Qualitative Elemental Analysis of Organic Compounds: Lassaigne's Test A systematic approach to the determination of the structure of a pure organic...
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Qualitative Elemental Analysis of Organic Compounds: Lassaigne's Test

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In the test for presence of halogen, what will happen if S or N is present and the solution is immediately added with AgNOa without boiling? [2 points] Give the equation for the following reactions in the test substances. [2 points each] a) silver nitrate with the halogen present b) lead nitrate with the sulfur present c) i) ferrous sulfate with the cyanide present ii) ferrocyanice with ferric chloride

Qualitative Elemental Analysis of Organic Compounds: Lassaigne's Test A systematic approach to the determination of the structure of a pure organic compounds begins with observations of its physical characteristics, e.g. melting point, etc. and the qualitative elemental analysis. These are followed by solubility tests, functional group determination and preparation of a derivative/s. It should be noted that melting and boiling point determinations were already taken up in previous experiments. In this experiment, students are expected to learn how to carry out qualitative elemental analysis. The student should be knowledgeable and familiar with the tests, procedure and theoretical principles involved inasmuch as these will be important in future work such as systematic identification of organic compounds and determination of elements present in plants or animal tissues. The objective in qualitative elemental analysis is to detect the presence of nitrogen, sulfur, halogen (chlorine, bromine, iodine) and oxygen in an organic compound. The first step is the treatment of a small sample of the compound with molten sodium metal, a powerful reducing agent, to convert all nitrogen, sulfur and halogen to cyanide, sulfide and halide respectively. Students should learn how to carry out this initial step correctly inasmuch as it is the fusion mixture that will be used in subsequent steps of the analysis. The sulfide is readily detected by the addition of lead acetate to an acidified solution of the fusion mixture. Formation of a black precipitate (lead sulfide) is the positive response to this test. To test for the presence of cyanide, the filtered basic solution is treated with aqueous ferrous sulfate and ferric chloride, boiled for a few minutes and acidified with sulfuric acid. If cyanide is present, a precipitate of prussian blue (ferric ferrocyanide) results (Jacobs, T. L., et. al. 1974). The presence of halides is detected by precipitating it as a silver halide by the addition of silver nitrate to the acidified test solution. Then the solubility of the precipitate in dilute ammonia is investigated. If the precipitate is soluble, chloride is present; if insoluble, iodine is present; if partially soluble, bromine is present.

MATERIALS: fusion bulblets (3) Bunsen burner test tubes (6) wire triangle 10-mL graduated cylinder test tube rack test tube brush wire gauze test tube holder filter papers (6) crucible tong centrifuge glass rod blue and red litmus paper watch glass filter funnel dropper 250-ml beaker (2) Tirrill burner evaporating dish or china dish CHEMICALS: sodium metal 10% lead acetate chlorobenzene 6M silver nitrate 0.1 sulfanilic acid 6M sodium hydroxide 0.1 6M nitric acid 6M ferric chloride 0.1 dil. ammonia 6M ferrous sulfate, (fresh-01. prepared) 3M sulfuric acid unknown compound glacial acetic acid Universal indicator PROCEDURE: (Landgrebe, 1972; Shriner, et.al., 1980) I. Preparation of Lassaigne's Extract Do sodium fusion (one at a time) for: solid sulfanilic acid and liquid chlorobenzene. A. Liquid Sample 1. Clean a piece of sodium metal (must be shiny) by pressing it between folded filter paper. Place in a clean bulblet. 2 . Heat the lower part of the bulblet gently until the sodium melts and vaporizes in the tube. Then add 2 drops of the liquid test sample directly into the sodium. Be sure the test compound does not touch the hot side of the tube, because it may pyrolyze before it reacts with sodium. 3. Heat gently then gradually increase the flame until the tube gets red hot for a minute. 4 Drop the tube into an evaporating did or China dish containing 10 mL distilled water and break up the bulblet with a stirring rod. 5. Boil then filter while hot. This filtrate is called Lassaigne's Extract. Use the filtrate for qualitative test for elements.

If precipitate is formed, centrifuge and decant. Note the color of precipitate. Add 1 mL dilute ammonia and note whether the precipitate is soluble, insoluble or slightly soluble. If the precipitate is white and solubleI it is AgCl. If pale yellow and partially soluble. it is AgBr. However, if it is yellow and insoluble. then it is AgI. Show your result to your instructor, record and have it signed by your instructor.

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