LECTURE DEMONSTRATIONS IN ELEMENTARY ORGANIC CHEMISTRY1

C. R. NOLLER Stanford University, Stanford, California

DEMONSTRATION APPARATUS

Apparatus Illustrating Separation by Distillation. To demonstrate separation by distillation an apparatus is used in which a multiule thermocouule renlaces the usual thermometer. I t is connected to a millivoltmeter hav- ing a large scale visible to the whole class. The mixture distilled is 10 ml. of ethanol (b. p. 78°C.) and 10 g. of azobenzene (b. p. 29S°C.). It is easy for the class to see the change in temperature when the different frac- tions distill. The volumes are kept small to permit completion of the experiment in a short time. The difference in the properties of the two fractions is visible readily to the class.

Surprisingly, the ethanol distillate is strongly colored yellow. In spite of the wide difference in boiling point some azobenzene distills with the alcohol. About half of the azobenzene is carried over with the alcohol by entrainment, but about half is carried over because of the vapor pressure exerted by the azobenzene. This fact can be demonstrated readily by placing a wad of glass wool in the neck of the distilling flask.

The yellow alcohol distillate can be used to demon- strate purification with adsorbent charcoal, since shak- ing with Norite and filtering gives a colorless filtrate.

Free Radical Formation. A colorless solution of tri- phenylmethyl chloride in dry benzene was shaken with zinc dust. Decantation gave a yellow solution. When a portion of the yellow solution was shaken with air the solution was decolorized, but on standing the color re- turned because of the dissociation of more hexaphenyl- ethane. The process was repeated several times until white tripbenylmethyl peroxide precipitated and the solution no longer turned yellow.

The preparation of sodium benzophenone ketyl was described. The apparatus consists of a flask having a ground-glass stopper sealed to a stopcock and small funnel. Sodium and toluene are placed in the flask and after all of the air has been expelled by boiling, the stop- cock is closed and the sodium is powdered by shaking and cooling to room temperature. A solution of henzo- phenone in absolute ether is admitted through the stop- cock, care being taken to exclude air. A deep blue solution of the metal ketyl forms. On admitting air the color is immediately destroyed.

Carbonzum Ion Formation. If a minute quantity of

tri~henvlcarbinol is dissolved in concentrated sulfuric A "

acid a yellow solution of the triphenylmethonium ion is formed which is almost indistinguishable from that of a solution of triphenylmethyl in benzene. If diphenyl- or-naphthyl-carhino1 is dissolved in sulfuric acid the color formed is dark green, because of the increased resonance. The same green color is produced by adding the carbinol to a solution of aluminum chloride or zinc chloride in nitrobenzene, or of boron trifluoride in ether, indicating that all of these reagents have electron- accepting properties like the proton.

Carbanion Formation. To a solution of potassium amide in liquid ammonia is added a small amount of triphenylmethane. The solution turns red because of the formation of triphenylmethide ion.

Vat Dyeing. White cotton cloth immersed in a solu- tion prepared by reducing indigo with sodium hydro- sulfite and alkali was yellow as long as it was kept in the stoppered containers. When the cloth was withdrawn i t rapidly became blue because of oxidation by the air to indigo.

White cotton cloth immersed in a reduced solution of flavanthrene yellow was a deep blue in color but on exposure to air slowly changed to a golden color. Be- sides illustrating vat dyeing, the experiment demon- strates that, in the case of indigo, resonance is less in the reduced form of the dye than in the oxidized form, while in the anthraquinone vats the reverse is tme.

Developed Dyeing and Diazo-type Dyes. Cotton cloth was direct-dyed with primuline, a yellow dye that con- tains armomatic amino groups. These groups were diazotized on the cloth by immersing it in a solution of nitrous acid. A portion of the cloth was exposed to the light of a photoflood lamp for one minute and then the cloth was immersed in an alkaline solution of 8-naph- thol. The portion of the cloth not exposed to light turned deep red, illustrating the process of developed dyeing. The portion exposed to light remained yellow because.the diazonium salt had been decomposed by the light, illustrating the principle of diazo-type papers.

Models of Enantiomorphic Crystals. Large-scale models of inactive, dextro, and levo ammonium acid malate were shown. Models of this salt are preferred to those of the classical sodium ammonium tartrate be- cause of the relative simplicity of the crystal forms and the resulting ease of distinguishing the hemihedral facets in the active forms.

Models of Atomic a d Molecular Orbitals. To demon- strate the value of improvising in devising illustrative material, models of p-orbitals and sp, sp2, and sp3 hybrid

430 JOURNAL OF CHEMICAL EDUCATION

orbitals made from old tennis balls were shown. Slides illustrating Walden inversion (THIS JOURNAL, 24, 277 were shown of models of ethylene and acetylene made (1947)) during substitution reactions, and a device to from rubber balloons to demonstrate u and 7 molecular be used in connection with a Delineascope for illustrat- orbitals in these molecules. ing optical activity and rotatory dispersion (THIS

Other pieces of apparatus demonstrated were a model JOURNAL, 26, 271 (1949)).