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The Identification of Organic Compounds
248. Throughout the laboratory course outlined in the previous chapters, the typical reactions of a number of important classes of compounds have been illustrated by experiments. These reactions are made use of in the identification of organic compounds. Practice in such identifications is of great educational value, as it requires continuous thought on the part of the student, is an excellent review of many facts which have been learned, and has a practical significance.
No set analytical scheme is available as is the case in inorganic qualitative analysis. The subject is more complicated than the latter, and each compound requires special study. By the application of a few simple tests it is often possible to determine to what class a compound belongs. In order to facilitate the work the student should prepare a table which summarizes the behavior of certain typical compounds with a few important reagents. To do this proceed as follows: Divide a large sheet of paper into squares by drawing on it twenty horizontal and fifteen vertical lines. Place in the first vertical column on the left side of the paper the names of the following important classes of compounds, using a square for each class: paraffin hydrocarbons, unsaturated hydrocarbons, aromatic hydrocarbons, alcohols, phenols, acids, ethers, anhydrides, esters, aIdehydes, ketones, amines, amides, nitriles, nitro compounds, halogen compounds, sulphonic acids, carbohydrates. In the squares that form the upper horizontal column write the following: water, cold solution of sodium hydroxide, hot solution of sodium hydroxide, solution of sodium carbonate, dilute hydrochloric acid, cold concentrated sulphuric acid, acetyl chloride, sodium, bromine, influence of substituents, influence of aromatic group, special reactions, and remarks. State briefly, as far as possible, in the appropriate squares, the behavior of the several classes of compounds with the reagents listed. It will take time and study to do this satisfactorily. The following references to the sections in the text-book and to the laboratory experiments will be of value in getting together the facts to be incorporated into the table. It is important that any notes under the experiments referred to should be read: paraffin hydrocarbons, 26, 27, and experiment 70, page 46; unsaturated hydrocarbons, 35, 43, and experiments 74 and 75, page 50; aromatic hydrocarbons, 352, and experiments 164d to h, page 129; alcohols, 67, 61, and experiment 79, page 55; phenols, 456, and experiments 197, 198, page 162; acids, 97, and experiments 90, 91, page 63; ethers, 119, and experiment 96, page 71; anhydrides, 123, and experiment 98, page 73; esters, 139, and experiment 103, page 77; aldehydes, 151, and experiment 108, page 83; ketones, 158, and experiment 111, page 84; amines, 170, 423, and experiments 113, 183, 184, 185, pages 86, 149, 151; amides, 186, and experiment 116, page 89; nitriles, 197, and experiment 127, page 94; alkyl halides, 206, and experiment 132, page 98; acyl halides, 223, and experiment 139, page 105; aryl halides, 402, and experiment 180, page 144; carbohydrates, 313, and experiments 148, 152, 156, pages 115, 119, 124; nitro compounds, 391, and experiment 171, page 136; sulphonic acids, 399, and experiment 174, page 140.
249. The properties of the members of any class are in general like those illustrated in the case of the simpler members, which have been studied by the student. It is important to remember, however, that increase in molecular weight generally leads to decreased solubility and chemical activity. For example, the anhydrides with low molecular weight of the homologues of acetic acid react promptly with water, whereas those of high molecular weight must be heated with water for some time to bring about hydrolysis. It is also important to remember that the presence of a second group in a compound often modifies the properties which a characteristic group gives to it; for example, monatomic phenols are insoluble in dilute hydrochloric acid, but aminophenol dissolves in this reagent on account of the presence of the amino group.
Before beginning to identify an unknown compound the student should study Sections 328 to 331 inclusive, and solve problem 2 at the end of Chapter XVII in the text-book. The work should be done carefully with small amounts of the substance to be studied. About 10 grams of a pure compound is ordinarily enough for complete identification, including the preparation of a derivative and the determination of its physical properties. Solubilities can be determined with a crystal or a single drop. The first substances given to the student should be practically pure.
The work is begun by testing the purity of the sample to be identified. If it is a liquid the substance should be distilled very carefully from a small flask (§44, page 29). If the substance is a solid its melting-point should be determined. Record the result obtained, as it is to be used in the final identification. If the substance is impure it should be purified before being studied further. Burn some of the substance (experiments 164d, note, and 180b, pages 130, 144,) and determine if possible if it is an aromatic compound (experiment 164e and f, note). Next make a qualitative analysis (§57-61, pages 38, 41). If any element other than carbon, hydrogen, and oxygen is found, it is well to make tests to discover in what group the element is present. Study next the behavior of the substance with the reagents given in the table prepared as described above, and test with Tollen's and Schiff's reagent. If the substance appears to be a carbohydrate, apply the Molisch test.
250. When the class or classes to which the compound belongs and the melting-point or boilingpoint have been determined, consult large texts, or other reference books on organic chemistry, which contain tables or lists of compounds with their physical properties. Such books as Richter, Organic Chemistry; Beilstein, Handbuch der organischen Chemie; Mulliken, Identification of Pure Organic Compounds; Chemiker Kalender; Meyer and Jacobson, Lehrbuch der organischen Chemie; and Van Nostrand's Chemical Annual are useful. Take into consideration compounds the melting-points of which lie within about four degrees on either side of the point observed. Such a range is necessary as the melting-points and boiling-points recorded are at times far from accurate. When one or more compounds are found the descriptions of which fit those of the substance under study, find out what derivative which possesses a satisfactory melting-point or boiling-point can be readily made. It is best to prepare a solid whenever possible, since in this case much smaller amounts of substance can be used. Look up the properties of this derivative and its method of preparation, and convert about 1 gram of the substance to be identified into the the derivative if the latter is a solid; if it is a liquid more will be required. Before making the preparation read carefully pages 28 to 31, where the special technique of handling small quantities is described. Determine the melting-point or boiling-point of the compound prepared; if this agrees with the number as given, the identification of the original substance may be considered satisfactory.
In the case of certain compounds it is difficult to transform them into other substances when only a small amount is available. In this case, if the compound is a liquid, it is often convenient to determine its specific gravity (§56, page 37) as an aid in its identification. At times both the melting-point and the boiling-point of a substance can be determined; these two constants often will serve to identify a compound. For the identification of a substance it is necessary to have at least two physical constants agree with those which have been recorded; these may be constants of the substance itself, or one may be a constant of the substance and one of a compound into which it has been converted. Reliance on one constant often leads to mistakes. For example, o-chlortoluene boils at 154° and m-chlortoluene at 156°. It would be very difficult to identify either substance by a determination of its boiling-point alone. The identification is readily accomplished, however, by converting the compounds into the corresponding benzoic acids; o-chlorbenzoic acid melts at 137° and m-chlorbenzoic acid at 153°.
251. In the following list, the derivative which can be conveniently prepared, in most cases, from the several classes of compounds is given. In some cases specific gravity determinations are made, although it is preferable to convert the compound into another substance if possible: paraffin hydrocarbons, specific gravity; unsaturated hydrocarbons, addition-products with bromine or hydriodic acid; aromatic hydrocarbons, solid nitrocompounds (experiment 172, note, page 138), or addition-products with picric acid (experiment 169c, page 133); alcohols, special tests or solid esters of aromatic acids; phenols, bromine substitution-products; monobasic fatty acids, solid anilides (experiment 91c, page 64), or esters; polybasic acids, characteristic salts; aromatic acids, amides (experiment 202, page 166) or nitroderivatives; ethers, specific gravity or conversion into iodides (experiment 96j, page 72); aromatic ethers, as above, or nitroderivatives; fatty anhydrides, anilides (experiment 98e, page 74); aromatic anhydrides, change to acids; esters, hydrolysis and identification of the acid or alcohol formed; fatty aldehydes, specific tests; aromatic aldehydes, nitroderivatives or special tests; ketones, oximes (experiment 210, page 172), or special tests (experiment 111f, page 85); fatty amines, salts with hydrochloric acid; aromatic amines, acetyl derivatives or special tests; amides, change to acids; nitriles, change to acids; nitro compounds, further nitration or reduction to amines; sulphonic acids, amides; carbohydrates, special tests, osazones, behavior with Fehling's solution, etc.
252. Study of Mixtures. - (See Section 331). - The mixture should be treated in turn with twice its volume of water, dilute hydrochloric acid, dilute sodium hydroxide, and concentrated sulphuric acid. When sulphuric acid is used care must be taken to keep the mixture cold. If the mixture is a liquid it should be treated with solvents in a graduated cylinder. The volume of the liquid should be noted before and after shaking with the solvents; in this way it can be easily seen whether any of the mixture dissolves. If this occurs the mixture should be treated with small quantities of the solvent as long as any of it dissolves. The several solutions should be examined separately. The water solution should be carefully distilled. It is tested first with litmus paper to determine whether an acid or an amine is present. If an acid is present the solution should be neutralized with sodium hydroxide before distillation. The acid will remain in the flask as a sodium salt and other volatile substances will pass over. The boiling-point should be carefully noted during the whole distillation. If there is evidence of the presence of anything in solution, tests should be applied to the distillate for the classes of substances soluble in water.
The extract obtained by treatment with acid should be rendered alkaline. If a basic substance has been removed it will be precipitated; this should be separated and tested.
The extract obtained with sodium hydroxide is next treated with dilute hydrochloric acid. If an acid or phenol is present it will be precipitated, and is examined.
The concentrated sulphuric acid solution is poured very carefully onto twice its volume of cracked ice. Any substance which is precipitated is washed with a little water, separated, and examined. If two substances which belong to the same class of compounds are present in a mixture, it is usually necessary to separate them by fractional distillation.
A full record of all tests used should be placed in the notebook. The observed physical properties (melting-point, boiling-point or specific gravity) of the substance and of the compound prepared from it should be given together with the physical properties as recorded in the reference books.