Qualitative Analysis of Organic Compounds

Qualitative Analysis of Organic Compounds - Qualitative...

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Qualitative Analysis of Organic Compounds. The analysis and identification of unknown organic compounds constitutes a very important aspect of experimental organic chemistry. There is no definite set procedure that can be generally applied to organic qualitative analysis. Various books have different approaches, but a systematic approach based on the scheme given below will give good results. Students should, however, consult the laboratory manual and Textbook of Practical Organic Chemistry, A.I. Vogel (4th Edition). Practical Notes Before outlining the general scheme, one or two points of practical importance should be noted. (a) Quantities of substance for tests. For most tests about 0.1 g solid or 0.1 - 0.2 mL (2 - 3 drops) of liquid material (NOT MORE) should be used. (b) Reagents likely to be met within organic analysis are on the reagent shelves. Students are advised to develop a general knowledge of the physical characteristics of common organic compounds. If in doubt about the expected result of a test between a certain compound and a reagent, carry out a trial test with a known compound and compare with the unknown. (c) Quantities of substance derivatives. Students have wasted much time and material in the past by taking too large a quantity of substance for preparation of a derivative. In general, 0.5 - 1 g (or 0.5 - 1 mL) of substance gives the most satisfactory results. If a practical book instructs one to use larger quantities (3 - 4 g or more), the quantities should be scaled down to 1 g or 1 mL of the unknown substance and corresponding quantities of reagents should be used. General Scheme of Analysis A. Preliminary Tests (a) Note physical characteristics - solid, liquid, colour and odour. (b) Perform an ignition test (heat small amount on metal spatula) to determine whether the compound is aliphatic or aromatic (i.e. luminous flame - aliphatic; sooty flame - aromatic). B. Physical Constants
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Determine the boiling point or melting point. Distillation is recommended in the case of liquids (see Appendix 3). It serves the dual purpose of determining the b.p., as well as purification of the liquid for subsequent tests. C. Analysis for elements present At C10 level, the elements present will be told to you, but read up the method. D. Solubility tests The solubility of the unknown in the following reagents provides very useful information. In general, about 3 mL of the solvent is used with 0.1 g or 0.2 mL (2 - 3 drops) of the substance. The class of compound may be indicated from the following table: SOLUBILITY TABLE REAGENT AND TEST CLASS GROUP OF COMPOUNDS Soluble in cold or hot water. (If the unknown is soluble do NOT perform solubility tests below) Neutral, acidic or basic. (Test with litmus or universal indicator paper) Lower members of series. Neutral, e.g. alcohols; Acidic,
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Qualitative Analysis of Organic Compounds - Qualitative...

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