Dielectric polarization and molecular interaction. Part 11. Acetylacetone,

Transcription

1 Dielectric polarization and molecular interaction. Part 11. Acetylacetone, - benzoylacetone, and dibenzoylmethane - triethyl amine - cyclohexane systems GAMAL R. SAAD, MAGDI M. NAOUM, AND HANN A. RIZK' Department of Chemistry, Faculty of Science, University of Cairo, Cairo, Egypt Received July 6, 1988 GAMAL R. SAAD, MAGDI M. NAOUM, and HANN A. RIZK. Can. J. Chem. 67,284 (1989). A modified form of Few and Smith equation applicable to dilute solutions of acetylacetone, benzoylacetone, and dibenzoylmethane in constant-ratio mixtures of triethylamine in cyclohexane is confirmed. The changes in dielectric polarization of diketones with amine concentration in the mixed solvents are used to deduce the equilibrium constants of the association process, and the dipole moments of the complexes formed. Further, an inference is made of the relative polarities of the keto and en01 tautomers of these diketones, and of the ketoanilides previously investigated (1). Key words: dielectric polarization, molecular interaction, P-diketones, triethylamine. GAMAL R. SAAD, MAGDI M. NAOUM ~~HANNA A. RIZK. Can. J. Chem. 67,284 (1989). On a confirm6 l'utilitk d'une forme modifike de l'kquation de Few et Smith pour ktudier des solutions dilukes d'acktylacktone, de benzoylacktone et de dibenzoylmkthane dans des mklanges a rapport constant de trikthylamine dans le cyclohexane. On utilise les changements de polarisation diklectrique des dicktones en fonction de la concentration d'amine dans les solvants mixtes pour dkduire les constantes d'kquilibre du processus d'association ainsi que les moments dipdlaires des complexes formks. De plus, on peut dkduire les polaritks relatives des tautomkres cktoniques ou knoliques de ces dicktones ainsi que des cktoanilides ktudiks antkrieurement (1). Mots clks : polarisation diklectrique, interaction molkculaire, P dicktones, trikthylamine. [Traduit par la revue] In part I (1) of this series the method of dielectric polarization measurements, first developed by Few and Smith (2), was used to investigate molecular interactions of acetoacetanilide and benzoylacetanilide with triethylamine in benzene solution. The present work extends this type of investigation to three P-diketones, namely, acetylacetone, 1, benzoylacetone, 2, and dibenzoylmethane, 3, using the same base and the non-polar solvent cyclohexane instead of benzene. However, since in an inert solvent, S, the acid, A, whether it is a P-diketones as in the present investigation, or P-ketoanilide as in the previous (I) one, exists in solution as an equilibrium mixture of the keto and en01 tautomers, the presence of an interacting base, B, such as triethylamine will not only cause complexation through hydrogen bonding between the enolic OH group and the amine nitrogen, but also a shift in the position of equilibrium towards en01 formation (3) as a consequence. This, in turn, might lead one to suggest that the standard value of polarization of the solute, which does not involve short-range interactioris leading to complex formation (2, 4, 5), would be better taken as the extrapolated value of the polarization, P z, in the solvent mixture, BS, in the limit when the concentration of the interacting base, B, approaches zero, rather than the polarization, PA, in the non-polar solvent, S, as previously suggested (1, 2). Thus denoting (PT,)wB=O as Pa, where WB is the weight fraction of the base in the constant-ratio solvent mixture, the equation of Few and Smith (2) can easily modified into I~uthor to whom correspondence should be addressed where AP = PAB - Pk - PB, the other terms having their usual significance (1). This equation has, therefore, been preferentially used in the present work for the calculation of both the equilibrium constants, K's of the association process between the P-diketone (1-3) and triethylamine in cyclohexane, and the dipole moments pab1s, of the complexes formed. Recalculation of these two parameters has also been made for acetoacetanilide, 4, and benzoylacetanilide, 5, which were previously investigated (1). Experimental and results Acetylacetone (PROLABO, France) was purified by repeated distillation, benzoylacetone and dibenzoylmethane (HOPKINS & WILLIAMS, England) by two-fold crystallization from ethanol, and cyclohexane and triethylamine according to recommended procedures (6). The purity of compounds was checked by comparing their physical constants with those cited in literature (6). Preparation of solutions and measurements of dielectric constant, E, density, d, and refractive index for the sodium-d line, n,,, were carried out as described before (1, 7). For the calculation of the equilibrium constant, K, of the association process and the dipole moment, ~ A B, of the complex, eq. [I] was used, the value of PI, being first obtained by graphical extrapolation of PT, at wb = 0. The results obtained are summarized in Tables 1-3 for acetylacetone-, benzoylacetone-, and dibenzoylmethane-triethylamine-cyclohexane systems, respectively.' Furthermore, since triethylamine has an appreciable dipole moment in cyclohexane, which is 0.83 D as determined in the present work, use was made of the modified Onsager equation (8, 9) for the determination of the apparent solution moment, '~ables 4 to 13 of original data have been deposited, A complete set may be purchased from the Depository of Unpublished Data, CISTI, National Research Council of Canada, Ottawa, Ont., Canada KIA os2.

2 SAAD ET AL. TABLE 1. Polarization data of acetylacetone (A) in triethylamine (Bj- cyclohexane % amine ~ B S Pi R i PZ (= WB) (g/cm3) (cm3) (cm3) 1 - PA l/~bd~s (D) MB/KAP A AP K PAB PA RAB PAB TABLE 2. Polarization data of benzoylacetone (A) in triethylamine (B)- cyclohexane % amine d~ s pi R: k2 (= WB) (g/cm3) (cm3) (cm3) / - I/WB~BS (D) MB/KAP l/ap A P K PAB PA RAB PAB TABLE 3. Polarization data of dibenzoylmethane (A) in triethylamine (B)- cyclohexane % amine d~ s Pi R 2 ~2 (= we) (g/cm3) (cm3) (cm3) 1I(PA - PA) l/wbdbs (D) MB/ KAP l/ap AP K PAB Pk RAB PAB 1.27 X 9.78 X FZ, of the P-diketone in the constant-ratio solvent mixtures of the arnines and cyclohexane. The results obtained are summarized in Tables Discussion First, since the equilibrium position between the keto and en01 tautomers of the acid A (whether a diketone in cyclohexane or ketoanilide in benzene) is shifted in favour of the en01 form in the presence of triethylamine, owing to the hydrogen bonding between the enolic OH group and the nitrogen lone pair of electrons, we would expect that its polarization, PA, at infinite dilution in the non-polar solvent, S, should be either higher or lower than its polarization, PL, at infinite dilution in the solvent mixture, BS, in the limit when the concentration of the base approaches zero, depending upon the relative polarities of the two tautomers. That is, if the keto form of the acid is of more polarity than the en01 form, PA will be greater than PL and vice versa. Based on this, an inspection of the results obtained as

3 CAN. J. CHEM. VOL. 67, 1989 TABLE 14. Apparent solution moments of acetylacetone in triethylamine-cyclohexane TABLE 15. Apparent solution moments of benzoylacetone in triethylamine-cyclohexane TABLE 16. Apparent solution moments of dibenzoylmethane in triethylamine-cyclohexane TABLE 17. Some physical properties of compounds 1 to 5 Compound PA PA PA P ~ O I Ptceto 6m K AP -AGO number (cm3) (cm3) (D) (D) (D) (D) (cm3/mol) pk,* (D) (kj/mol) *Measured in 50% dioxane water mixture (12). TABLE 18. Fractions of the diketone introduced in triethylamine-cyclohexane mixtures which pass into the form of complex at infinite dilution Acetylacetone Benzoy lacetone Dibenzoylmethane (2) (2) aamine %.mine.,mine (2) (WB) w,=o ( WB) w,=o ( WA) w,=o

4 SAAD ET AL. 287 shown in Table 17 would lead to the inference that whilst the keto tautomer is more polar than the en01 in the case of P-diketones, the opposite takes place in the case of P-ketoanilides, which is consistent with previous findings (10). Subsequently, use can be made of the value Pi to calculate by the refractivity method, the corresponding molecular dipole moment, pi. Thus the results obtained are also listed in Table 17, together with the dipole moments, penal and pketo, which were previously (10) deduced from other lines of evidence. It will be evident from this table that in the case of diketones there is a fair agreement between the values of pi and penal, and that in the case of ketoanilides there is a marked difference between these values, namely, pi are appreciably lower than penal. This in turn might suggest that in extreme dilute solutions of the acid A, the effect of the base B through the hydrogen bonding action is either to cause practically complete conversion of this acid into its en01 form if it is a diketone in cyclohexane solution, or to shift largely the position of equilibrium in favour of the en01 form (but not to completion) if it is a ketoanilide in benzene solution. This conclusion might be accepted in view of the experimental facts (10, 11) that in the case of P-diketones the amount of en01 present at infinite dilution in cyclohexane (as well as in carbon tetrachloride) is of the order of mol%, whereas in the case of P-ketoanilides at infinite dilution in the less inert solvent benzene, in which preferential solvation of the keto tautomer takes place, it is less than 40 mol%. Consequently, in the case of diketones a rough estimate has been made of the dipole moment, Sm, of the intramolecular hydrogen bond of the en01 tautomer by applying the method of bond moments (1 1) and making use of the value pi which was taken as equal to keno,. The results obtained are again listed in Table 17. It will be evident from this table that Sm increases in the order [2] 1<2<3 signifying that the acidity of the diketones decreases in the opposite order; that is, This is consistent with the observations that the values (12) of pka increase in the order given by eq. [2], whereas those of the equilibrium constant, K, as determined in the present work, decreases in the sequence given by eq. [3] (again see Table 17). It,may be pointed out here that in the case of ketoanilides an estimate of the dipole moment of the intramolecular hydrogen bond of the en01 form could not be made as in the case of diketones, since pla, cannot be equated to penal, as noted above. Also that the values of the equilibrium constant, K, of the association process in benzene solution, as obtained by applying eq. [I], which is a modified form of Few and Smith (2) original equation, are 364 and 720 cm3/mol for acetoacetanilide and benzoylacetanilide, respectively. These values might be regarded as more correct than those previously (1) obtained. On the other hand, the fact that they are still much lower than those of the association process of the diketones with the same amine in cyclohexane solution (see Table 17) although the ketoanilides are of higher acidity, as can be evidenced from the pka values, can be accounted for on the basis that since benzene is more active than cyclohexane the formation of hydrogen bonded complexes in the former solvent necessitates the breaking of the bonds which the ketoanilides (and probably the base triethylamine) form with it. It might be added here that although the apparent solution moments, pz7s, of the diketones in the various constant-ratio mixtures obtained by either the usual Debye equation, or the modified Onsager (8,9) equation could be regarded "incorrect", owing to the fact that no account is taken in any of these equations of the short-range forces that lead to complex formation, their apparent increase with increasing concentration of the amine (see Tables 1-3, and 14-16) suggests an increase in complex formation. An estimate of this increase can, in turn, be made by the calculation of the fraction, AwA/wA, of the diketone introduced in the constant-ratio mixture of triethylamine and cyclohexane, which is complexed with the amine at infinite dilution. For this purpose, the following equation also suggested by Few and Smith (2) except that Pi is substituted for PA, as pointed out above, has been used. [4] Pi - Pi = AP(AwA/wA) The results obtained are shown in Table 18. It is obvious from this table that an increase in the proportion of triethylamine in the solvent mixture favours complex formation; as is to be expected. Moreover, for a given solvent mixture, the tendency toward complex formation with the amine is highest for acetylacetone, 1, and least with dibenzoylmethane, 3. This agrees with the above conclusion drawn from the values of the dipole moment, Sm, of the intramolecular hydrogen bond, as well as from those of the equilibrium constant, K, of complexation, and of the acidity, pka, of the diketones. Owing to the limited number of investigated compounds in the present work, it is difficult to investigate the effect of change in acidity of these compounds in a given solvent upon the equilibrium constant of the association process. For the same reason, it is also difficult to check whether there is a correlation between the magnitude, Ap, of the dipole moment change associated with the formation of the intermolecular hydrogen bond, N---H-0, and the equilibrium constant expressed in terms of the free energy change, AGO, of formation of the complex. If, however, in a preliminary investigation one confines oneself just to the three diketones investigated here, it can be easily seen from Table 17 that there is a correlation between AGO, and the acidity, pka, the trend being given by the relation and that there is no correlation between Ap, which was roughly calculated by the vector summation method of bond moments (1 1, 13-15), and AGO. This is to be expected on the basis that a breaking of the intramolecular hydrogen bond in the en01 tautomer of the acid should precede the formation of the intermolecular hydrogen bond. Obviously, a clear justification of these inferences could only be obtained by extending measurements on a larger number of compounds. I. M. M. NAOUM, G. R. SAAD, M. M. ABDEL MOTELEB, H. G. SHINOUDA, and H. A. RIZK. Can. J. Chem. 65, 2760 (1987). 2. A. V. FEW and J. W. SMITH. J. Chem. Soc (1949). 3. L. W. REEVES. Can. J. Chem. 35, 1351 (1957). 4. D. P. EARP and S. GLASSTONE. J. Chem. Soc (1935). 5. D. L. HAMMICK, A. NORRIS, and L. E. SUTTON. J. Chem. Soc (1938). 6. A. WEISSBERGER and E. PROSKAUER. Organic solvents, physical constants and methods of purification. The Clarendon Press, Oxford

5 288 CAN. J. CHEM. VOL A. R. TURKY, H. A. RIZK, and Y. M. GIRGIS. J. Phys Chem. 64, 565 (1960). 8. L. ONSAGER. J. Am. Chem. Soc. 58, 1486 (1936). 9. H. A. RIZK and I. M. ELANWAR. Can. J. Chem. 46, 507 (1968); H. A. RIZK, N. Y. YOSSEF, and H. GRACE. Can. J. Chem. 47, 3767 (1969). 10. M. M. NAOUM, H. G. SHINOUDA, A. S. SHAWALI, and H. A. RIZK. J. Chim. Phys. 78, 155 (1981); M. M. NAOUM and M. M. ABDEL MOTELEB. Gazz. Chim. Ital. 111, 473 (1981); M. M. NAOUM and G. R. SAAD. Indain J. Chem. 26,510 (1987); M. M. NAOUM and G. R. SAAD. J. Soln. Chem. 17, 67 (1988); M. M. NAOUM and M. G. BOTROS. Indain J. Chem. 25,427 (1986); M. M. NAOUM and M. G. BOTROS. Indain J. Chem. 26,607 (1987). 11. C. P. SMYTH. Dielectric behaviour and structure. McGraw-Hill, London H. J. HARRIES, R. K. HUGHES, and T. SMITH. J. Inorg. Nucl. Chem. 34, 1609 (1972). 13. A. COULSON. In Hydrogen bonding. Edited by D. Hadzi and H. W. Thompson. Pergamon Press. New York p W. BLACK, J. G. D. DE JONGH, TH.G. OVERKEEK, and M. J. SPARNEAY. Trans. Faraday Soc. 56, 1597 (1960). 15. P. L. HUYSKENS, W. CLEUREN, H. M. VAN BRADANT-GOVAERTS and M. A. VUYLSTEKA. J. Chem. Phys. 84, 2740 (1980).