Purification and Characterisation of Organic Compounds – JEE Mains Chemistry
1. Introduction to Purification and Characterisation
Purification is the process of removing impurities from organic compounds. Characterisation involves identifying the properties and structure of the compound. Various methods like crystallization, sublimation, distillation, differential extraction, and chromatography are used for purification, and qualitative and quantitative analyses are essential to determine the composition and purity of compounds.
2. Methods of Purification
Crystallization
Crystallization is a process in which a solid forms from a liquid or gas phase. It involves the formation of crystals from a solution. This technique is useful for purifying compounds based on their solubility in different solvents.
Sublimation
Sublimation is the process in which a solid changes directly into a gas without passing through the liquid phase. It is used to purify solid compounds that can undergo sublimation, such as iodine and naphthalene.
Distillation
Distillation is a technique used to separate liquids based on their boiling points. It is commonly used to purify liquids and separate volatile substances. Fractional distillation is used when the boiling points of the liquids are close.
Chromatography
Chromatography is a technique used to separate and purify components from mixtures. It works by passing a mixture through a stationary phase and a mobile phase. Common types include paper chromatography, thin-layer chromatography (TLC), and column chromatography.
Differential Extraction
Differential extraction is used to separate compounds based on their solubility in different solvents. It is often used to extract compounds from aqueous mixtures by adding an immiscible solvent.
3. Qualitative Analysis
Qualitative analysis involves detecting the presence of specific elements or groups in a compound. The tests for detecting nitrogen, sulfur, phosphorus, and halogens are some of the common qualitative tests in organic chemistry.
Detection of Nitrogen
The presence of nitrogen can be detected by the Lassaigne’s test, where sodium fusion is performed, followed by the addition of sodium picrate.
Detection of Sulphur
Sulphur can be detected by the lead acetate test, where a white precipitate of lead sulphide is formed when the compound is heated with lead acetate.
Detection of Phosphorus
Phosphorus is detected by the formation of a yellow precipitate of ammonium phosphomolybdate when a compound containing phosphorus is treated with molybdate solution.
Detection of Halogens
Halogens can be detected using the silver nitrate test. When organic halides are treated with silver nitrate, the halogen ion reacts with silver ion to form a precipitate of silver halide.
4. Quantitative Analysis
Quantitative analysis involves determining the amount of specific elements or compounds in a sample. Basic principles include the estimation of carbon, hydrogen, nitrogen, halogens, sulfur, and phosphorus.
Estimation of Carbon, Hydrogen, Nitrogen
The estimation of carbon and hydrogen is done using the combustion method, where the organic compound is burned in excess oxygen, and the products are measured. Nitrogen estimation can be carried out using the Kjeldahl method.
Estimation of Halogens
Halogens are estimated by the method of argentic salt formation, where the halogen reacts with silver nitrate to form a precipitate, which is weighed.
Estimation of Sulphur and Phosphorus
Sulfur is estimated by combustion and absorbing the sulfur dioxide in a suitable solvent, while phosphorus estimation is typically done by forming phosphomolybdic acid.
5. Calculations of Empirical and Molecular Formulae
The empirical formula of a compound is determined by calculating the simplest whole-number ratio of atoms in the compound. The molecular formula is determined by using the empirical formula and the molar mass of the compound. Numerical problems in organic quantitative analysis often involve these calculations.