An Effective and Facile Demonstration of Organic Photochemistry Trevor M. Brown and Alan T. Dronsfield Derbyshire College of Higher Education, Derby, DE3 1GB. England Christopher J. Cooksey and David Crich University College, London, WC1H OAJ, England The number of experiments that illustrate photochemically induced change and that are suitable for student use is limited. Althoueh manv thousands of photolvses are described in the ~it~rature,~alrnost all suffer horn &e or moreof the followine drawbacks. Thev require expensive specialist equipment, often made from qua& glassware, to permit the passage of higher energy ultra-violet radiation. Many require extremely long photolysis times; starting materials may be relatively inaccessible or very costly and the products difficult for students to characterize by the usual spectroscopic techniques. Yields can be low due to competing reactions, and the product may only be marginally different from the starting material. The ~hotolvsisex~erimentdescribed here is carried out very quickly using tungsten-filament lamp irradiation and has none of the above drawbacks, It is dramatic in that rhe reaction progress is accompanied by an obvious color change. The starting material is an organocobaloxime,one of a recently discovered class of compound based on a cobaltcarbon covalent bond. Although the structure looks daunting (figure 1) it may he cheaply prepared in a matter of minutes in a one-pot reaction from dimethylglyoxime, p p i dine, sodium hydroxide, cobalt(I1) chloride, and an alkyl or benzyl bromide.' On photolysis the cohaloxime derived from the latter yields benzyl radicals (Ar-CH2 .) that are readily captured by the spin trap 2,2,6,6-tetramethyl-1-piperidinyloxyl ("Tempo"), as shown in the reaction below. This crystalline commercially available radical trapping agent is relatively inexpensive. The products from benzylic radicals and Tempo are particularly suitable for characterization by proton NMR spectroscopy.
.
ime starting material rather than any other substituted benzvl bromide has several significant advantaees. Most benzvl bromides are lachrymator; liquids. The nitro-derivative is-a solid and has a much lower vapor pressure. Thus it is possible to work with this material in the open laboratory whereas the use of other benzyl hromides necessitates the use of a fume hood. (The use of gloves is recommended when handling any henzyl bromide). Crystalline 4-nitrobenzyl bromide is inexpensive and has the added advantage that not only is the derived organocobaloxime a solid, but also the hydroxylamine (radical-Tempo adduct) photochemistry end product is, thus facilitating isolation and purification a t all stages of the experiment. Many other benzyl-Tempo adducts are oils that are difficult to separate from residual solvent except by short-path distillation. Finally the use of this p-substituted benzyl bromide gives instant recognition (in the 'H NMR spectrum) of the aromatic protons from their characteristic coupling pattern.
Preparation of 4-Nltrobenryl Cobaloxime A I-L 3-necked flask is fitted with a pressure-equalizingdropping funnel attached to an outlet gas bubbler and a B24 stopper. The
The exercise can be presented either as a fully detailed routine preparation simply to show photochemistry in action, or it may be offered as a miniresearch project in which the student has to carry out the photolysis according to instructions and then apply chemical detective work to elucidate the structure of ;he product. The use of 4-nitrobenzyl bromide to prepare the cohalox-
434
Journal of Chemlcal Education
third neck is equipped with an inlet tube for nitrogen gas via a gas bubbler. This allows the system to be purged with nitrogen and for the later stage of the reaction to be conducted under strict exclusion of air. Pyridine (3.2 g, 40 mmol, 3.3 mL) and methanol (400 mL) are
' Brown, T. M.; Cooksey, C. J. Educ. Cham. 1987,24(3), 77
added to the flask and m a g n e t i d y stirred under nitrogen flow for dissolved oxveen. Takine care to remove -several - .-.-.minntpa -.... ...tn ..exoel -the R24 stopper only briefly (and increasing the %supply t e m p rarily) dimethylgyl~mime(9.2R g, 80 mmoll is added follawed by finely powdered cobalt(ll) chloride hexahydrate (9.60 g. 40 mmol). ~ o t - a l the l dimethylglyoxime will dissolve, but the red-brown color of a cobaloxime(I1) complex should develop immediately on the addition of thesecond rea&nt. If it does not then one or more of the reactantsis missing. Sodium hydroridesolution (6.4 g, 160 mmol dissolved in ahout 20 cm7 watt.,) is added via the funnel drupwise over about 20 s. Stirring is continued until the intense blue-black color of the mbdoxime(1) develops. This takes 2-8 min. If the brown color remains, it means that air has entered the apparatus and the experiment should be repeated. To the blue-black suspension is added via the dropping funnel 4nitrobcnzvl hromidc solution (3.89 g. 18 mmolJ, diaaolved in the minimumvalume (approx. 15mL) ofpropanone. The amount of the benzyl bromide used is 90% of that demanded by the reaction stoichiometry. This minimizes contamination of the product by unreacted bromide starting material, which is troublesome to remove except by preparative column chromatography. The color of the mixture should immediately change to yellowbrown with the formation of 4-nitrobenzyl eobaloxime. The mixture is allowed to stand for 5 min and then poured into a slush of ice and water (about 2 L) with vigorous stirring to oxygenate and soluhilize unwanted products. The 4-nitrohenzyl cohaloxime precipitates out over the next few minutes. I t is filtered off, washed with water until the washings are colorless, sucked dry, and finally air-dried on a filter paper pad. The product is then sufficiently pure for the pho7~~
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Typically this preparation yields about 2.7 g of 4-nitrohenzyl cohaloxime. The product is stable if stored in the dark (brown glass bottles are recommended) hut should not he stored in solution for anv l e n d of time. Cohaloximes are believed to be safe to handle anh n o t t o present undue toxicological hazard. Reactlon of 4-Nltrobenzyl Cobaloxime with Tempo The crude 4-nitrobenzyl cohaloxime (0.99g, 1.91mmol) and Tempo (0.31 g, 1.99 mmol) are separately dissolved in diehloromethane (10 mL) and combined in a 250-mL flask equipped with a magnetic stirrer and reflux condenser. Tetrachloramethane (40 mL) is added and the stirred mixture phatolyzed using two tungsten-filament Lamps. The heat from the Lamps will cause the mixture to reflux. Typically we use 200-W bulbs with integral reflectors, but lower intensity lamps can be used. The lamps are placed about 2 em from the reaction flask. As the photolysis proceeds a jade-green deposit appears round the inside of the flask, and the deep brown color of the initial reaction mixture lightens considerably. The photolysis is complete (under the conditions described above) in 30 min; however, the extent of the reaction may he monitored, if required, by thin-layer chromatography [25- X 60-mm foilbacked silica-gel plates: elution using a mixture of ethyl acetate and cydohexane (1:s)). The product spot (visualized by UV) migrates ahead (Ri approx 0.6) of the Tempo spot (Rr approx 0.4). When the reaction is considered to he complete, the solvents are removed by rotary evaporation and the residue dissolved in the minimum volume of dichloromethane. This solution is added to a silica-gel column (10 cm X 2 cm) prepared using dichloromethane and eluted with the same solvent (see appendix). The colorless and pale straw-color eluant is colleded; the brown unwanted products are retained in the column. Solvent removal by rotary evaporation should yield the solid nitrobenzyl-Tempoadduct (0.49g, 0.17 mmol, 84%).
If further purification is required, for example, to obtain good analytical data the above material may be freed from trace impurities (such as 4-nitrohenzaldehyde) by means of a more careful chromatographic fractionation, this time using ethyl acetate/cyclohexane (1:s) as eluant and collecting thwe fractions that are shown by thin-layer chromatography as being "one spot pure". Alternatively (or in addition) purification may be achieved by recrystallization from methanol or short-path distillation to give a light yellow product, mp 6 3 4 4 %. However, the initial product is sufficiently pure to yield an NMR spectrum of acceptable quality for interpretation purposes. Proton NMR Spectroscopy Data The spectrum of the 4-nitrahenzyl cobaloxime shows peaks a t 6 = 2.0 (methyl groups), 6 = 2.7 (benzylic CH2 group), and 6 = 6.88.6 (the pyridine ring and the nitrobenzylaromatie ring). 4-Nitrohenzyl bromide impurity is readily spotted from its CH2 resonance at 6 = 4.4, but, using the quantities suggested in the above preparative details. the cobdoxime mav be nrenared free from this im~uritv. The -~~~so'eetrum of the 4-nitrdbenzvl-Temoo adduct has ~ e a k s a6t 1.2 (methyl groups on the Tempo), d = 1.5 (CHr groups on the Tempo, 6 = 4.9 (benzylic CH: gruup). and 6 = 7.4-8.2 (two doublets rharavterisric of 1.4-disubstitution on a benzene ring). All chemical shift data is given in ppm. ~
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Acknowledgment W e would like to t h a n k students from Queen Elizabeth's School Wimborne, Woodlands School ~ l l & r e e , Derby a n d Ecclesbourne School. Duffield for their involvement in t h e work. Appendb
Filtmtion on Silica Gel Silica gel (7&230 mesh) is covered with dichloromethane and stirred for ahout aminute to farm a slurry and to release trapped air. The mixture is poured into a glass 2- X 25-cm chromatography column equipped a t the hottom with a sintered glass frit and tap. The silicagel willsettle through the dichloromethane trapping some air hubbles on the way. These should he removed by tapping along the length of column as the slurry settles. The aim should be to prepare an even 10-em column, free from bubbles. The dichloromethane is run off until the top of the column just dries out, and then the solution of the 4-nitrohenzybTempo adduct is carefully added without disturbing the top of the column. More dichloromethane is again run off to expwe the top of the column, which is then topped up with dichloromethane. Again care should be taken not to disturb the top layer of the silica gel: the addition of solvent is best performed with a pipet, the tip of which should he held about 0.5 cm above the surface of the adsorbent. The tap is opened and all the colorless or pale straw-colored eluant is collected in a single vessel. The column should be topped up with more dichloromethane from time to time andnot allowed to dry out. More details about the technique of preparative column chromatography can be found in books by Ault2and Moore and D a l r ~ m p l e . ~
Auk, A. Techniques and ExpBImenb for hganic Chemisby: Holbrook: Boston, 1973; pp 48-52. Moore, J. A,: Dalrymple, D. L. Experimental Mefhods in Organic Chemistry: Saunders: Philadelphia. 1976; pp 67-72.
Volume 67
Number 5
May 1990
435
