A Puule-Solving Experiment Utilizing a Grignard Reagent Ernest F. Silversmith Morgan State University, Baltimore, MD 21239 A number of organic experiments that involve puzzlesolving, and therefore are not of the "cookbook" variety, have been published ( I , 2). Furthermore, a n experiment involving a Grignard reagent is sine qua non in most organic laboratory courses. We have developed a Grignard experiment that presents the student with a puzzle and is suitahle for beginning organic chemistry. Whenever a nucleophile is added to an a,@-unsaturated carbonyl compound, the problem of 1,2- vs. 14-addition arises (3). The 1.4-addition is sometimes called "conjugate additidn;', and, when the nucleophilic atom is carbon, ;t is "Michael addition". Our experiment involves the reaction between phenylmagnesium bromide and 1,3-diphenyl-2propen-l-one (chalcone, 1). There are precedents for four possible reaction pathways. These are shown in the figure; the melting ~ o i n of t each product is given under its struce show that the melting point alone can ture. ~ h e s ;dues prove whether a product is 2,5, or either 3 or 4. The last two have identical melting points, but they are easily distinguished by infrared spectroscopy. Details are in the experimental section. When phenylmagnesium bromide and chalcone are used in a molar ratio of 2 1 , 1,4-addition occurs to give 3 (4.5). When the ratio is 1:1, two successive 1,4-additions occur to give 5 (6). Thus, the experiment can be made more interesting by having some students use a 2:l ratio and others a 1:l ratio. l,2-Addition occurs only if more reactive organometallics such as phenyllithium or phenylpotassium are used (7). The formation of 4. which involves 1.2- and 1.4-addition, requires a 141 ~ r i & a r dreagent:chalcone ratio, as well as hieher temoeratures and longer reaction times than our experiment uses (8). he form&.ion of both 3 and 5 involves Michael addition: two other experiments involving this reaction have been described recently (9,lO). A historical note that would interest students is the fact that the renowned chemist, Elmer P. Kohler, a t first thought that the structure of the product using a 2 1 ratio was 2 (4). thereafter (5). chemical He corrected - - ~ ~ ~ i t to -3 shortlv ~ ~ ~ ~ . . Onlv methods for determining structures were known a t the time, and his results could be interoreted in terms of either structure! Now, of course, a n infrared spectrum would solve the problem in no time. p
-
~ ~
~
~~ ~
~
~
~
~
~
Experlmental Procedures Caution: Bromobenzene and chalcone are skin irritants. Ether is highly flammable. 1,2,2-Triphenyl-l-propanone (3). A solution of phenylmagnesium bromide is prepared from 0.54 g (22mmol) of magnesium, 2.20 mL (21 mmol) of bromohenzene, and 20 mL of anhydrous ether. After the reaction subsides, the solution is stirred and refluxed for
888
Journal of Chemical Education
Possible pathways for the addition of phenylrnagnesium bromide to chalcone.
15min. Stirrine and refluxine are continued. while a solution of 2.08 prher ia addeddropwise.Themixture isstirredand boiled for Sminnirer the addition iscomplete. Hearing is disrontinued,and 6 M HCI ir added drapwise until two clear layers form. The layers are separated, the aqueous layer is washed with 10 mL of ether, and the combined ether solutionsare dried over MgS04.Ether removal and recrystallization from 20 mL of methanolgive 1.97 g (69%)of very pale yellow crystals, m.p. 92-94 'C. The infrared spectrum (CC4 solution)has a band at 1691em-' (C=O) but none in the 3200-3600 cm-' (O-H) region. The spectrum of 4, on the other hand, has bands at 3525 (O-H) and 1115 cm-I (C-0) but none in the C=O region (8). 1,3,5-Triphenyl-2-(diphenylmethyl)-1.5-pentnnedione (5). The above procedure is repeated except that 0.27 g (11.1mmol) of magnesiumand 1.10mL (11.3mmol) ofbromobenzene in 10 mLof ether are used. Addition of 10 mL of HCI results in the formation of two liquid layers and a colorless solid. Vacuum filtration gives 1.03 g (42%)of colorless solid, m.p. 173-179 "C. The product can be purified by boiling it with 10 mL of methanol; it does not dissolve completely. The mixture is cooled and filtered by vacuum, giving 0.93 g of colorless micromystals, m.p. 180-181 'C. g (10.0 md) ofi,halrone(~ldyirh Chemical ~ o . in 1 15 ml. of
Literature Clted 1. Todd,D.; Pickering, M. J. Cham. Educ 1388. 65, llW-1102 and references 4-14 therein. 2. Pickering, M. J. Chem.Educ. 1930,67,52&525. 3. Msreh, J. Adooncad O~gonicChemislry.3rd ed.: Wiley: New Yolk, 1985;pp 664-686. ">" ,.u-..,"," . 4. Kuhler, E. P. Am. Chem Soc. 1903.29.352-363. S. Kohior. E. P. Am. Cham. Sor. 1904,31,642-661. 6. K0hier.E. P.:Peterson. W. D. J . Am. Chem.Sur.1333.55. 1073-1082.
