional distillation

When two volatile liquids have boiling points that lie close together, simple distillation cannot resolve them, because both contribute substantially to the vapour. A fractionating column is inserted between the flask and the condenser. As vapour rises and condensate falls inside the column, repeated cycles of vaporisation and condensation occur; the more volatile component is progressively enriched in the rising vapour, while the less volatile one drains back. This is the basis of separating components of petroleum and of air.

Distillation under reduced pressure

A liquid boils when its vapour pressure equals the external pressure. If the external pressure is lowered, the liquid boils at a lower temperature. This is exploited for compounds that decompose at or below their normal boiling point: by distilling under reduced pressure (vacuum distillation), the compound is volatilised at a temperature below its decomposition point. A standard industrial example is the recovery of glycerol from spent lye in soap manufacture.

Steam distillation

Steam distillation is applied to compounds that are steam-volatile and immiscible with water, the impurities being non-volatile in steam. Steam is passed through the heated impure liquid. Because the compound and water are immiscible, the mixture boils when the sum of the vapour pressures of the two equals the atmospheric pressure; this occurs below the boiling point of water, so the compound distils over with the steam at a comparatively low temperature and is recovered from the condensate, which separates into two layers. Aniline is the classic compound purified this way.

NEET trap

Steam distillation vs fractional vs simple — read the conditions

These three are the most heavily examined distinctions. Pin them to their conditions:

Steam distillation → compound is steam-volatile and water-immiscible, impurity non-volatile (e.g. aniline; separating ortho- and para-nitrophenol, where the intramolecularly H-bonded ortho isomer is steam-volatile).

Fractional distillation → two volatile liquids with a small boiling-point gap (a fractionating column is essential).

Simple distillation → volatile liquid + non-volatile impurity, or two liquids with a large boiling-point gap.

Differential Extraction

When an organic compound is present in an aqueous solution, it can be recovered by solvent (differential) extraction, which relies on the compound distributing itself unequally between two immiscible liquids. The aqueous solution is shaken in a separating funnel with an organic solvent — such as ether — in which the compound is more soluble than it is in water. The compound passes preferentially into the organic layer, which is then separated and the solvent evaporated to recover the compound.

Extraction is more efficient when carried out with several small portions of solvent rather than one large portion, because each fresh portion re-establishes the favourable distribution. If the compound is only sparingly soluble in the organic solvent, a continuous extraction apparatus is used so that the same solvent is repeatedly cycled through the aqueous solution.

Chromatography

Chromatography separates the components of a mixture by their differing distribution between a stationary phase and a moving (mobile) phase. It is valuable for separating closely related compounds, for purifying small quantities, and for isolating compounds that are not easily resolved by the other methods. Two broad classes are distinguished by the kind of interaction with the stationary phase.

Adsorption chromatography

Here the components are adsorbed to different extents on the surface of a solid stationary phase — commonly silica gel or alumina — as a mobile liquid (the eluent) carries the mixture along. The more strongly adsorbed a component, the more slowly it moves. Two important forms exist.

eluent (mobile phase) silica / alumina (stationary phase) band A (faster) band B (slower) collect fractions
Figure 2. Column chromatography (adsorption). The mixture is loaded at the top of a packed column; as eluent flows down, the less strongly adsorbed component (band A) travels faster and emerges first, while the more strongly adsorbed component (band B) lags behind — separating into distinct bands collected as successive fractions.

In column chromatography the adsorbent is packed into a vertical glass column. The mixture is placed at the top and the eluent is run through; components separate into bands that move down at different rates and are collected separately as they emerge. In thin layer chromatography (TLC) a thin film of adsorbent is coated on a glass or plastic plate; a spot of the mixture is applied near one end, and the plate is stood in a solvent that rises by capillary action, carrying the components to different heights as separate spots.

Partition chromatography

In partition chromatography the components distribute (partition) themselves between two liquid phases — a stationary liquid held on a solid support and a moving liquid. Paper chromatography is the standard example: the moisture trapped within the cellulose fibres of the chromatography paper acts as the stationary liquid phase, and the developing solvent is the mobile phase. A spot of the mixture is applied on a baseline, the solvent ascends the paper, and the components are carried to different distances according to how they partition between the two phases.

Worked idea — Rf value

How are separated spots identified in TLC and paper chromatography?

Each component is characterised by its retardation factor, $$R_f = \dfrac{\text{distance moved by the component (spot)}}{\text{distance moved by the solvent front}}$$ measured from the same baseline in the same run. Under fixed conditions $R_f$ is characteristic of a compound and lies between 0 and 1, so comparing $R_f$ values lets you identify the components of a mixture.

Master Table: Technique → Principle → Use

The following table is the single most useful revision artefact for this subtopic. NEET questions on purification almost always reduce to choosing the right row for a stated separation problem.

TechniquePrinciple (property exploited)Suitable for
Sublimation Direct solid → vapour → solid transition on heating Solids that sublime (camphor, naphthalene, benzoic acid) with non-volatile impurities
Crystallisation Difference in solubility in a chosen solvent (hot vs cold) General method for purifying solids; soluble impurities stay in mother liquor
Simple distillation Difference in volatility (boiling point) Volatile liquid + non-volatile impurity, or liquids with a large boiling-point gap
Fractional distillation Repeated vaporisation–condensation in a fractionating column Two (or more) volatile liquids with close boiling points
Distillation under reduced pressure Boiling point falls when external pressure is lowered Liquids that decompose at or below their normal boiling point (e.g. glycerol)
Steam distillation Mixture boils when combined vapour pressures equal atmospheric pressure Steam-volatile, water-immiscible compounds with non-volatile impurities (e.g. aniline)
Differential extraction Unequal distribution between two immiscible liquids Recovering a compound from its aqueous solution using an organic solvent
Adsorption chromatography (column, TLC) Differential adsorption on a solid stationary phase Separating components of a mixture; small-scale purification and monitoring
Partition chromatography (paper) Differential partition between a stationary and a mobile liquid phase Separating and identifying closely related compounds by Rf
Quick recap

Purification of Organic Compounds in one screen

  • Every method magnifies a difference in a physical property — solubility, volatility, sublimability or phase affinity — between the compound and its impurities.
  • Solids: sublimation (sublimable solid, non-volatile impurity) or crystallisation (the general solubility-based method).
  • Liquids: simple (large bp gap / non-volatile impurity), fractional (close bp), reduced-pressure (decomposes near bp), steam (steam-volatile, water-immiscible).
  • Differential extraction recovers a compound from aqueous solution into an immiscible organic solvent; small repeated portions work best.
  • Chromatography: adsorption (column, TLC) on a solid; partition (paper) between two liquids. Identify spots by Rf = distance moved by spot ÷ distance moved by solvent front.
  • Caveat: the supplied NCERT source was encoding-corrupted, so these are standard-syllabus descriptions; no numeric data has been fabricated.

NEET PYQ Snapshot — Purification of Organic Compounds

Real NEET questions where the task is to match a separation problem to the correct technique.

NEET 2017

The most suitable method of separation of a 1 : 1 mixture of ortho- and para-nitrophenols is:

  1. Steam distillation
  2. Sublimation
  3. Chromatography
  4. Crystallisation
Answer: (1) Steam distillation

ortho-Nitrophenol has intramolecular H-bonding and so a lower boiling point and is steam-volatile, whereas para-nitrophenol is associated by intermolecular H-bonding and has a higher boiling point. The steam-volatile ortho isomer distils over with steam, separating the two.

Concept

A liquid organic compound decomposes a little below its normal boiling point. Which technique purifies it without decomposition?

  1. Simple distillation
  2. Distillation under reduced pressure
  3. Steam distillation
  4. Sublimation
Answer: (2) Distillation under reduced pressure

Lowering the external pressure lowers the boiling point, so the compound can be distilled below its decomposition temperature — the principle used to purify glycerol from spent lye.

Concept

Which technique is best suited to separate two volatile liquids whose boiling points differ by only a few degrees?

  1. Simple distillation
  2. Fractional distillation
  3. Differential extraction
  4. Crystallisation
Answer: (2) Fractional distillation

A small boiling-point gap requires a fractionating column, which provides repeated vaporisation–condensation cycles to enrich the more volatile component in the rising vapour.

Concept

In paper chromatography, the stationary phase is:

  1. The cellulose of the paper itself
  2. The water held in the pores of the paper
  3. The developing solvent that ascends the paper
  4. Silica gel coated on the paper
Answer: (2) The water held in the pores of the paper

Paper chromatography is a partition method: the moisture trapped in the cellulose acts as the stationary liquid phase, while the developing solvent is the mobile phase.

FAQs — Purification of Organic Compounds

Common doubts on choosing and explaining the right purification technique.

What is the basic principle behind purification of organic compounds?
Every purification technique exploits a difference in some physical property between the desired compound and its impurities — solubility, volatility (boiling point), the tendency to sublime, or the relative affinity for two immiscible phases. By selecting a method that magnifies one such difference, the compound is made to behave differently from its impurities and can be separated. The choice of method therefore depends on the nature of the compound and the impurities present, and on the quantity available.
When is sublimation used instead of crystallisation?
Sublimation is used when the desired compound passes directly from the solid to the vapour state on heating (and the vapour reverts to solid on cooling), while the impurities are non-volatile. Crystallisation, by contrast, is used when the compound is a solid whose solubility in a suitable solvent differs markedly from that of the impurities. So sublimation is reserved for the comparatively small set of solids that sublime — such as camphor, naphthalene, anthracene and benzoic acid — whereas crystallisation is the general-purpose method for purifying solids.
What is the difference between simple and fractional distillation?
Simple distillation separates a volatile liquid from a non-volatile impurity, or two liquids whose boiling points differ by a large margin. Fractional distillation is required when the components are both volatile and have boiling points that are close together; a fractionating column is inserted between the flask and the condenser, providing repeated cycles of vaporisation and condensation so that the more volatile component is progressively enriched in the vapour rising up the column.
Why is distillation under reduced pressure used for some compounds?
Some compounds decompose at, or even below, their normal boiling point, so they cannot be distilled at atmospheric pressure. Lowering the external pressure lowers the temperature at which the liquid boils, because a liquid boils when its vapour pressure equals the external pressure. The compound can then be distilled at a temperature below its decomposition point. This technique is used industrially in the purification of glycerol from spent lye in soap manufacture.
Which compounds can be purified by steam distillation?
Steam distillation is suitable for compounds that are steam-volatile and immiscible with water, while the impurities are non-volatile in steam. Such a compound is distilled along with water at a temperature below 100 °C because the mixture boils when the sum of the vapour pressure of the compound and that of water equals the atmospheric pressure. Aniline is a classic example purified by steam distillation.
What is the difference between adsorption and partition chromatography?
In adsorption chromatography the components of a mixture are adsorbed to different extents on the surface of a solid stationary phase such as silica gel or alumina; column chromatography and thin layer chromatography are of this type. In partition chromatography the components distribute themselves to different extents between two liquid phases — a stationary liquid held on a solid support and a moving liquid; paper chromatography is an example, where the moisture trapped in the cellulose of the paper acts as the stationary liquid phase.
What is Rf value in chromatography?
The retardation factor, Rf, is the ratio of the distance moved by a component (the spot) from the baseline to the distance moved by the solvent front from the same baseline, measured in the same run. It is a characteristic of a given compound under fixed conditions and is used to identify components in thin layer and paper chromatography. Because it is a ratio of two distances it has no units and lies between 0 and 1.
Related Topics
Organic Chemistry — Basic Principles & Techniques Back to the full chapter overview and all subtopics. Qualitative & Quantitative Analysis Detecting and estimating the elements in a purified organic compound. Classification of Organic Compounds How organic compounds are grouped by skeleton and functional group.