Introduction
Organic compounds are isolated either from natural sources or from reaction mixtures. These compounds are seldom pure and are usually contaminated with small amounts of other similar compounds, which are found to exist together or formed during the reaction. In order to characterize them, it is important to purify them. Identification and characterization of any organic compound formed, is essential to postulate the mechanism of the reaction. Determination of the structural formula of an organic compound is one of the most important aspects in organic chemistry.
Processes for Purification
The identification and characterization of an organic compound such as molecular formula etc., must have the compound available in pure form. The process of obtaining the compound in its purest form from an impure sample or from any reaction mixture is called purification.
Crystallization
This technique is used for obtaining a compound in pure solid form of well-defined geometrically shaped crystals. This process of crystallization can be used for the purification of a compound only when the compound and the impurities present in it differ considerably in their solubilities in any particular solvent.
Sublimation
Substances that get converted to vapors without passing through the liquid state on heating, e.g. naphthalene, benzoic acid and camphor, undergo sublimation. The vapor on cooling, gives crystals of the pure solid compound. The crystalline material so obtained is called sublimate.
Distillation
This method is used to purify liquids that boil without decomposition and have non-volatile impurities. Impure liquid is boiled and the resulting vapors are condensed to get the pure liquid.
Chromatography
To identify, purify and/or separate constituents of a mixture that are present in very small amounts.
Differential Extraction
To extract a compound that is present in very small amounts in an aqueous solution with the help of an organic liquid in which that compound is highly soluble.
Qualitative Analysis
The detection of various elements present in an organic compound is called qualitative analysis. Carbon and hydrogen are present in almost all the organic compounds. Other commonly present elements in organic compounds are oxygen, nitrogen, halogens, sulphur and sometimes phosphorus.
Qualitative Analysis - Continued
The presence of halogens in any organic compound is detected by using sodium extract (Lassaigne's extract) with silver nitrate as follows.
Quantitative Analysis
Determination of the relative proportions (as percentages) of various elements present in an organic compound is called quantitative analysis of the compound. The percentages of the various elements present in the compound are estimated by suitable chemical methods as described.
Quantitative Analysis - Continued
Estimation of Halogens
The amount of halogen in an organic compound is estimated by Carius method.
Determination of Molecular Mass
The molecular mass of a compound is essential to determine its molecular formula. Molecular mass of an organic compound is determined by a number of methods.
Determination of Molecular Mass - Continued
Silver salt method is used for determining the molecular masses of carboxylic acids. This method is based on the fact that they form insoluble silver salts, which upon heating decompose to leave a residue of metallic silver.
Empirical and Molecular Formulae
The simple formula that gives the simplest whole number ratio between the atoms of the various elements present in the compound is called its empirical formula. For example, in a molecule of benzene (C6H6), carbon and hydrogen are in the ratio 1:1. So, the empirical formula of benzene is CH.
