1. A. McGrew and T. L. Kruser Ball State University Muncie, Indiana 47306 -
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Kinetic Venus Thermodynamic Control An organic chemistry experiment
In a reaction system in which two products are reversibly formed by competing reaction processes the more stable product is not always the one that is more rapidly formed. Indeed, a number of organic reactions are known in which the thermodynamically more stable product is produced at a much slower rate than the less stable product. Such a reaction system can be made to favor either the kineticallycontrolled or the thermodynamically-controlled product by the proper choice of conditions. The addition of hydrogen bromide to 1,3-butadiene and the sulfonation of naphthalene are the most common textbook examples. An examination of thirteen recently published laboratory manuals from the authors' bookshelves revealed that only two of these manuals include an experiment designed to illustrate the important principle of kinetic versus thermodynamic control. Both manuals use the same reaction, namely, the competitive semicarbazone formation of cyclohexanone and furfuraldehyde first reported by Conant and Bartlett (1). We wish to report here an experiment which does not involve the competition of different starting materials for a common reagent, but rather one in which a kinetically-controlled and a thermodynamically-controlled product may be obtained from exactly the same starting materials. The experiment has the added advantage of illustrating certain principles of the use of infrared and ultraviolet spectra for the identification of compounds of similar structures. It is a well-known fact that certain organic bases catalyze the polymerization of phenyl isocyanate (I) and that either a cyclic dimeric structure (11) or a cyclic trimeric structure (111) may be produced.
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Hofmann (8) was the first to report the apparently exclusive dimer formation caused by the action of triethylphosphine catalyst. Other workers have since reported the formation dimers by the reaction of triethylphosphine with ring-substituted aromatic isoEastman Highest Purity, used without purification. Available from K & K Laboratories, Inc., Plainview, N. Y., and u ~ e dwithnut purificntion.
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Journal o f Chemical Education
cyanates ( S ) , and it has been found that the catalyst tri-n-bntylphosphine gives similar results (4). A mechanism for the tri-n-hutylphosphine catalyzed reaction has been proposed and is supported by the results of kinetic studies (6). Triethylamine, on the other hand, appears to catalyze trimer formation exclusively (4, 6 , 7 ) . These reports concerning the activity of tertiary amine and tertiary phosphine bases have created the impression that the two catalysts are quite specific in their action. It has been indicated, however, that tri-n-butylphosphine is not a specific catalyst for dimerization, and that either a dimer or trimer may be produced from phenyl isocyanate, depending on reaction conditions (5). We have found that electronwithdrawing groups on the aromatic ring activate the isocyanate toward both reaction pathways. Thus, with m-cblorophenyl isocyanate either dimer or trimer may be produced in a time easily adapted to a labor* tory situation. The two sets of conditions required to produce the dimer and trimer such that each product is relatively free of the other involve only moderate variations of reaction time and temperature. It has recently been demonstrated that the dimeriza tion reaction is indeed reversible with an unfavorable equilibrium position, and that dimer can be converted to trimer by basic catalysts in the absence of monomeric isocyanate (5). It is therefore concluded that this reaction system represents a true case of competition between two similar reaction pathways. The dimer is the kinetically-controlled product, but is less stable because of ring strain. The nnstrained trimer is thermodynamically favored, but is formed at a slower rate. Since the same starting materials take part in both reactions, and since the rates of formation and stabilities of the products may be easily related to their structures, this reaction system is an excellent one for the illustration of kinetic versus thermodynamic control. The Experiment Stock solutions of m-chlorophenyl isocyanate' and tri-n-butylphosphinez in sodium-dried toluene will be provided. The isocyanate stock solution (A) will contain 30.0 ml (37.6 g) of m-chlorophenyl isooyanate in 100 ml of solution. The catalyst stock solution (B) will contain 20.0 ml of tri-n-butylphosphine in 100 ml 9o17,tinn . ...nf -.. -. - ..... CAUTION! Both reagents must be handled with care. The imcyanste is poisonous and lachrymatory, while the phosphine is poisonous and possesses a. very disagreeable odor. All transfer of reagent d o t i o n s shorild he made in the presence of good ventilation. Residual isocyanate in measuring vessels and other glassware may be destroyed by ddition of ethanol. Residual addition of 3% hydrogen perphasphine may he destroyed oxide. Isocyanates react readily with water to form diarylureas and carbon dioxide. For this reason, all measuring and reaction glassware should be scrupulously dry, and resotion mixtures should be protected from atmospheric moisture at all times.
Part (I). Place 50 ml of Stock Solution A in a dry 125-ml conical flask, stopper the flask, and cool it in an ice-salt bath a t -10'-O'C. To initiate reaction add 10 ml of Solution B, with swirling, and allow the mixture to stand in the cooling bath. White crystalline material should begin to appear almost immediately upon addition of the catalyst solution. After 30 min add 20 ml cold solvent, break up the solid product, isolate it by suction filtration, and wash it with a s m d amount of cold ethanol. Residual phosphine catalyst must he removed a t this point by triturating the product with enough 3y0 hydrogen peroxide to cover it well (Note. 1). After reisolrttion and washing with cold ethanol, allow the product to dry in the atmosphere. Determine the weight of the product, its melting point, and take its infrared and ultraviolet spectra (Note 2). Save the product for use in Part 3. Part (8). Add 10 ml of Solution B to another 50-ml portion of Solution A in a dry 125-ml conical flask. Stopper the Bask and allow it to stand at room temperature. Magnetic stirring is desirable if equipment is available. After two days, add 20 ml of petroleum ether and cool the flask and its contents in an ice bath. Isolate the product ar before, wash with cold petroleum ether, and allow the product to dry in the atmosphere. Determine its weight, melting point, and spectral properties as above. P a ~ t(5). Place 3.0 g of the product from Part 1 in a 125-ml conical flask, add 20 ml of dry toluene and 10 ml of Solution B, stopper the flask and allow it to stand s t room temperature, with magnetic stirring, if possible. After two days, add 20 ml of petroleum ether and isolate the product as in Part 2. Determine its weight, melting point, and spectral properties as before. Note (1). Failure to completely remove the catalyst from the product of Part 1 may result in its degradation by reaction with atmospheric moisture and in its decomposition at a temperature well below its true melting point. Note (2). Infrared spectra may he taken using mull or pellet technique. Ultraviolet spectra may be taken for the region 320-190 nm in 95% ethanol a t a concentration of approxi-mately 0.008 glliter. Note (5). The products isolated as described are of acceptshle ouritv. Further ~urifiestionmav he effected for either compoundby mystdlia~tionfromto~ubne. Results
The following set of questions is provided in order to guide the students toward the correct interpretation of the exverimental results Which compound, the dimer or the trimer of m-chlorophenyl isocyanate, would you expect to exhibit the higher melting point? Whet is the major difference in the infrared spectra of the products of Perts 1 and 21 Which of the two possible compounds would you expect to exhibit the higher carhonyl et al., "The absorption frequency? (Hint: See SHRINDR, Systematic Identification of Organic Compounds," John Wiley & Sons, Inc., New York, 1964, p. 184.) The ultraviolet absorption in the 250-270 nm region is due to the -N-C=O system common to both products. I
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h. Calculate the percent of the theoretical yield for each part. 3) a. What du the term* "kinetic control" and "rhcrw,dynamic wmtnd" Incan as applied u,organic reactions? b. I n t h ~ scxpcrin~ent,which product is kinctically-controlled and which is thcrmods~~amieallv-controlled? c. Csn you offer a. quali&tive expianation for these results?
Although we prefer to present the experiment as outlmed above, with the set of questions included, i t would of course be possible to present the experiment in much more open-ended fashion. Students could be asked to draw their own conclusions about the relationships between product structure and reaction conditions after being presented with the experimental details and some information about the type of product to be expected. The physical data for the products typically obtained by students in our laboratory may be summarized as follows Part 1. m-Chlorophenyl isocyanate dimer-Yield 80-W%; mp 171-173'; ir 1780(s), 1600(m), 1490(m), 1450(m), 1400(m), 420(w); X,.254 (loge4.23). 210 (loge4.61). Part 8. m-Chlorophenyl isocyanate trimer-Yield SO-91%; mp 219-221'; ir 1725, 1690(s), 1595(m), 1480(m), 1430, 268 (fine structure, loge 3.04),208 (loge4.76). 1400(s); A., Part 5. m-Chlorophenyl isocyanate trimer-Yield 40-60%; identical to the product of Part 2.
We have found that students are readily able to assign the correct structures from a consideration of the melting point, the position of the carbonyl absorp tion, and the intensity of the long wavelength ultraviolet absorption. If the catalyst is not com~letely removed from the dimer productof Part 1,an appare;t melting point of 80-100" may be observed. The major contaminant of the products is 1,3-di-m-ohlorophenyl urea, which may arise from contact of the reaction mixtures with moisture, or by reaction of the dimer product with atmospheric moisture. The latter process is also catalyzed by the phosphine base. The presence of this contaminant is signaled by N-H absorption a t 3300 cm-I and C=O absorption a t 1635 cm-' in the infrared spectra. In no case have the spectra of dimeric or trimeric products indicated the presence of significant amounts of the alternate product. Normal precautionary measures will thus insure the production of dimer or trimer cleanly with no contaminating byproducts. Literature Cited
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In dimers this system may be coplanar with the aromatic rine. I n trimers. molecular models reveal that it cannot in this region? What are the identities of the products ahtsined in the various parts of this experiment?
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(2) HOFMANN, A. J a h m b ~ ~349 . , (1858). (3) R ~ w o n oL. . C.. ANDFBEYERMUTB. H. B.. J. 010. Chem., 8.174. (1843). ( 4 ) AarroLn. R. G..NELSON,J. A., AND VERBANC, J. J., Cham. Re.,57. 47 (1957). B ,E.,AND MCGROW,L. A,, J. A m . Chem. Soc., 88, 3583 (5) B O C K ~ ER. (1866). (6) HOFMINN, A. W., J a h r b ~ ~ h P m t 8 ~ A ~ e C h335 ~ D(1862). lie~ (7) H o r u * m , A. W., Be,., 18,765 (1885).
Volume 48, Number 6, June 1971
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