Aline M. Harrison The York Campus The Pennsylvania State University York, 17403
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"Show 'N' T d " An organic chemical presentation for the layman
M u c h has recently been said about the need to commllrricatean understanding of science to the layman, and fe\v ~ h have o tried will argue that one of the greatest problems is to get his attention. This presentation of seoaration techniaues from organic chemistry has been developed in an attempt to k p t this need and thus far reception by lay audiences has been most gratifying. The place and importance of the separation techniques is set forth initially by a simplified outline of the kinds of problems which chemists investigate in general and the nature of the organic chemists' work in particular. The need to carefully construct the introductory outline cannot be overemphasized since it plays a dual role in the talk. Obviously it leads into the demonstration portion, and therefore sufficient but not extra background must be given if the audience is to remain interested. R4uch more subtly but of at least equal importance, the introduction offers a lecturer the rare opportunity of conveying a thumbnail sketch of chemists' activities to an audience whose interest has been momentarily captured in anticipation of the forthcoming show. Following the introduction, each of the demonstrations below is performed with approximately the indicated explanation. No attempt is made to give thorough explanations-in fact, such rigor is deliherately avoided. For simplicity, the bases for selecting a particular technique are confined to a consideration of whether the components to be separated are solid, liquid or gaseous and whether they would be soluble in aqueous or organic solvents. The demonstrations used are Thin-Layer Ch7omatogmphy. (Given asuseful for separation of solid-solid, solid-high-boiling liquid, high-boiling liquid from high-boiling liquid.) The coating process is demonstrated, stare ing with a display of some unused Silica Gel G powder and some solvent (usually alcohol). These are then mixed (in small quantity) and five or six 13 X 100 mm test tubes are coated. Where time and circumstances permit, members of the audience have been successful in coating some of the tithes and this has seemed to add interest to tho talk. Some previously coated and airdried tubes are then spotted with a washable black ink (Skrip has been used by this lecturer) and the red, yellow, and blue colors have been separated in an aqueous chamber. This demonstration is performed first since by the end of the 20-30 min series, the chromstogram is usually complete. The chamber is set so that the audience can see the course of the separation. Distillation. (Given as useful for separation of gas-liquid, liquid-liquid, and solid-liquid.) Unless a laboratory bench is available, only a set-up is shown but the process is explained and diagrammed on the chalkboard. Vacuum distillation is also briefly described at this time Eztraetion. (Given as useful far separation of liquid-liquid, solid-liquid, and solid-solid where one of the pair is more soluble in water and the other is more soluble in organic solvents.)
612
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Journal of Chemical Education
Fimt some water and carbon tetrsehloride are introduced into the ~ep-tory funnel and shaken to show that they separate easily. Carbon tetrachloride is then med to extract iodine from solution of iodine.potassium iodide. hi^ lecturer hm preferred to use a low enough concentration of iodine that virtually all of the color will be removed from the aaueous laver during t h e demonstration, Two or three extractions of ihe iodine are then made with carbon tetrachloride and the techniques of solvent layer removal and replenishment are demonstrated. For purposes of the lecture problem to be given at the end the audience is told that in general one of the two layers will be water and the other will be some organic solvent which does not dissolve in water. Filtration. (Given as useful to separate solid-liquid if solid is insohtble in 1iouid.i Aeain onlv a. drv model is shown since
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it. Centrifugation. (Given as useful to separate insoluble solidliquid or in special cases liquid-liquid.) This technique might either be mentioned or demonstrated depending on the time available. Specific Reagent. (Given as useful to separate any type of compound.) Use of a specific reagent to form a derivative which is insoluble in the mixture or a solution of it is described. This process is not described as derivative formation as such but it is simply explained that occasionally some chemical is available which will react specifically with one compound in the mixture thereby converting it to a, compound which is not soluble in the mixture and which may therefore be removed by filtretion-hence the term specific reagent. I t is also mentioned that if the reagent used is specific for the desired compound, the desired compound must later he released from the reagent without having been harmed.
The following problem, given here for example purposes, has been used by this lecturer for summary. The
liquid bp near 200°C sol in org solvents insol in water decomp. a t hp
liquid bp near 50'C sol in arg solvents sol in water no decamp. a t bp SOLVENT liquid bp near 50°C sol in org solvents insol in water no decomp. a t bp
R-CH-CH-R'
liquid bp near 200°C decamp. prior to bp sol in org solvents insol in water
+
2
RrNH2Br
solid mp near 250°C insol in org solvents sol in water no decomp. at mp
audience is asked to senarate Comuound I11 from all others and if possible to separat,e all items from one another. Often more t,han one acceptable separation scheme is developed based on the information given t o the audience. H~~~~the opportunity to discuss the pros and cons of different sequences naturally evolves.
The entire vresentation should take no loneer than 75 ,in since the problem solving session is usually complete in 15 min. The audience seems t o like the problem solving portion, and individual members have expressed a feeling of accomplishment.
Volume 48, Number
9, September 1971
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613
