Langmuir 1996, 12, 3881-3884
3881
Aggregating Tendencies of Some Phosphonates and Phosphinates Xi-Kui Jiang,* Ji-Liang Shi, and Xin Chen Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Lu, Shanghai 200032, China Received March 27, 1996X Simple aggregations of phosphorus-containing compounds have been investigated for the first time. The aggregators studied are O-alkyl O-4-nitrophenyl methylphosphonates (1-n and 2), 4-nitrophenyl alkylphenylphosphinates (3-n), and O-n-octyl O-4-nitrophenyl phenylphosphonate (4). Their aggregating tendencies have been evaluated by measuring their critical aggregate concentrations (CAgC’s) in dioxaneH2O binary mixtures of graded compositions. Structural effects on aggregating tendencies have been discussed.
Hydrophobic-lipophilic interactions (hereafter abbreviated HLI) together with Nature’s other forces create simple aggregates, micelles, and vesicles, etc. from organic molecules in solvents with solvent aggregating power (SAgP). Simple aggregates of electrically neutral organic molecules are formed almost solely by HLI. Therefore, they may serve as one of the simplest models for studying HLI.1,2 Organic molecules which tend to form simple aggregates in solvents with SAgP are called aggregators (Agr’s).2,3 The aggregating tendencies of aggregators are generally evaluated by measuring their critical aggregate concentrations, or CAgC’s,4 in aqueous or in aquiorgano binary mixtures with their composition designated by the symbol Φ, the volume fraction of the organic component of the binary mixture. CAgC is the concentration of aggregator at the onset of aggregation. Under similar conditions, a smaller CAgC value signifies a greater aggregating tendency. In general, in the absence of pronounced shape effect,5 the more hydrophobic (or lipophilic) is a particular molecule, as roughly estimated from its Rekker’s ∑f value,6 the greater is its aggregating tendency. In the past, CAgC’s of many carboxylic ester aggregators have been evaluated by plotting their hydrolytic rate constants (kob) against their initial concentrations ([Agr]i).1,4 However, all these previously studied simple aggregators are purely carbon compounds, and no study of the phosphorus- or sulfur-containing aggregators has ever been reported. In view of the facts that the phenomena of aggregation are inseparably related to life processes and that phosphorus-containing compounds are one of the essential building blocks of life,7 we decided X
Abstract published in Advance ACS Abstracts, July 15, 1996.
(1) (a) Jiang, X. K. Acc. Chem. Res. 1988, 21, 362 and pertinent references cited therein. (b) Tung, C. H.; Xu, C. B. In Photochemistry and Photophysics; Raber, J. F., Ed.; CRC Press: Boca Raton, FL, 1991; Vol. 4, Chapter 3. (2) Jiang, X. K. Pure Appl. Chem. 1994, 66, 1621 and pertinent references cited therein. (3) Jiang, X. K.; Ji, G. Z.; Zhang, J. T. Langmuir 1994, 10, 122. (4) Zhang, J. T.; Nie, J.; Ji, G. Z.; Jiang, X. K. Langmuir 1994, 10, 2814. (5) Jiang, X. K.; Ji, G. Z.; Tu, B.; Zhang, X. Y.; Shi, J. L.; Chen, X. J. Am. Chem. Soc. 1995, 117, 12679. (6) Rekker's f values or hydrophobic fragmental constants are calculated from octanol-water partition coefficients. The total hydrophobicity of an organic molecule can be roughly estimated from a summation of the f values of all the “fragments” of that molecule. For illustration, the parenthesized f values are given by Rekker for the following fragments: CH3 (0.701), CH2 (0.519), CH (0.337), Ph (1.840), COO (-0.962), C6H4 (1.658), NO2 (-0.053). See: (a) Rekker, R. F. The Hydrophobic Fragmental Constants; Elsevier: Amsterdam, 1977; Vol. 1. (b) Rekker, R. F.; de Kort, H. M. Eur. J. Med. Chem. 1979, 14, 479.
S0743-7463(96)00372-1 CCC: $12.00
to start a study on the aggregation behavior of some representative phosphorus-containing compounds. In this paper, we report the aggregation behavior in the dioxane (DX)-H2O solvent system at 35 °C of two types of organophosphorus esters, namely, O-alkyl O-4-nitrophenyl methylphosphonates (1-n, n ) 4, 8, 12, 16; and 2) and 4-nitrophenyl alkylphenylphosphinates (3-n, n ) 4, 8, 10, 16), as well as O-octyl O-4-nitrophenyl phenylphosphonate (4) and 4-nitrophenyl decanoate (5). Compounds
4 and 5 were used to serve as common standards for comparison of aggregating tendencies of aggregators with the same chain length (such as 1-8, 3-8, 4, and 5) in specified mediums. Pseudo-first-order hydrolytic rate constants (kob) of these p-nitrophenyl esters in alkaline solutions were measured in order to evaluate the critical aggregate concentrations of these Agr substrates.1,4 There is a great difference in hydrolytic reactivity between the carboxylates and phosphonates or phosphinates,8 and large substituent effects on the rate of hydrolysis of phosphonates and phosphinates have been reported8 and (7) (a) Biochemistry of Natural C-P Compounds; Hori, T., Horiguchi, M., Hayashi, A., Eds.; Maruzen: Kyoto, 1984. (b) Corbridge, D. E. C. Phosphorus: An Outline of its Chemistry, Biochemistry and Technology, 3rd ed.; Elsevier: Amsterdam, 1985. (8) (a) Cook, R. D.; Diebert, W. S.; Turley, P. C.; Haake, P. J. Am. Chem. Soc. 1973, 95, 8088. (b) Hudson, R. F.; Keay, L. J. Org. Chem. 1956, 2463. (c) Yuan, C.; Li, S.; Liao, X. J. Phys. Org. Chem. 1990, 3, 48.
© 1996 American Chemical Society
3882 Langmuir, Vol. 12, No. 16, 1996
observed in the present work. Therefore, it was found that three different buffers (cf. Experimental Section) had to be used for the present study, and this state of affairs made it impossible to compare the aggregating tendencies (CAgC’s) of all the studied aggregators in one common medium. However, comparisons can be made among aggregators studied in the same medium. Experimental Section Apparatus. 1H NMR spectra were obtained at 90 MHz on a Varian FX-90Q spectrometer or at 200 MHz on a Varian XL-200 spectrometer with TMS as the internal standard, and 31P NMR spectra were obtained on a Varian FX-90Q spectrometer with 85% H3PO4 as the external standard. Chemical shifts are expressed in ppm (δ), and coupling constants (J) are quoted in hertz ((0.3 Hz). Mass spectra (MS) were taken by using an HP 5989A spectrometer (EI, 70 eV). Infrared spectra (IR) were recorded on a Shimadzu IR 440 spectrometer. Reagents and Substrates. All target compounds were prepared or synthesized in this laboratory and identified later by elemental analysis and 1H NMR and 31P NMR spectra. Alkyl p-nitrophenyl methylphosphonates (1-n and 2) were synthesized from methylphosphonic dichloride, itself prepared according to the literature.9 A typical procedure for preparing 1-n and 2 is as follows: methylphosphonic dichloride (0.05 mmol) was first half-esterified by the action of alkanol (0.05 mmol) in the presence of triethylamine (0.05 mmol) in benzene (60 mL) at 40 °C for 0.5 h, followed by addition of a mixture of nitrophenol (0.05 mmol) and triethylamine (0.05 mmol) in benzene (10 mL) for an additional hour. Compounds 1-8 and 2 were previously reported,5 and compounds 1-4 and 1-8 were reported by Osa.10 pNitrophenyl alkylphenylphosphinates (3-n) were synthesized from dichlorophenylphosphine, itself obtained by the well-known reaction between benzene and phosphorus trichloride.11 A typical procedure is as follows: under N2 atmosphere, a mixture of dichlorophenylphosphine (18 mmol), aluminium chloride (anhydrous, 18 mmol), and alkyl halide (chloride or bromide, 18 mmol) was stirred at room temperature for 2 h; then 60 mL of CH2Cl2 was added to dissolve the mixture, followed by addition of a solution of nitrophenol (18 mmol) and pyridine (36 mmol) in benzene (10 mL). Nitrophenyl octyl phenylphosphonate (4) was synthesized from phenylphosphonic dichloride, which was prepared by the action of dichlorophenylphosphine and thionyl chloride, and the procedure was similar to the synthesis of 1-8. p-Nitrophenyl decanoate (5) was prepared from decanoic acid as reported.12 All the esters were purified by flash column chromatography on silica gel with petroleum-ethyl acetate as eluent. Phosphonate 1-4. 1H NMR (CDCl3): δ 8.24 (2H, d, J ) 9.3), 7.40 (2H, d, J ) 9.3), 4.12 (2H, m), 1.71 (3H, d, J(P-CH3) ) 17.7), 1.22-1.53 (4H, m), 0.93 (3H, m). 31P{1H} NMR (36 MHz, CDCl3): δ 28.19. MS: m/z (relative intensity) 274 (M + 1)+ (100), 218 (91.6), 201 (17.2). IR (film): νmax 1250 (PdO), 1018 (PsOsC). Anal. Calcd for C11H16NO5P: C, 48.35; H, 5.90; N, 5.13; P, 11.34. Found: C, 47.87; H, 6.17; N, 4.87; P, 11.61. Phosphonate 1-12. 1H NMR (CDCl3): δ 8.14 (2H, d, J ) 8.8), 7.29 (2H, d, J ) 8.8), 4.05 (2H, m), 1.61 (3H, d, J(P-CH3) ) 17.8), 1.15 (20H, m), 0.78 (3H, m). 31P{1H} NMR (36 MHz, CDCl3): δ 28.14. MS: m/z (relative intensity) 386 (M + 1)+ (78.7), 218 (100), 201 (16.7). IR (film): νmax 1250 (PdO), 1015 (PsOsC). Anal. Calcd for C19H32NO5P: C, 59.20; H, 8.37; N, 3.63; P, 8.03. Found: C, 59.08; H, 8.68; N, 3.68; P, 8.00. Phosphonate 1-16. 1H NMR (CDCl3): δ 8.18 (2H, d, J ) 9.0), 7.33 (2H, d, J ) 9.0), 4.05 (2H, br), 1.66 (3H, d, J(P-CH3) ) 18.0), 1.21 (28H, m), 0.84 (3H, m). 31P{1H} NMR (36 MHz, CDCl3): δ 28.24. MS: m/z (relative intensity) 442 (M + 1)+ (13.6), 218 (100), 201 (8.0). IR (film): νmax 1250 (PdO), 1120 (9) (a) Kinnear, A. M.; Perren, E. A. J. Chem. Soc. 1952, 21, 3437. (b) Clay, J. P. J. Org. Chem. 1951, 16, 892. (10) Osa, A.; Arukaevu, H.; Aaviksaar, A. J. Chromatogr. 1977, 135, 196. (11) Buchner, B. Organic Syntheses; Wiley: New York, 1963; Collect. Vol. IV, p 784. (12) (a) Jiang, X. K.; Hui, Y. Z.; Fan, W. Q. J. Am. Chem. Soc. 1984, 106, 7202. (b) Hui, Y. Z.; Wang, S. J.; Jiang, X. K. J. Am. Chem. Soc. 1982, 104, 347.
Jiang et al. (PsOsC). Anal. Calcd for C23H40NO5P: C, 62.56; H, 9.13; N, 3.17. Found: C, 62.75; H, 9.27; N, 3.35. Phosphinate 3-4. 1H NMR (CDCl3): δ 8.11 (2H, d, J ) 9.0), 7.80 (2H, m), 7.53 (3H, m), 7.31 (2H, d, J ) 9.0), 0.93-1.41 (9H, m). 31P{1H} NMR (36 MHz, CDCl3): δ 49.99. Anal. Calcd for C16H18NO4P: C, 60.18; H, 5.68; N, 4.39; P, 9.70. Found: C, 60.08; H, 5.98; N, 4.38; P, 9.02. Phosphinate 3-8. 1H NMR (CDCl3): δ 8.10 (2H, d, J ) 9.1), 7.79 (2H, m), 7.53 (3H, m), 7.30 (2H, d, J ) 9.1), 0.84-1.40 (17H, m). 31P{1H} NMR (36 MHz, CDCl3): δ 50.16. MS: m/z (relative intensity) 376 (M + 1)+ (100), 290 (86.6), 237 (27.9). IR (film): νmax 1208 (PdO), 1020 (PsOsC). Anal. Calcd for C20H26NO4P: C, 63.99; H, 6.98; N, 3.73; P, 8.25. Found: C, 64.01; H, 6.89; N, 3.88; P, 8.11. Phosphinate 3-10. 1H NMR (CDCl3): δ 8.10 (2H, d, J ) 8.0), 7.81 (2H, m), 7.52 (3H, m), 7.30 (2H, d, J ) 8.0), 0.80-1.54 (21H, m). 31P{1H} NMR (36 MHz, CDCl3): δ 50.28. MS: m/z (relative intensity) 404 (M + 1)+ (60.1), 301 (100), 265 (10.5). IR (film): νmax 1215 (PdO), 1030 (PsOsC). Anal. Calcd for C22H30NO4P: C, 65.49; H, 7.49; N, 3.47. Found: C, 65.31; H, 7.43; N, 3.16. Phosphinate 3-16. 1H NMR (CDCl3): δ 8.11 (2H, d, J ) 9.0), 7.80 (2H, m), 7.50 (3H, m), 7.29 (2H, d, J ) 9.0), 0.80-1.62 (33H, m). 31P{1H} NMR (36 MHz, CDCl3): δ 50.23. MS: m/z (relative intensity) 488 (M + 1)+ (100), 385 (44.0). IR (film): νmax 1210 (PdO), 1020 (PsOsC). Phosphonate 4. 1H NMR (CDCl3): δ 8.15 (2H, d, J ) 9.0), 7.80 (2H, m), 7.52 (3H, m), 7.30 (2H, d, J ) 9.0), 4.20 (2H, m), 1.10-1.80 (12H, m), 0.86 (3H, t). 31P{1H} NMR (36 MHz, CDCl3): δ 15.80. MS: m/z (relative intensity) 392 (M + 1)+ (100), 280 (76.9), 253 (7.8). IR (film): νmax 1208 (PdO), 1020 (PsOsC). Anal. Calcd for C20H26NO5P: C, 61.37; H, 6.70; N, 3.58; P, 7.91. Found: C, 61.55; H, 6.79; N, 3.62; P, 7.83. Carboxylate 5. 1H NMR (CDCl3): δ 8.22 (2H, d, J ) 9.0), 7.23 (2H, d, J ) 9.0), 2.57 (2H, t), 1.60-1.84 (2H, m), 1.10-1.50 (12H, m), 0.87 (3H, t). MS: m/z (relative intensity) 294 (M + 1)+ (6.0), 155 (100). Anal. Calcd for C16H23NO4: C, 65.51; H, 7.90; N, 4.78. Found: C, 65.32; H, 7.89; N, 4.87. Kinetics. Water was deionized, and DX (dioxane) was purified by a standard procedure.13 Kinetic measurements in DX-buffer systems with graded Φ values were performed on Perkin-Elmer 559 and Perkin-Elmer Lambda 5 UV-vis spectrometers equipped with a thermostated cell holder at 35 °C, by monitoring the formation of the p-nitrophenol at 410 nm, as previously described.3-5,14-16 The experimental uncertainty for the kob values was within (5%. The aqueous buffer solutions used were as follows: Buffer I: 0.01 M NaOH, 0.01 M NaHCO3, and 0.34 M NaCl (pH ) 11.7 at 18 °C). Buffer II: 0.13 M NaOH and 0.05 M KCl (pH ) 13.0 at 18 °C). Buffer III: 0.064 M NaOH and 0.05 M KCl (pH ) 12.6 at 18 °C). Buffer I was first used because it had been used in most of the previous CAgC measurements,1a,4,14,15 but it was found that the hydrolyses of the phosphonates and phosphinates were much too slow in this buffer. Buffer II was found convenient only for phosphinates, but hydrolyses of carboxylates and phosphonates were found to be too fast for accurate kinetic measurement in this buffer. Buffer III was later found to be the most convenient buffer systems and was used in most of our CAgC measurements.
Results and Discussion The hydrolytic rate constants of substrates in DX-H2O mixtures of graded Φ values at various initial substrate concentrations were measured and plotted against initial substrate concentrations ([Agr]i), and for each substrate and solvent system a figure with curves corresponding to different Φ values can be obtained. Whenever there is a break point in these curves, a CAgC value can be determined.4,15 As exemplified by Figure 1, the relationship between the initial concentration (10-6 to 10-4 M) of 1-16 ([Agr]i ) and kob at different Φ values has been (13) Riddick, J. A. Organic Solvents; Vol. II, p 939. (14) (a) Jiang, X. K.; Hui, Y. Z.; Fan, W. Q. J. Am. Chem. Soc. 1984, 106, 3839. (b) Zhang, J. T.; Nie, J.; Sun, S. X.; Ji, G. Z.; Jiang, X. K. Chin. J. Chem. 1994, 12, 179. (15) Jiang, X. K.; Li, X. Y.; Huang, B. Z. Proc.sIndian Acad. Sci., Chem. Sci. 1987, 98, 409; 1987, 98, 423. (16) Jiang, X. K.; Ji, G. Z.; Luo, G. L. Chin. J. Chem. 1991, 9, 448.
Aggregation of Some Phosphonates and Phosphinates
Figure 1. Log kob vs log [Agr]i plots for 1-16 in DX-H2O solvent systems with different Φ values. Table 1. CAgC Values (10-5 M) of 1-n (n ) 4, 8, 12), 2, and 5 in DX-H2O Systemsa with Graded Φ Values at 35 °C Φ (DX-H2O) Agr
0
0.05
0.10
0.20
0.30
1-4 b 1-8 3.90 ( 0.10 7.00 ( 0.31 >13.3 1-12 5.00 ( 0.11 2 c 5 0.42 ( 0.21 1.72 ( 0.08 4.98 ( 0.11 a The aqueous component is a buffer of 0.01 M NaOH, 0.01 M NaHCO3, and 0.05 M KCl. b No CAgC can be found up to [1-4]i g 14.0. c No CAgC can be found up to [2]i g 10.0
investigated. Three curves are typical: curve C shows that at Φ ) 0.50 there is only a monomeric region in which no aggregation of the target molecule (1-16) occurs. Curve A indicates that at Φ ) 0.25 there is only an aggregated region in which kob decreases with increasing [Agr]i; i.e., the degree of aggregation increases with the increasing [Agr]i. Curve B shows that at Φ ) 0.35 there are a monomeric region, a transition region, and an aggregated region; thus, the CAgC value of 1-16 at Φ ) 0.35 can be evaluated to be 4.52 × 10-5 M. These results also show that the occurrence of aggregation of 1-16 depends on the Φ value of the medium. This observation is very similar to the observation on the behavior of the carboxylates.1a,2,4 Our other substrates, i.e., 1-n, 3-n, 4, and 5, all have similar curves; in other words, all our data indicate that the occurrence of aggregation depends on the substrate concentration [Agr]i and the solvent aggregation power (SAgP) of the medium,1 because the SAgP of one specified solvent system is directly correlatable with the Φ value.2,15,16 The CAgC values of 1-8, 1-12, and 5 in the DX-H2O system in which the aqueous component is buffer I (vide supra) are listed in Table 1. The CAgC values of 3-8 and 3-10 in the DX-H2O system in which the aqueous component is buffer II are listed in Table 2. The CAgC values of 1-n, 3-n, 4, and 5 in the DX-H2O system in which the aqueous component is buffer III are listed in Table 3. Examination of Tables 1-3 reveals the following observations: (1) As expected, CAgC values increase with increasing Φ values; e.g., in Table 1, 1-8 at three Φ values and 5 at three Φ values; in Table 2, 3-8 at two Φ values and 3-10 at five Φ values; in Table 3, 1-8 at two Φ values, 1-12 at five Φ values, 1-16 at four Φ values, 3-8 at three Φ values, 3-10 at four Φ values, and 4 at five Φ values.
Langmuir, Vol. 12, No. 16, 1996 3883
(2) The following linear relationships between CAgC and Φ have been found. In Table 1, for 5, log CAgC ) 5.37Φ - 5.89, n ) 3, and r ) 0.997. In Table 2, for 3-10, log CAgC ) 7.51Φ - 5.68, n ) 4, and r ) 0.999. In Table 3, for 1-12, log CAgC ) 9.78Φ - 6.30, n ) 3, and r ) 0.999; for 1-16, log CAgC ) 12.2Φ - 8.67, n ) 4, and r ) 0.995; for 3-10, log CAgC ) 7.90Φ - 5.84, n ) 3, and r ) 1.00; and for 4, log CAgC ) 6.04Φ - 5.62, n ) 5, and r ) 0.998. The existence of these relationships also speaks well for the reliability of our CAgC measurements.2,4 (3) The conventional chain-length effect is confirmed by the following orders of increasing CAgC values. In Table 2, at Φ ) 0, 3-10 < 3-8 < 3-4. In Table 3, at Φ ) 0.30, 1-16 < 1-12, 3-16 < 3-10; at Φ ) 0.10, 3-10 < 3-8; and at Φ ) 0, 1-8 < 1-4, 3-8 < 3-4. Persuasive evidence for the HLI driving force for the aggregation of our phosphorus-containing Agr’s is the observation that the aggregating tendencies of these aggregators are directly related to the ∑f values6 of the alkyl groups. Although the f values of PdO and OPdO fragments are not available from Rekker’s table,6 the relative hydrophobicities of our compounds, i.e., 1-n, 3-n, and 4, can still be assessed by comparing the ∑f values of the alkyl groups because ∑f values of identical groups cancel out during comparison between two compounds. The ∑f values of the alkyl groups are as follows: C16H33, 8.486; C12H25, 6.410; C10H21, 5.372; C8H17, 4.334; C4H9, 2.258. Evidently this ordering is exactly the same as the ordering of the aggregating tendencies for the 1-n and 3-n series. The most interesting piece of information from Table 3 is a structure effect observed by comparing 3-8 and 4, both of which possess the n-octyl chain. In both Φ ) 0.05 and 0.10 systems, the CAgC values of 4 are much smaller than those of 3-8. This indicates that the fragment P(O)OR has a greater aggregating tendency (smaller CAgC) than that of P(O)R. If one could subtract the P(O) fragment from both, one might be led to an unbelievable or unreasonable conclusion, which says that the OR fragment possesses a greater aggregating tendency than that of R (the f constant for the O atom is negative; i.e., it is a hydrophibic fragment). Therefore, the P(O)OR (in phosphonates) and the P(O)R (in phosphinates) groups should be considered as indivisible hydrophobic fragments in HLI considerations. The above-mentioned conclusion is also supported by the comparison of aggregating tendency between 1-12 and 3-10. Except for one O atom (the oxygen atom of the P-O-C12H25 fragment of 1-12), the difference of ∑f for 3-10 and 1-12 is rather small; (i.e., ∆∑f ) {f(C6H5) + f(C10H21)} - {f(CH3) + f(C12H25)} ) (1.84 + 5.37) - (0.70 + 6.41) ) 0.10), but their CAgC values in solvent mixtures of Φ ) 0.10, 0.15, and 0.20 are quite different (Table 3), with the CAgC’s of phosphonate 1-12 smaller than that of phosphinate 3-10. Since a small difference in the ∑f value is usually not a deciding or important factor,5,16 these differences could be a reflection of the difference of the aggregating tendencies of the P(O)OR and P(O)R fragments, with the former larger than the latter. Another structural effect was that of the effect of a side chain; that is, branching of the chain reduces its aggregating tendency.5 A look at the CAgC values (Φ ) 0, Table 1 and Table 3) of the two esters 1-8 and 2, both with the same ∑f values, shows that the aggregating tendency of the phosphonate 1-8 (CAgC ) 3.90 × 10-5 M in Table 1 and 6.87 × 10-5 M in Table 3) is much larger than that of its branch-chained isomer 2 (CAgC > 10.0 × 10-5 M). One of the main objectives of this work is to compare the phosphorus-containing compounds with the carboxylates. This can be done now by comparison of the aggregating tendencies of 5 (Table 1, Φ ) 0.10, CAgC )
3884 Langmuir, Vol. 12, No. 16, 1996
Jiang et al.
Table 2. CAgC Values (10-5 M) of 3-n (n ) 4, 8, 10) in DX-H2O Systemsa with Graded Φ Values at 35 °C Φ (DX-H2O)
a
Agr
0
0.05
3-4 3-8 3-10
b 2.64 ( 0.06 0.22 ( 0.02
5.78 ( 0.13
0.10
0.15
0.20
0.30
1.08 ( 0.03
2.79 ( 0.12
7.00 ( 0.15
>11.1
The aqueous component is a buffer of 0.13 M NaOH and 0.05 M KCl. b No CAgC can be found up to [3-4]i g 14.0 Table 3. CAgC Values (10-5 M) of 1-n, 2, 3-n, 4, and 5 in DX-H2O Systemsa with Graded Φ Values at 35 °C Φ (DX-H2O)
Agr
0
1-4 1-8 1-12 1-16 2 3-4 3-8 3-10 3-16 4 5
b 6.87 ( 0.02
a
b b 2.35 ( 0.02
0.05 >13.2 12.8 0.90 ( 0.04
2.21 ( 0.02
5.55 ( 0.03
0.90 ( 0.08 13.7 1.11 ( 0.02
4.52 ( 0.05
13.2 ( 0.2
>14.7 13.3 × 10-5 M), which has the same hydrocarbon chain.
Evidently, the carboxylate possesses a larger aggregating tendency than the phosphonate does. Another very rare and interestingly phenomenon should be described; namely, the order of aggregating tendencies of 1-12 and 4 has been found to be reversed when Φ changes from 0.05 to 0.25, as shown in Figure 2. For example, when Φ e 0.15, the aggregating tendency of 1-12 is greater than that of 4; however, when Φ ) 0.20, the order is reversed. In other words, the order of aggregating tendencies of 1-12 and 4 can be reversed by SAgP. This is the consequence of the fact that the sensitivity of their CAgC values to SAgP (slopes of the straight lines in Figure 2) is different. Up to now, this is the only example that we are aware of. Finally, we may conclude that alkyl phosphonates and phosphinates form simple aggregates in solvents with SAgP just like purely carbon Agr’s and that the phosphonate and phosphinate groups, just like the carboxylate group, can also be used as kinetic probe groups for evaluating CAgC values. LA960372E
