BP305P – Organic Chemistry-II Lab Manual Practical No 1
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Table of Contents
Practical No 1
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Aim
To explore the use of different distillation and recrystallization procedures for the purification of solvents, liquids, and synthetic compounds.
Learning Objectives
After completing this practical, I will be able to:
- Understand the principles of recrystallization and distillation techniques.
- Select suitable purification methods for organic compounds.
- Perform purification of solvents, liquids, and synthetic compounds using standard laboratory procedures.
Evaluate the purity and percentage yield of the purified products.
Learning Outcomes
Upon successful completion of this practical, I will be able to:
- Apply recrystallization and distillation techniques for purification.
- Select appropriate purification methods based on compound properties.
- Assess the purity and recovery of purified products.
- Demonstrate safe laboratory practices while performing purification techniques.
Reference
- Furniss B. S., Hannaford A.J., Smith P.W.G., Tatchell A.R., Vogel’s Textbook of Practical Organic Chemistry, Fifth Ed, Longman Sci and Technical, 1989, page 963.
- Ahluwalia V.K., Agrawal Renu., Organic Synthesis Special Techniques, Nerosa publishing house, 2001, p. no. 90 – 114
Theory
- Purification in chemistry refers to the physical separation of a desirable product from impurities or contaminants. A successful purification produces a pure substance (isolate). To get the isolate, many purifying procedures may be performed, either singly or in combination.
- Filtration:
Filtration is a mechanical process for separating particles from liquids or gases by passing the combination through a porous media that holds the particulates while allowing the fluid to pass. - Centrifugation:
Centrifugation separates small suspended particles that cannot be removed by filtering by spinning the suspension at high speeds, forcing solids to settle and allow for simple decanting. - Evaporation:
Evaporation separates volatile solvents from nonvolatile solutes. - Liquid-liquid extraction:
Liquid-liquid extraction separates or recovers components based on their preference solubility in an immiscible solvent.
- Crystallization:
By taking advantage of the variations in solubility between the substance and contaminants, crystallization purifies solids. When a concentrated solution is cooled or a precipitant is added, the pure compound crystallizes and is separated using centrifugation or filtration. When the solubility of the impurities is identical, repeated crystallization is employed. - Recrystallization:
In analytical and synthetic chemistry, recrystallization is frequently used to increase the purity of compounds by dissolving them in a pure solvent and enabling controlled crystallization. - Distillation:
Distillation is a common process used in petroleum refining, ethanol purification, and the extraction of volatile components from non-volatile contaminants. It does this by separating volatile liquids according to differences in boiling points.
Purification of organic compounds
Many methods viz., Simple and fractional crystallisation, sublimation, distillation (simple/fractional/reduced pressure/steam/ azetropic), chromatography, differential extraction and chemical methods are available for the purification of substances. The choice of method, however, depends upon the nature of substance (whether solid or liquid) and the type of impurities present in it.
Simple Crystallization
This is the most common method used to purify organic solids. It is based upon the fact that whenever a crystal is formed, it tends to leave out the impurities. For crystallization, choice of suitable solvent depends on
- Solvent that dissolves more of the substance at higher temperature than at room temperature
- In which impurities are either insoluble or dissolve to an extent that they remain in solution (in the mother liquor) upon crystallization,
- Solvent which is not highly inflammable
- And which does not react chemically with the compound to be crystallized.
The most commonly used solvents for crystallization are: water, alcohol, ether, chloroform, carbon- tetrachloride, acetone, benzene, petroleum ether etc.
Examples:
- Sugar having an impurity of common salt can be crystallized from hot ethanol since sugar dissolves in hot ethanol but common salt does not.
- A mixture of benzoic acid and naphthalene can be separated from hot water in which benzoic acid dissolves but naphthalene does not.
Fractional Crystallization
The process of separation of different components of a mixture by repeated crystallizations is called fractional crystallization. The mixture is dissolved in a solvent in which the two components have different solubility’s. When a hot saturated solution of this mixture is allowed to cool, the less soluble component crystallises out first while the more soluble substance remains in solution. The mother liquor left after crystallisation of the less soluble component is again concentrated and then allowed to cool when the crystals of the more soluble component are obtained. The two components thus separated are recrystallized from the same or different solvent to yield both the components of the mixture in pure form.
Fractional crystallisation can be used to separate a mixture of KCl and BaCl2, KCl (less soluble) and BaCl2 (more soluble).
Sublimation
Certain organic solids on heating directly change from solid to vapour state without passing through a liquid state, such substances are called sublimable and this process is called sublimation.
The sublimation process is used for the separation of sublimable volatile compounds from non sublimable impurities. The process is generally used for the purification of camphor, naphthalene, anthracene, benzoic acid, Iodine and salicylic acid etc containing non-volatile impurities.
Distillation
Distillation is the joint process of vaporisation and condensation. This method is used for the purification of liquids which boil without decomposition and contain non-volatile impurities. This method can also be used for separating liquids having sufficient difference in their boiling points. This method can be used to separate a mixture of
- Chloroform (b. p. 334 K) and Aniline (b. p. 457 K)
- Ether (b. p. 308 K) and Toluene (b. p. 384 K)
Fractional distillation
This process is used to separate a mixture of two or more miscible liquids which have boiling points close to each other. Since in this process, the distillate is collected in fractions under different temperatures, it is known as fractional distillation. This process is carried out by using fractionating columns. Fractionating column is a special type of long glass tube provided with obstructions to the passage of the vapour upwards and that of liquid downwards. This method may be used to separate a mixture of acetone (b. p. 330 K) and methyl alcohol (b. p. 338 K) or a mixture of benzene and toluene. One of the technological applications of fractional distillation is to separate different fractions of crude oil in petroleum industry into various useful fractions such as gasoline, kerosene oil, diesel oil, lubricating oil etc.
Distillation under reduced Pressure
This method is used for the purification of high boiling liquids and liquids which decompose at or below their boiling points. The crude liquid is heated in distillation flask fitted with a water condenser, receiver and vacuum pump. As the pressure is reduced, the liquid begins to boil at a much lower temperature than its normal boiling point. The vapour is condensed by water condenser and the pure liquid collects in the receiver.
Glycerol which decomposes at its boiling point (563°K) under atmospheric pressure can be distilled without decomposition at 453°K under 12 mm of Hg. Similarly, sugarcane juice is concentrated in sugar industry by evaporation under reduced pressure which saves a lot of fuel.
Steam Distillation
This method is applicable for the separation and purification of those organic compounds (solids or liquids) which (a) are insoluble in water (b) are volatile in steam (c) possess a high vapour pressure (10-15 mm Hg) at 373 K and (d) contain non-volatile impurities.
Aniline (b. p. 457 K) can be purified by steam distillation since it boils at a temperature of 371.5 K in presence of steam. Other compounds which can be purified by steam distillation are: nitrobenzene, bromobenzene, o-nitrophenol, salicylaldehyde, o-hydroxyacetophenone, essential oils, turpentine oil etc.
Azetropic distillation
Azeotropic mixture is a mixture having constant boiling point. The most familiar example is a mixture of ethanol and water in the ratio of 95.87: 4.13 (a ratio present in rectified spirit). It boils at 78.13oC. The constituents of an azeotropic mixture can’t be separated by fractional distillation. Hence a special type of distillation (azeotropic distillation) is used for separating the constituents of an azeotropic mixture.
In this method a third compound is used in distillation. The process is based on the fact that dehydrating agents like diethyl ether etc. depress the partial pressure of one of the original components. As a result, the boiling point of that component is raised sufficiently and thus the other component will distil over.
Dehydrating agents having low boiling point (e.g. ether) depress the partial pressure of alcohol more than that of water; on the other hand, dehydrating agents having high boiling point (glycerol, glycol) depress the partial pressure of water more than that of alcohol.
Recrystallization
Recrystallization is a widely used purification technique for solid organic compounds based on differences in solubility at different temperatures. An impure solid is dissolved in a suitable hot solvent to form a saturated solution. Upon cooling, the pure compound crystallizes out, while impurities remain dissolved in the mother liquor or are removed by filtration. The choice of solvent and recrystallization method plays a crucial role in achieving high purity and yield. The principle of recrystallization depends on the temperature-dependent solubility of a compound in a selected solvent. The compound should be highly soluble in the solvent at elevated temperature and sparingly soluble at lower temperature. Impurities are separated either by filtration (if insoluble) or remain in solution during crystallization.
Recrystallization Procedures
- Simple Recrystallization
The impure compound is dissolved in a minimum quantity of hot solvent, filtered if necessary, and allowed to cool slowly to form crystals.
- Recrystallization Using Activated Charcoal
Activated charcoal is added to the hot solution to remove coloured impurities, followed by filtration and crystallization.
- Recrystallization by Solvent Pair
Two miscible solvents are used: one in which the compound is soluble and another in which it is insoluble. Crystallization occurs upon adjusting the solvent ratio.
- Recrystallization by Cooling and Ice Bath
Crystallization is enhanced by gradual cooling followed by cooling in an ice bath to improve yield.
Result
The purification techniques of solvents/liquids and synthesized products by various methods have been studied.
Synopsis Questions
- Define purification of organic compounds.
- State the principle of recrystallization.
- Define distillation. Mention its principle.
- Differentiate between crystallization and recrystallization.
- What are the characteristics of an ideal recrystallization solvent?
- Why is the minimum quantity of solvent used during recrystallization?
- What is the role of activated charcoal during recrystallization?
- Define mother liquor.
- Why is hot filtration performed during recrystallization?
- Differentiate between simple crystallization and fractional crystallization.
- What is sublimation? Give two examples of sublimable compounds.
- State any two applications of sublimation.
- What is simple distillation? Mention one suitable application.
- What is fractional distillation? When is it preferred over simple distillation?
- Define steam distillation. State any two conditions required for steam distillation.
- What is distillation under reduced pressure? Why is it used?
- Define azeotropic distillation with one example.
- State any four methods used for the purification of organic compounds.
- Why is slow cooling preferred during recrystallization?
- Mention any four precautions to be followed during recrystallization.
Viva Voce Questions
Basic Concepts
- What is purification?
- Why is purification necessary in organic chemistry?
- Name different methods used for purification of organic compounds.
- How do you select a purification method for an organic compound?
- Which purification method is suitable for solids?
- Which purification method is suitable for liquids?
Recrystallization
- Define recrystallization.
- State the principle of recrystallization.
- Why is recrystallization widely used?
- What is a saturated solution?
- What is a supersaturated solution?
- What is mother liquor?
- What are the properties of an ideal recrystallization solvent?
- Why should the solvent not react with the compound?
- Why is excess solvent avoided?
- Why is the solution filtered while hot?
- Why is a fluted filter paper used?
- Why is activated charcoal added?
- Why should activated charcoal be used in small quantities?
- Why is slow cooling preferred over rapid cooling?
- Why is an ice bath used after slow cooling?
- Why are crystals washed with ice-cold solvent?
- Why should crystals be dried before weighing?
- What factors affect crystal size?
- How can the purity of recrystallized crystals be checked?
Crystallization
- Define crystallization.
- What is fractional crystallization?
- On what principle does fractional crystallization work?
- Give one example of fractional crystallization.
Sublimation
- Define sublimation.
- Name four compounds that can be purified by sublimation.
- Why do only certain compounds undergo sublimation?
- State one pharmaceutical application of sublimation.
Distillation
- Define distillation.
- State the principle of distillation.
- What is simple distillation?
- When is simple distillation used?
- Give one example of simple distillation.
- What is fractional distillation?
- What is the function of a fractionating column?
- Why is fractional distillation preferred for liquids with close boiling points?
- Give two examples of mixtures separated by fractional distillation.
- What is steam distillation?
- What are the requirements for steam distillation?
- Name four compounds purified by steam distillation.
- What is distillation under reduced pressure?
- Why does lowering pressure reduce the boiling point?
- Which compounds are purified by vacuum distillation?
- What is azeotropic distillation?
- What is an azeotrope?
- Give one example of an azeotropic mixture.
- Why can’t azeotropic mixtures be separated by fractional distillation?
Laboratory Skills
- What is the purpose of filtration during purification?
- What is centrifugation?
- What is evaporation?
- What is liquid-liquid extraction?
- How does liquid-liquid extraction differ from distillation?
- What apparatus are required for simple distillation?
- What is the purpose of a condenser?
- What is the function of a thermometer during distillation?
Application-Based Questions
- Which purification method is suitable for camphor?
- Which purification method is suitable for glycerol?
- Which purification method is suitable for essential oils?
- Which purification method is used in petroleum refining?
- Which purification method is commonly used in pharmaceutical industries for solid drugs?
- Why is purity important in pharmaceutical compounds?
- Which purification method gives the highest purity for solid organic compounds?
- What factors affect the percentage recovery during purification?
Higher-Order Questions
- How would you choose between recrystallization and distillation for purification?
- What happens if an unsuitable solvent is selected for recrystallization?
- Why is repeated recrystallization sometimes required?
- Why should crystallization be allowed to occur slowly?
- What are the common sources of error during recrystallization?
- What precautions should be followed during distillation?
- Which purification technique would you recommend for a thermally unstable liquid? Why?
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