Michael R. VanDeMarkl and Philip L. Kumler
Soginow Valley College University Center, Michigon 48710
A Combined Photochemistry, Organic Qualitative Analysis Experiment
The lahoratory project described below has been used with two groups of undergraduate students during the second semester of an organic laboratory sequence. This experiment involves carrying out a photochemical irradiation and subsequent determination of the number and types of products formed. The student is asked to make as complete an identification as possihle. During the first semester of the lahoratory sequence the students have been trained in the common methods of isolation and purification (including vapor-phase chromatography and thinlayer chromatography) and have utilized infrared spectroscopy for determination of the structure of an unknown. This particular experiment is offered as one alternative for the final experiment of the second semester.2 The student has a certain amount of latitude in the choice of this experiment; however, the reaction chosen or the apparatus utilized should be of a type with which he has had little or no previous experience. Introduction
Although the use of photochemical techniques in preparative organic chemistry has increased considerably during the last twenty years, only a few limited examples of experiments involving photochemistry are to be found in most of the organic lahoratory manuals. A selection of the lahoratory experiments available includes: (a) photoisomerization of azuhenzene ( I ) , (b) reduction of benzophenone to henzpinacol (21, ( c ) photocycloaddition of pbenzoquinone to cyclo-octene (31,and ( d ) photocycloadditiun of chloranil to cyclo-octene (4). We felt that most of these experiments presented photochemical methods as isolated techniques removed from the mainstream of "normal" lahoratory operations. I t was decided, therefore, to devise an experiment which would combine photochemical techniques with other laboratory operations. Specifically we hoped to include further training in "wet" and instrumental methods of analysis for the structural determination of organic compounds. In considering possibilities for this experiment a number of limitations were kept in mind. The experiment should he capable of completion in 2-3 3-hr laboratory periods, should not require elaborate photochemical apparatus, should give training in techniques for structure determination, should contain at least one "solvable" unknown, and should utilize inexpensive chemicals and reagents. The Experiment
The experiment chosen is a modification of that described by Akhtar and Barton (5), and involves the irradiation of cyclopentanol in the presence of mercuric oxide and iodine. The reaction is conveniently carried out and
' Undergraduate research participant. 2The two-semester organic laboratory sequence in current use at Saginaw Valley College emphasizes training in the common techniques of isolation and purification during the first semester and experience in synthetic and physical organic techniques (with heavy reliance an the chemical literature) during the second semester. 512
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proceeds in good yield (both isolated yield and quantum yield) to give 5-iodovaleraldehyde; workup procedures are quite straightforward and the crude product can be used as such for characterization without further purification
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The particular way in which we have utilized this experiment is as follows: The student is given a mimeographed handout sheet which includes a brief discussion of photochemistry and photochemical techniques with appropriate literature references. This sheet also includes explicit details concerning the safety precautions to be used when working with ultraviolet light sources. An additional handout is given the student which contains the detailed instructions for irradiation of cyclopentanol and work-up of the photolysate. This handout also contains the specific questions posed to the student in this experiment. These questions are: (a) how many products are formed?, (b) what is the yield of any products formed?, and ( c ) what kind of functional group is present in any major photochemical product? The irradiation and workup procedure suggested leads to a 50-70% yield of 5-iodovaleraldehyde as a colorless sweet-smellinp oil which can he characterized without further purification. It is suggested that the student devise his own experimental techniques for identification and characterization and identify the product as completely as possible within the framework of his own expertise. He is not required to make a complete structural identification of the major product hut most students who have performed this experiment have done so. As each student devises his own experimental procedure, there is considerable variation in the actual performance of this experiment. On the basis of our results, however, certain points are worthy of discussion in more detail. Source of Ultraviolet Light We have utilized four different types of lizht sources for performing the experiment (with c&nparabie results and yields) and thus this experiment is readily adaptable to whatever (if any) photochemical apparatus is available. The only requirement is a source of 3600 A ultraviolet light. The types of light sources possible include a Rayonet photochemical reactor, a medium-pressure mercury arc, a sun lamp, or even bright sunlight (see section below on Experimental Details for specific suggestions fur the use of any of these sources). Because the choice of light source will usually determine the time required for actual irradiation the most intense light source available should he utilized; however, lack of suitable ultraviolet sources does not preclude the use of this experiment because it can conveniently be performed using natural sunlight. Chromatographic Techniques The usual technique employed to answer the question concerning the number of products is some form of chromatography. During the first semester of the organic lahoratory sequence our students have been exposed to vapor-
phase chromatography, column chromatography, paper chromatoma~hv,and thin-laver chromatoma~hv.At this stage of ihe experiment a ceitain amount-of diiection on the part of the instructor is suggested. Use of vapor-phase . . chromatography should he avoided because of an apparent thermal instabilitv of the iodo-aldehvde (see below). Column chromatography (on either silica Gel or alumina) can he used but thin-layer chromatography should be the method of choice. Our students generally make their own tlc plates.. .prepared on microsco~eslides via the slurw . method, using silica gel containing both long wavelength and short wavelength fluorescent indicators (Merck Silica Gel PF-254 + 366.3 This part of the experiment also demonstrates the desirability of using more than one method to visualize tlc plates; the iodo-aldehyde is conveniently visualized under ultraviolet light hut cyclopentano1 is not visualized under these conditions. Visualization of both cyclopentanol and the iodo-aldehyde is carried out by exposing the developed plate to iodine vapors. In a typical tlc experiment, using chloroform as the developing solvent, the Rr values for cyclopentanol and the iodo-aldehyde are 0.4 and 0.7, respectively. We have not investigated the use of other types of tlc plates hut it is expected that other types of plates (commercially available plates on glass or polyester) would be equally satisfactory. It is very important to emphasize to the student that he must he ahle to visualize the cyclopentanol (in addition to any products) to determine whether there is any residual cyclopentanol remaining in the crude photolysate (see Spectroscopy section). Spectroscopy Most students will rely on infrared spectroscopy to determine the types of functional groups present. The ir spectrum of the crude photolysate (taken either as a neat liquid or in solution) should be very revealing to the student. The iodo-aldehyde exhibits strong carhonyl absorption a t 1725 cm-1; in some cases small amounts of residual cyclopentanol cause the appearance of a moderate to strong absorption hand in the 3300-3500 cm-1 range, which can increase the difficulty of spectrum interpretation. Many of the students are also ahle to identify the aldehyde C-H stretching absorption a t 2720 cm-1 (which is quite obvious in most of the spectra) which greatly simplifies the identification. Some of the students miss this important absorption hand and are forced to rely on "wet" methods of analysis to identify the aldehyde functionality. Also, failure to "look for" residual cyclopentan01 by tlc has resulted in a number of misassignments of alcohol functionality in the major product. Our students have not utilized nmr spectroscopy (due to lack of availahle instrumentation) hut use of this technique would greatly simplify the complete identification. Purification As indicated ahove, the crude product is sufficiently pure for characterization and identification and we suggest that further purification not he attempted. If further purification is desired it can he done by column chromatography or very careful short-path vacuum distillation. At temperatures sienificantlv ahove room t e m ~ e r a ture the iodo:aldehyde isconverted hack to cyclopent'anol; for example. attemoted distillation a t 60°C a t a nressure of 10 m i results in-essentially complete conversion of the crude iodo-aldehvde to cvclo~entanol. We have. however. - . been able to distill the crude product successfully at 3 r d a t a pressure of 1.0 mm. This amarent thermal reversihil.. ity of the reaction (which we have not as yet investigated in any detail) also precludes the use of normal gas chromatography methods. Column chromatography can also be used to purify the crude iodo-aldehyde. We have successfully chromato-
graphed the product on 80-200 mesh alumina (Matheson, Coleman and Bell; 29296) using a weight ratio of adsorbent to crude product of 30:l. Elution of the column with chloroform results in the iodo-aldehyde being the first significant product eluted. Other impurities and any residual cyclopentanol are eluted much later than the desired aldehyde. Characterization and Identification By the time the student has carried out some or all of the above techniques he will usually rely on classical "wet" methods for structural characterization. Use of techniques such as elemental analysis by sodium fusion (the product gives a strong positive test for halogen) and performance of classification tests (Fehling's test, Schiffs test, reaction with 2,4-dinitrophenylhydrazine, etc.) should allow identification as a halogen-containing aldehyde. If the student is successful in identifying the maior functional group as an aldehyde he is enc&riged to p k pare the 2,4-dinitrophenylhydrazonederivative. In our experience this derivativeis the easiest of the common derivatives to prepare and allows an additional opportunity for the student to use ultraviolet spectroscopy to differentiate aldehydes and ketones (6). For this derivative we have observed a mp of 125-6T and an ultraviolet maximum (in 95% ethanol) of 358 nm ( 6 20,000); the original literature (5) cites the melting point of this derivative as 127-218"C, an obvious transposition of numerals(?). This fact can offer the student a valuable learninn ex~erience concerning the fallability of the chemical literature if he is successful in findine " the literature reference for this compound (see below). By this stage of his experimental investigation the average student suspects that the product is an iodine-containing aldehyde and simple structural considerations (of the starting material) suggest that the iodine atom is located a t the terminal position of the chain. Although the mechanism of the reaction may not be ohvious to the average student he can see that the ring has been broken and the terminal position is an "obvious" location for the iodine atom. Use of nmr spectroscopy, if availahle, could clarify this assignment and confirm the structural identification. Use of the Chemical Literature If desired, the student can he given the exercise to locate 5-iodovaleraldehyde in the original literature. The student will not find this compound listed in any of the common compendia (various handbooks, "The Merck Index," Rodd's "Chemistry of Carbon Compounds," Heilbron's "Dictionary of Organic Compounds," or even in Beilstein through the third supplement). If the student is to find a literature reference for this compound he must resort to a search of Chemical Abstracts, utilizing the name or formula indices, which will lead him to the one literature reference for this compound (5). Miscellaneous Details The amount of time required for completion of this experiment varies considerably with the capabilities of the students ~erforminethis experiment. hut an averaee student req;ires three 3-hr laboratory periods to complete the exoerimental work (librarv time and other sunaested readings are in addition to thk above times). herea are a number of areas that an instructor might wish to include for an interested student. A study of the generality of the reaction hy observing the behavior of other cyclic alcohols under the same experimental conditions is one possible 3Available from Brinkmann Instruments, Westbury, New York 11590.
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area. In our experience, however, cyclopentanol is the only alcohol (of the three t h a t we have tried) t h a t gives a relatively "clean" product in high yield. Another type of question which t h e interested student might want t o investigate is whether ultraviolet light is really required for this reaction. It is a simple procedure for a student to run a control reaction with all the reactants in a n Erlenmeyer flask t h a t has been completely wrapped with aluminum foil. T h e student carrying out this procedure finds t h a t no reaction a t all occurs in the absence of light. Experimental Details A mixture of 5.08 g (0.02 mole) of iodine and 4.30 g (0.02 mole) af mercuric oxide is placed in a 250-ml Erlenmeyer flask (Pyrex) which contains a magnetic stirring bar. Carbon tetrachloride (100 ml) is added followed by cyclopentsnol (0.425 g, 0.005 mole). The flask is stoppered with a plug of cotton and placed on a magnetic stirrer and irradiated with an appropriate light source by one of the followingprocedures. Use of a Rnyonet Reactor. The mixture is placed within a Rayonet reactor and irradiated at 3600 A (RUL-3600 lamps) for 1 hr. Make certain that safety glasses are worn during this operation and avoid looking directly at the ultraviolet lamps. Use oj a Medium-Pressure Mercuv Are. The sample is placed approximately six inches from a water-cooled quartz immersion well which contains a 450-W Hanovia mercury arc lamp (or a comparable lamp) surrounded by a Pyrex absorption sleeve. Use of the sleeve can be avoided by using an immersion well constructed of Pyrex. The irradiation should be carried out for 1hr. Use of o Sun Lamp. The sample is placed approximately six inches from an appropriate sun lamp (we have used a G.E. 275W sun lamp) and irradiated for 1.5 hr. Use of Sunlight. The sample is placed as close as possible to a window receiving the direct rays of the sun. We have utilized both a northeast window in the morning and a northwest window in the afternoon. The length of irradiation required will vary considerably with prevailing climatic conditions but will probably he complete in 2-4 hours during an average sunlit day.
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After irradiation by one of the above procedures the solution is filtered through fluted filter paper (to remove residual mercuric oxide) and the residue in the filter paper is washed with an additional 25 ml of carbon tetrachloride. The resultant deep-red filtrate is washed twice with 10% sodium thiosulfate solution to remove excess iodine. The organic phase is washed once with saturated salt solution and filtered through cotton. Carbon tetrachloride is removed under reduced pressure at room temperature (avoid heating the crude product above 40°C because of the thermal decomposition discussed above); use of a rotary evaporator is suggested, but not required, far this operation. This irradiation and work-up procedure leads to a sweet-smelline oil (colorless to oak in vields of 50-7070. The moduct . vellowl . is pure enough for further characterization and is to be analyzed by expernnents of the student's own choice and deslgn. Summary and Conclusions We have presented a n experiment which allows photochemical techniques t o be presented in conjunction with other normal organic laboratory operations. This approach allows the student t o see photochemical techniques a s simply a n extension of other common laboratory methods. T h e experiment is quite versatile and can he adapted t o a wide variety of laboratory situations a t the discretion of a particular instructor. labora ate photochemical apparatus is not required b u t can be utilized if available. Material costs a r e k e p t t o a minimum. The experiment is flexible enough t h a t it can he used with large classes of students of diverse capabilities. Literature Cited (11 Ja-n. J. F. J.CHEM. EDUC..d6.117(19691. (21 Mohri.. J. 8.. and Neckem. D. C.. "Laborator" Exwriment3 in oreanie Chomistry,' Reinhold Bmk Corporation, New Yolk, 1968, P: 138. (31 isaacs. N. S.. "Exwrimenls in Physical Oreanic Chemistw." The MacMiIlan Company, London. 1969. p. 252. (4) Ref. 13). p. 251. (51 Akhfar, M.,and Bahon,D. H. R..J. Amer. Chem. Soc.. 86,1528 (19641. (6)Dyer, J. R.. '"Applicationn of Absorption Spec~olcapyof Orgsruc Compound=," Plentic~~Hall Inc.. Englevocd Cliffs. N a r Jersey. 1965. p. 10.
