J . Phys. Chem. 1985,89. 2082-2086
2082
positive. This would warrant the validity of t h e extended algorithm.
Acknowledgment. The technical assistance of Guy Preau in the construction of t h e instrument is greatly acknowledged. mass flow rate of the gas stream when X = 0, Le., when pure carrier gas is allowed to flow through the column (eq 1) mass flow rate of the gas stream containing a n-hexane concentration X (eq 1) time mesh size of the network of points used in the Godunov integration scheme (eq 6) space mesh size of the network of points used in the Godunov integration scheme (eq 6) mobile phase flow rate flow rate of the mobile phase during the injection of a large size band carrier gas flow rate during the elution of a large band left-hand side of eq 1, results from experimental measurements column capacity factor with a stream of pure carrier gas (eq 1) column capacity factor for a n-hexane partial pressure equal to PX (eq 1)
largest value measured for the column capacity factor amount of n-hexane adsorbed when its partial pressure ispX (Figure 3) column length (eq 8) inlet to outlet pressure ratio (eq 1) average column pressure (eq 4) ratio of the local pressure to the column outlet pressure (eq 1) local pressure in the column (eq 8) time total analysis time t,(x) for X = 0, Le., conventional inert gas retention time retention time of a pulse of inert gas injected on a concentration step X of n-hexane (eq 1) width of the rectangular injection t R ( x ) for X = 0, Le., conventional retention time (eq 1) retention time of a small pulse of n-hexane injected on a concentration step X (eq 1) lowest value of the mobile phase velocity (eq 6) concentration step height, Le., concentration of n-hexane vapor in the mobile phase (eq 1) n-hexane concentration during the injection of a large concentration band concentration profile of a n-hexane band in the column boundary condition, Le., profile of the injected band abcissa along the column (eq 8)
A Method for Estimating Anionic Reactivity of Organic Salts.
Nuclear Relaxation
Gerald Remaud, Gerard J. Martin,* and Maryvonne L. Martin Laboratoire de Chimie Organique Physique ERA-CNRS 315, UniversitC de Nantes, 2 rue de la Houssini2re. 44072 Nantes Cedex, France (Received: August 9, 1984) 35C1NMR offers a powerful tool for obtaining information concerning microorganization in the vicinity of the chlorine anion. The variation in the transverse relaxation of 35Cl as a function of temperature has been examined for 13 chloride salts in acetonitrile solutions. The line-width parameter is very sensitive to changes in the electric-field gradients at the chlorine nucleus, and a significant lowering in the symmetry of the chlorine environment is observed when two ammonium species are mixed at a fixed concentration in CH3CN. Strong perturbations are introduced when phenols are added to a given organic salt, and a plateau is generally observed in the curves which relate to the temperature for equimolar mixtures of phenols and salt in acetonitrile. These results enable a typical parameter, to be defined, which exhibits a very good correlation with the pK, of the phenols. The method can be used for characterizing the anionic reactivity of chlorine in various organic salts. By means of complementary experiments performed in methanol, lithium chloride has also been tentatively situated within the reactivity scale determined for the ammonium, iminium, amidinium, guanidinium, and phosphonium salts.
Organic salts and mainly ammonium, iminium, or phosphonium chlorides behave as catalysts in reactions between Lewis acids (POC13,C 0 C l 2 , ...) and basic substrates,Ip2 and it has been recognized that, in certain cases, solutions of these salts possess particular properties. Thus long-chain ammonium salts may exhibit micellar structures which undergo transitions under various experimental and it has been concluded that tetralkylammonium cations can introduce a structure-forming effect on the water lattice.E-10 NMR spectroscopy is a convenient tool for studying the short-range structure of salts, and the resonances ~~
(1) Martin, M. L.; Ricolleau, G.; Poignant, S.; Martin, G. J. J . Chem. Soc., Perkin Trans. 2 1976, 182. (2) Poignant, S.; Gauvreau, J. R.; Martin, G. J. Can. J. Chem. 1980, 58, 946. (3) Lindman, B.; Forsen, S. In "NMR-12 Basic Principles and Progress"; Diehl, P.,Ruck, E.,Kosfeld, R., Eds.; Springer-Verlag, Heidelberg, 1976. (4) Lindblom, G.; Lindman, B. J . Phys. Chem. 1973, 77, 2531. (5) Lindblom, G.; Lindman, B.; Mandell, L., J. Colloid Interface Sci. 1973, 42, 400. (6) Elseoud, 0. A,; Fendler, E. J.; Fendler, J. H.; Medary, R. T. J . Phys. Chem. 1973, 77, 1876. (7) Kahn, A,; Fontell, K.; Lindblom, G. J . Phys. Chem. 1982, 86,383. (8) Fister, F.; Hertz, H. G. Ber. Bunsenges. Phys. Chem. 1967, 71, 1032. (9) (a) Hertz, H. G.; Holz, M. J. Phys. Chem. 1974, 78, 1002. (b) Braun, B. M.; Holz, M. J. Phys. Chem. 1983, 135, S77. (10) Lindman, B.; Forsen, S.; Forslind, E. J . Phys. Chem. 1968, 72,2805.
0022-3654/85/2089-2082$01.50/0
of various nuclei pertaining to the cation, such as 'H, 2H, 13C, I4N,...,have been the source of information on the environment and motion of organic cations in s ~ l u t i o n . ~ * "Halogen -~~ NMR and especially bromine and chlorine relaxation rates have also been used for investigating changes in binding or ionic interactions involving metal ions and organic cations.35~9~10~143 In most cases, (11) Henriksson, V.; Odberg, L.; Eriksson, J. C. Westman, L. J . Phys. Chem. 1977,81, 76. (12) Dando, V. R.; Gold, H. S.; Dybowski, C. Appl. Spectrosc. 1983, 37, 29. (1 3) Dando, N. R.;Dybowski, C.; Gold, H. S. Anal. Chem. 1982.54, 1101. (14) Ulmius, J.; Lindman, B.; Lindblom, G.; Drakenberg, T. J . Colloid Interface Sci. 1978, 65, 88. (15) Hall, C . ; Haller, G. L.; Richards, R. E. Mol. Phys. 1969, 16, 377. (16) Wennerstrom, H.; Lindman, B.; Forsen, S. J . Phys. Chem. 1971, 75, 2936. (17) Fabre, H.; Kamenka, N.; Khan, A,; Lindblom, G.; Lindman, B.; Tiddy, G. J. T. J. Phys. Chem. 1980,84, 3428. (18) Yudasaka, M.; Sugawara, T.; Iwamura, H.; Fujiyama, T. J . Chem. SOC.Jon. 1981. 54. 1933. (1$ YudaLka, M.;Sugawara, T.; Iwamura, H.; Fujiyama, t. Bull. Chem. SOC.Jpn 1982, 55, 3 11. (20) Sugawara, T.; Yudasaka, M.; Takahashi, K.; Tamamushi, R.; Iwamura, H.; Fujiyama, T. Bull. Chem. SOC.Jpn 1982, 55, 1959. (21) Sugawara, T.; Yudasaka,M.; Yokoyama, Y.; Fujiyama, T.; Iwamura, H. J. Phys. Chem. 1982.86,2705. (22) Eisenstadt, M.; Friedman, H. L. J . Chem. Phys. 1966, 44, 1407. (23) Langford, C. H.; Stengle, T. R. J. Am. Chem. Sot. 1969, 91, 4014.
0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 10, 1985 2083
Anionic Reactivity of Organic Salts
TABLE I: Half-Height HCl Line Widths for Various Chlorides in CH3CN TMCA
THAC
Ali336
TBAC
BBAC
TPPC
MMTPPC
Guani
BEAC
BMAC
A27
MDCA
concn, M 0.25 A V I / ~ , ~ ~ H1290 Z AvlpbHz 1370
0.25 1120 1270
0.25 24 265
0.25
0.25 34 180
0.25 24 230
0.125 35 164
0.125 110 227
0.125 37 175
0.125 20 170
0.05 16 98
0.25c 100 195
0.25 460 470
“Line width of the
signal at 295 K. b35Clline width for an equimolar solution of salt and phenol in CH$N
TBCA
88 272
at 295 K. CInequiv.
these studies were concerned with purely aqueous solutions of the salts or with mixed solutions containing water. However, much less is known about the interactions in other protic or aprotic Moreover, until now, few investigations have been devoted to the problem of nucleophilic assistance of ammonium salts, which are known as powerful phase-transfer agents.24 In the present work attention is focused on the role of the anion in various organic salts, and a direct examination of the chlorine resonance is expected to provide more reliable results concerning the microorganization in the vicinity of this anion in nonaqueous solutions. It will be shown that chlorine-35 relaxation offers a new approach to estimating the anionic reactivity of organic salts in catalyzed halogenation reactions. Experimental Part Reagents. Tetrabutyl- (TBAC), tetrahexyl- (THAC), and benzyltributylammonium (BBAC) chlorides are considered as model reagents for the investigation of the dynamic behavior of the chloride anion (section a in Results and Discussion), but other ammonium, iminium, amidinium, guanidinium, and phosphonium salts behave similarly. Anhydrous solutions of TBAC, THAC, and BBAC in CH3CN (0.5 M) were carefully prepared from recrystallized salts and distilled solvent. Several different phenols p-XC6H40H (X = NOz, CN, Br, C1, CH30, CH3, H ) and 2,4,6-trimethylphenol were used as 0.5 M solutions in CH3CN, the purity of the phenols exceeding 99%. Other chlorides were investigated with a view to comparing their reactivity (section c of Results and Discussion): benzyltriethyl- (BEAC) and benzyltrimethyl- (BMAC) ammonium salts, aliquat 336 (Ali 336) methylmethoxytriphenyl- (MMTPPC) and tetraphenyl- (TPPC) phosphonium salts, a Mannich’s salt, methylenedimethylammonium chloride (MDACE(CH~)~N+=CH~,C~-), tetramethylchloroformamidinium chloride (TMCA), tetrabutylchloroformamidmium chloride (TBCA),Z5the amberlyst resin A27, and the guanidinium salt (guani). All these compounds were used as 0.5 M (0.5 equiv/L for A27) solutions in CH3CN, with the exception of BEAC, MMTPPC, TPPC (0.25 M), and BMAC (0.1 M) due to solubility limitations in CH3CN. LiCl was used as 0.5 M in C H 3 0 H . Escosity Measurements. These measurements were performed with an Ostwald viscosimeter. NMR Determinations. 35Clspectra were recorded at 24.5 and 8.8 MHz by using Briiker W M 250 and W H 90 spectrometers equipped with wide band 15-mm-0.d. probes. The pulse lengths corresponding to a r / 2 pulse are 120 X 10” and 28 X 10” s, respectively. A spectral width of SO00 Hz and a sampling memory size of 8K associated with an acquisition time of 0.82 s were selected. A satisfactory signal-to-noise ratio was usually obtained with 1000 scans. For line widths larger than 300 Hz an exponential broadening of 20 Hz was applied. In some cases better results were obtained by means of spin-echo experiments. A variable temperature unit was used to control the temperature of the air flow in the N M R probe containing the N M R cell, and the temprature of the probe was measured by means of a thermocouple. The spectrometer was used in the sweep-off mode without field frequency locking. Results and Discussion ( a ) Organic Salts in Acetonitrile Solutions. If we ignore exchange processes, the relaxation of the chlorine nucleus in the (24) Stark, C. M. J . Am. Chem. Soc. 1971, 93, 195. (25) Gauvreau, J. R.; Martin, G. J. J. Chem. Soc., Perkin Trans. 2 1983, 1541.
Figure 1. Variations in the line width of the 35Clsignal as a function of the relative concentrations of the salts for mixtures of two ammonium chlorides in CH3CN (0.5 M) at 298 K.
salts obeys a pure quadrupolar mechanism, and, in the extreme narrowing conditions (w2r:
