A LABORATORY STUDY of HOMOLOGY GEORGE W. BENNETT
AND
FRANCIS ELDER
Gmve City College, Grove City, Pennsylvania
I
N THE beginning course in organic chemistry the laboratory treatment of the important principle of homology commonly consists in test-tube examinations of "properties" of the members of the given series. If it is desired to illustrate homology and its corollaries in the aliphatic division of the subject by means of prefiarations (which will then also include one of the "properties" of the starting materials) the choice of homologous series for student preparation will be quite limited. The preparation of members of the paraffin series is hardly suitable; the olefins are gaseous a t room temperatures for the first four members; the alkynes are not suitable; and serious difficulties would beset the student in the preparation of the f i ~ s tfour or five members of the aldehydes, acids, acid chlorides, and amines. The alcohols might be prepared, but they would serve better as starting materials than as end materials. Choice of a series to prepare would probably narrow to the alkyl halides, esters of a given acid, say acetic, or esters of a given alcohol, say ethyl. If the alkyl halides are prepared, the lower chlorides and one bromide are gaseous a t room temperatures, and only the alkyl iodide series could be prepared complete in liquid form a t room temperatures. If, moreover, it is desired to prepare compounds involving only a single organic molecule, the esters will not serve. The ketone series, however, will meet the desiderata admirably. The first four members of the series, a t least, can be readily prepared in liquid form by the dichromate oxidation of the corresponding secondary alcohols. Preparation of the homologous series most likely would be undertaken as a class project with four student groups each preparing a different ketone. Directions for such experiments are not to be found in the more familiar laboratory manuals of organic chemistry because very few manuals give details for the preparation of any of the aliphatic ketones. Fortunately, however, details for student preparations of aliphatic ketones have appeared in THISJOURNAL. Yohe, Louder, and Smith (1)described the preparation of pentanone-2 and pentanone-3. Robertson (2) presented details for the student preparation of acetone, and Wagner (3) amplified the details for the preparation of acetone in regard to temperature conditions. By slight modifications of the procedures advised by these authors one can also prepare butanone and hexanone9 in good yields. The four-carbon ketone is prepared by the same procedure as pentanone-2 except that the oxidizing solution may consist of the same quantity of sodium
dichromate (50 grams) in 50 cc. of water and 15 cc. of concentrated sulfuric acid, and the one-half mole of alcohol is dissolved completely in 100 cc. of water and 30 cc. of concentrated sulfuric acid. For hexanone-2 one may use the same volumes as are recommended for pentanone-2 with the exception, of course, of the hexanol-2. The preparation of the homologous ketones will illustrate satisfactorily several corollaries to the principle of homology. Thus the behavior of the secondary alcohols illustrates that in a series of compoun dshaving the same kinds of atomic linkages the various members read alike against a common reagent, and that they thereby produce a different series of compounds, all members of which have similar atomic linkages. This group of preparations will also show that the reactivity of the higher members in a series becomes increasingly sluggish. Thus hexanone9 cannot be prepared in as good yield as pentanone-2 by about 10 per cent., using the same procedure. In the third place the four preparations illustrate how experimental conditions must be varied to suit the reactants. Isopropyl alcohol is completely soluble in water, and butanol dissolves in about 30 per cent. sulfuric acid. For the preparation of the first two ketones, then, the oxidation is carried on in a homogeneous liquid mixture. Pentanol-2 and hexanol-2, on the other hand, require such concentrated acid for solution (roughly 45 per cent. and 60 per cent.) that there is danger of increasing the oxidation potential of the acidified dichromate solution to a value greater than that required for the oxidation of alcohol to ketone. Yields would thereby be diminished by further oxidation of the ketones to acids of lesser carbon content. The experimental conditions for the preparation of pentanone4 and hexanone-2, therefore, produce a twophase mixture to be subjected to oxidation. As regards oxidation temperatures to be employed, the study by Wagner (3) shows that results for student experiments are about equally good when the oxidation is done a t room temperatures or a t the boiling temperature. The directions for pentanone-2 (1) specify low-temperature oxidation. In the preparation of butanone the present authors and their students get about the same results with temperatures carefully kept a t 40 degrees and with temperatures allowed to run up to 50 or 70 degrees. For hexanone-2, however, the higher temperatures diminish the yields. Acetone, moreover, illustrates that the first member of an homologous series differs from the second more than do any other consecutive air in the series. Thus
acetone is freely soluble in water, and forms no azeotropic mixture with water, whereas none of the other ketones are miscible with water and they do form azeotropic or pseudoazeotropic mixtures with water (4). Finally, the physical properties of the series of ketones prepared and the series of secondary alcohols used illustrate the regular variation in physical properties as one ascends an homologous series. These data can be tabulated for the molecular weights, boiling points of the alcohols and of the ketones, boiling-point differences of the corresponding alcohols and ketones, densities, refractive indices, and solubilities in sulfuric acid-water mixtures. A study of the data thus listed has a special interest to the student because most of
them have been used in the preparations carried out in the laboratory class project. For laboratories not a t sea level observed boiling points are conveniently corrected by the method described by Hoyt (5). LITERATURE CITED (1) YOHE,LOUDER, AND SMITH. "New laboratory preparations for the course in organic chemistry," J. CHEM. EDUC., 10, 374-6 (1933). (2) ROBERTSON. "The fractionating column in the preparation of acetone," {bid., 10, 704-5 (1933). "Oxidation of isopropyl alcohol to acetone," ibid., (3) WAGNER, 11,309-10 (May, 1934);, (4) LECAT, "L'azeotropisme, Lamertine, Bmxelles, 1918, pp. 80-1. (5) HOYT, -simple boiling point correction," J. CHEM.EDUC., 11, 405 (July, 1934).
