exam que~tion exchcmge Practical Questionsfrom the Chemical Literature Harold Wilson John Abban College Ste. Anne de Bellevue, Ouebee, Canada
Chemistry is an experimental science and this fact should be emphasized a t all levels of instruction. This emphasis can be achieved bv teaching lab in a nontraditional problemsolving manner and h;careful integration of thkory and experiment in lecture. All efforts a t establishing the central rofe of the experiment are doomed to failure, however, if the same emphasis is not evident in evaluation. There is a paucity of questions of the required type. A cursory glance a t questions in even the most popular textbooks reveals that the maioritv of auestions do not deal with practical problems. This hefiiienc; can be overcome by devisine"uuestions from the experimental sections of oublished . papers. Consider, for example, experimental report 1: 1) Hydrogen bromide was slowly bubbled through a solution of 6.02 g of styrene oxide in 50 mL of chloroform at -15 'C. After two houn the flask was capped and placed in a freezer overnight. The chloroform was then removed in vacuo and the residue was taken up in diethyl ether. After washing with water, 10% aqueous HCI, a saturated NaHCOz solution, and again with water, the ether extract was dried over MgSOn. The ether was removed in vacuo and the residue was recrystallized from hexane to yield 5.8 g of product as white crystals (mp 38 "C). Properties of Product: IR: 3400 em-' NMR: S 7.1-1.5 (m,5H) 4.9(t,lH) 3.9(d,2H) 2.l(s,lH) %Composition:41.50%C 4.39%H Dehydrohromination of the product yielded phenylacetaldehyde From this simple experimental procedure i t is possible to generate many questions of a wide variety. Examples: 1) Draw a labelled sketch of the apparatus used to react styrene
oxide with HBr(g). 2) Describe a procedure for generating HBr(g). 3) The impure product of the reaction was dissolved in ether and then washed with various reagents. Explain the purpose of each step in the washing procedure.
484
Journal of Chemical Education
JOHN
J. ALEXANDER
University of Cincinnati Cincinnati. Ohio 45221
4) How are solvents removed in "vacuo"?
5) From the known mechanism of addition of HBr to an epoxide suggest possible structures for the product of the reaction. 6) Determine the structure of the product from its given properties. Explain your choice. 7) Write amechanism for the dehydrohromination of the product to give phenylacetaldehyde. 8) What is the % yield in the reaction? etc. Consider also examole 2. This can be used to generate questions of a quantitative nature suitable for c6urses in ohvsical as well as organic - chemistry. This exercise is left t o the reader. 2) Samples ( 4 . 4 g) of pure thiol (RSH) were transferred to 50mL volumetric flasks. Some solvent was then added followed by sufficient standardized base to neutralize between 20 and 60%of the thiol. The solution was then made up to the mark with solvent. In order to determine the stoichiometric concentration of thiol a 5-mL sample was withdrawn and added to an excess of a standard iodine solution which was then backtitrated with standard sodium thiosulfate solution (2RSH Iz R2S2 2HI). The pH of a second sample of the partially neutralized thiol was measured with a pH meter whieh had been calibrated with buffers of pH = 7.00 and 9.20. Results
+
-
+
[RSHlst~i
IRS-I
PH.~
Osn H~N-0-S-H 0.0528 M
0.0712 M
0.0274 M
9.30
0.0183 M
9.97
The preparation of questions of this type is very beneficial for the instructor. Questions for organic courses can be designed from experimental sections of current papers (which keens one abreast of recent develooments) while one has to del;e into historical referencesto create q"estions for introductory courses. Be warned, however, some experimental sections are obscure and errors are not rare.
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