Studies on nitroaromatic compounds. Part 8. A kinetic and spectroscopic study of the reaction of di- and tri-ethylamine with 1,5-dimethyl-2,4,8-trinitronaphthalene

摘要

J.C.S. Perkin I1 Studies on Nitroaromatic Compounds. Part 8.t A Kinetic and Spectro- scopic Study of the Reaction of Di- and Tri-ethylamine with 1.5-Dimethyl- 2,4,8-trin itronaphthalene By Eric Buckley, John E. Everard, and Clifford H.J. Wells," School of Chemical and Physical Sciences, Kingston Polytechnic, Kingston upon Thames, Surrey KT1 2EE The reaction of 1,5-dimethyI-2,4,8-trinitronaphthalene with diethylamine and triethylamine in DMSO and in DMSO-MeOH solvent systems has been studied. lH N.m.r. investigations have shown that a benzyl type anion is formed due to hydrogen abstraction from the l-methyl group of the nitroaromatic compound. Kinetic and thermodynamic data for the systems have been determined. The results are explained in terms of the difference in solvation of the two amines and the charged products and the difference in solvating ability of the solvent media used.The effect of added triethylammoniun chloride on the reaction of 1,5-dimethyI-2,4,8-trinitronaphthalene with triethylamine in DMSO has been studied and the change in kinetics is attributed to ion association between the added salt and the charged products. KINETIC,thermodynamic, and structural investigations of the reaction of bases with nitroaromatic compounds have received considerable attention and a variety of products have been proposed including Meisenheimer complexes, radical ions, and carbanions.1-6 The kinetics of the reaction of 2,4,6-trinitrotoluene with base to give the 2,4,6-trinitrobenzyl anion has been investigated by various ~orkers,~-ll but Only recently has there been a successful n.m.r.investigation of the form- ation of this anion in flow and static systems.12 N.m.r. TND 8 amine with 1,5-dimethy1-2,4,8-trinitronaphthalene (TND) in DMSO and DMSO-MeOH solvent systems. RESULTS Spectroscopy.-The 'H n.m.r. spectrum of a mixture of TND and one equivalent of triethylamine in 2H,dimethyl sulphoxide a set of product peaks which were assigned to the benzyl type anion, TNL)-, shown in the Scheme. No peaks attributable to products other than those of the Scheme were observed in the spectrum. A similar 1H n.m.r., spectrum was obtained for the product TND-6H' SCHEME spectroscopy has also been used to show the formation of a benzyl type anion in the reaction of 1,5-dimethyl- 2,4,8-trinitronapht halene with methoxide As far as is known, no other evidence for the formation of benzyl type anions from alkylpolynitronaphthaleneshas been reported.Previous studies of the reaction of amines with nitro- aromatic compounds have indicated the formation of the zwitterionic form of a Meisenheimer complex and the conjugate base of this complex. For example, primary and secondary amincs react with 2,4,6-trinitro-benzene and -anisole in DMSO to give a Meisenheimer complex with addition at C-l.14-17 The reaction of butylamines with ethyl 2,4-dinitro-l-naphthyl ether in DMSO proceeds through an intermediate zwitterionic Meisen- heimer complex by addition at C-1. The complex then loses a proton to the base to give the conjugate base of the neutral zwitterionic form.l* Recently a flow n.m.r.study of this system confirmed the presence of the inter- mediate complexes. l9 The present paper reports on the formation of a benzyl type anion in the reaction of diethylamine and triethyl- Part 7 is ref. 13. formed when diethylamine was added to TND in 2He-dimethyl sulphoxide. The product resonances in these spectra were in similar positions to those reported for the product formed on reaction of TND with methoxide ion.13 lH N.m.r. spectroscopy showed that the reactions did not go to completion when methanol was present in the solvent system. Thus, the lH n.m.r. spectrum of a reaction system containing TND and three equivalents of triethylamine in a 3 1 mixture of W,dimethyl sulphoxide-2H,methanol exhibited peaks due to both TND and TND-.Addition of further 2H,methanol to the system shifted the equili- brium position as evidenced by an increase in the relative intensity of the peaks due to TND and a concomitant decrease in intensity of the peaks due to TND-. That is, the equilibrium constant for the system shown in the Scheme decreases as the methanol content of the solvent increases. Visible absorption spectra of mixtures of TND and either di- or tri-ethylamine are typified by that shown in Figure 1. Since neither the reactants or the BH+species absorb in the visible region, the band with A,,,,. at 560 nm can be assigned to the TND- anion.The values obtained for the molar decadic absorptivity, E, of this species at 560 nm by reacting TND with excess of diethylamine and substituting the final absorbance (Abs) into the expression (Abs) = ETND,Z are 1970, 1960, and 1940 m2 mol-l at 303.5, 314.0, and 324.5 K, respectively. There is good agreement between E for the charge-transfer complex formed between hexa- these vaIues and the respective values of 2 000, 1 970, and methylbenzene and tetracyanoethylene.20 1 940 m2 mol-l, determined at the same temperatures, from Kinetics.-Kinetic results were obtained by monitoring the change with time of the absorbance due to the TND- anion at 560 nm. Kinetic analyses were then carried out using the appropriate equations given in the Experimental section.In the majority of cases the initial concentration of base was in excess, usually between 100 and 1 600 times the initial concentration of TND, so that pseudo-first-order kinetics applied to the forward reaction and second-order kinetics to the reverse reaction of the Scheme. For this situation, the pseudo-first-order rate constant, hobs, for the forward reaction was evaluated using either equations (1) or (2). The values of hobs for the reaction of TND with diethylamine and with triethylamine in different solvent systems are listed in Tables 1 and 2 respectively. The second-order rate constant, K,, for the forward reaction is related to hobs by the expression hobs = K,Base, 1 I 1 and the values for k, listed in Tables 1 and 2 were obtained 350 450 550 650 from the slope of the straight line plots of hobs veYsus Base.hlnm Values for the equilibrium constant, K for the reaction of FIGURE Absorption spectrum of a solution containing TND TND with base under various conditions are also listed in 1 (6.8 x 10-5~)and 800 equiv. of diethylamine in DMSO-MeOH Tables 1 and 2. These values were derived using equations (2 : 1). Cell path length I = 10 mm (3) or (4),or, in the case of solvent containing 67 DMSO, Benesi-Hildebrand type plots on the results of the reaction from the ratio of K,/h-l. Typical plots according to equ- of TND with an excess of triethylamine. A point of note is ation (4) are shown in Figure 2. The values for the second- that the values for E decrease with increase in temperature, order reverse rate constant, K-,, given in Tables 1and 2 were a trend which has previously been reported for the values of derived in most cases from the relationship k-, = k,/K.In TABLE1 Kinetic data for reaction of TND with diethylamine yoDMSO t/"C by vol. in solvent 1O5TND/M lo3HNEt, /M 103kobs/s0 kl/dm3mol-l s? k-l/dm3 molP s-l K 21.4 100 5.11 5.11 1.93 0.37 0.45 0.82 a 5.11 10.22 3.81 5.11 15.30 5.62 30.5 100 5.11 5.11 3.01 0.59 0.89 0.65 5.11 5.11 3.12 5.11 5.11 3.13 5.11 10.22 5.93 5.11 15.30 9.33 41.4 100 5.11 5.11 5.38 0.97 1.67 0.59 a 5.11 5.11 5.50 5.11 10.22 10.30 5.11 10.22 10.23 5.11 15.30 15.10 5.11 15.30 14.50 31.5 67 6.82 6.82 0.792 0.112 0.86 0.130 6.81 13.60 1.44 6.80 34.00 4.00 6.78 54.30 5.99 41.0 67 6.82 3.41 0.588 0.210 2.35 0.089 6.82 6.82 1.16 6.81 13.60 2.89 6.78 54.30 11.00 51.5 67 3.41 3.41 1.14 0.36 5.99 b 0.060 6.81 13.60 4.83 6.80 34.00 12.26 30.5 50 5.11 10.22 0.567 0.06 0.50 0.12 5.11 15.30 0.882 5.09 50.90 3.02 5.09 50.90 3.10 41.0 50 5.11 5.11 0.405 0.088 3.14 0.028 5.11 10.20 0.784 5.11 15.30 1.26 5.10 30.60 2.66 5.09 50.90 4.11 52.3 5u 5.11 10.22 1.97 0.213 10.00 0.021 5.11 15.30 2.98 5.09 50.90 10.82 a Value determined using low base concentrations.b Value determined using equation (5).Value determined from k,jk-l. J.C.S. Perkin I1 TABLE2 Kinetic data for the reaction of TND with triethylamine DMSO t/"C by vol. in solvent 105TND/M 103NEt3/M 103Fz,amp;-1 k1/dm3 mol-' s-l kl/dm3 mol-1 s-1 K 30.0 100 5.11 5.11 0.441 0.081 0.64 0.126 5.11 10.22 0.862 5.09 40.07 3.34 5.06 80.93 6.53 40.8 100 5.11 5.11 0.854 0.133 1.13 0.118 5.11 10.22 1.62 5.11 10.22 1.63 5.10 20.40 3.05 5.09 40.70 6.12 5.06 80.93 10.20 5.06 80.93 11.05 51.3 100 5.11 2.56 0.684 0.268 3.01 0.089 5.11 3.83 1.08 5.11 5.11 1.41 5.11 10.22 2.82 5.11 10.22 2.80 5.10 25.50 6.82 60.3 100 5.11 2.56 1.19 0.449 4.93 0.091 5.11 5.11 2.18 5.11 5.11 2.41 5.11 10.22 4.59 31.3 90 4.60 4.60 0.275 0.068 0.51 0.134 4.60 9.20 0.621 4.60 10.50 0.782 4.59 23.00 1.59 4.58 36.70 2.42 4.57 54.80 3.77 4.56 72.90 4.96 41.0 90 4.60 4.60 0.507 0.113 1.40 0.081 4.60 11.49 1.28 4.58 36.70 4.14 52.0 90 4.60 4.60 0.877 0.201 4.18 0.048 4.60 11.49 2.41 4.58 36.70 7.91 4.56 72.90 14.40 31.6 80 5.11 5.11 0.241 0.049 0.702 0.070 5.11 10.22 0.453 5.10 20.40 1.09 5.09 40.70 2.07 5.07 60.90 3.02 5.06 81.00 3.94 31.5 67 6.82 5.11 0.176 0.038 2.28 0.0167 6.81 6.81 0.201 6.81 10.22 0.273 6.79 27.20 0.827 6.78 40.70 1.52 6.77 54.20 2.07 41.0 67 6.82 5.11 0.216 0.070 6.84 a 0.0079 6.81 10.22 0.497 5.10 20.40 1.09 5.07 60.90 3.28 51.5 67 6.82 5.11 0.383 0.117 17.8 0.0066 6.81 10.22 0.815 6.79 27.20 2.47 5.09 40.70 3.68 5.07 60.90 5.62 60.3 67 6.82 6.82 32.3 a 32.0 50 5.11 10.22 0.182 0.021 4.88 0.0043 5.11 15.30 0.300 5.10 20.40 0.455 5.09 30.60 0.611 41.0 50 5.08 5.11 50.80 10.22 1.06 0.217 0.027 15.9 0.0017 5.09 30.60 0.868 5.08 50.80 1.33 52.3 50 5.11 10.22 0.635 0.070 25.9 0.0027 5.09 30.60 2.17 5.68 50.80 3.50 a Value determined using equation (5).b Value determined from k,/Ll. those systems where the solvent contained 67 DMSO, values for k1were determined using equation (5). Arrhenius plots for the change with temperature of Iz, and FIGURE Benesi-Hildebrand plots according to equation (4)2 for the reaction of TND with (a) tricthylamine in DMSO-MeOH (9 : 1) at 304.3 I, and (b) diethylamine in DMSO-MeOH (1 : 1) at 303.5 K K-, in different solvent tnixtures gave good straight lines. Values for the enthalpy of activation, AHI, and entropy of activation, AS, calculated from such plots are listed in DISCUSSION Tables 1 and 2 show that the rate constants, k,, euro;or the forward reaction increase as the percentage of DMSO in the solvent system increases, whilst Figure 3 shows that a reasonably linear relationship is observed when log k, is plotted against yoDMSO.The rate constants, k-1, for the reverse reaction decrease as the percentage of DMSO is increased although the results for the composition range 90-100yo DMSO are not entirely consistent with this behaviour.Generally in the formation of Meisenheimer complexes from lyate ions and nitroaromatic compounds in DMSO- ROH solvent mixtures, the increase in the forward rate constant as the amount of DMSO is increased is attri- buted to the enhanced nucleophilicity of the lyate ion which results from the lower solvation of the ion as the percentage of DMSO increase^,^?^.^ whilst the decrease in the reverse rate constant is attributed to the enhanced solvation of the Meisenheimer complex in solvents containing a high percentage of DMS0.5 Normally when anions react with aromatic molecules in Meisenheimer complex formation or in nucleophilic displacement reactions, AH$ decreases and AS often increases as the percentage of DMSO in the solvent increases.2122 How- ever the opposite trend in AH: was observed for the reaction between hydroxide ion and 2,4-dinitrophenyl 3-nit rophenyl ether .23 In the present study of hydrogen abstraction by a neutral base from a nitroaromatic compound, the in- TABLE 3 Thermodynamic parameters for the reaction of TND with di- and tri-ethylamine in DMSO-MeOH at 298 K Base .DMSO AH,$/kJ mol-' ASltl J K-' mol-'AH-iI/ AH/kJ mol-l kJ mol-l AS-,t/J K-1 mol-l AS1J K-l mol-l. Et,NH 100 67 34.9 f-3.0 45.6 f3.0 48.4 f4.0 -13.5 f5.0 -135 f6 76.0 f3.5 -30.4 f4.6 -113 f6 -87 f8 3f7 -48 f10 -117 f9 50 45.2 4.5 -120 amp; 10 Et,N 90 67 100 45.9 f3.0 40.4 4.0 43.7 f3.0 56.4 amp; 4.0 -10.5 f5.0 -115 f6 82.3 f4.0 -41.9 f5.7 -135 f8 81.0 + 3.5 -37.3 f4.6 -129 f6 -63 amp; 8 12 f8 28 f7 -52 f10 -147 f11 -157 f9 50 47.0 f4.5 -124 f10 The standard state in AS calculations taken as 1 mol dm-s.Table 3. Also listed in certain cases are the calculated values for the overall enthalpy change, AH, and the overall entropy change AS, for the two reaction systems. Kinetic data for the reaction of TND with triethylamine in the presence of triethylammonium chloride are given in Table 4. TABLE4 Kinetic data for the reaction of TND (5.12 x 10-5~) with triethylamine in DMSO at 313.4 K in the presence of triethylammonium chloride (5.12 x 10-3~) 1O2Et3N/~1Oqkobs/s-l k, a/dm3 niol-' k-l/dni3 mol-l s-l 0.512 0.661 0.129 0.076,b 0.067 1.024 1.350 0.132 8.190 9.910 0.121 Derived from expression kf = kob,/Base.Derived from expression k-l = k,/K where the value of K as determined experimentally using equation (7) is 1.67. Determined by addition of triethylammonium chloride (5.12 x ~O-,M) to a previously equilibriated mixture of TND (5.12 x lOP5~)and triethylamine (1.02 x lW3~) followed by analysis usingequation (9). crease in k, with increase in the DMSO content of the solvent for the reaction of TND with triethylamine appears to be due to an increase in AS,$, as indicated by the trend in AHl$ values (see Table 3). However, for the reaction with diethylamine the corresponding in- crease in k, appears to be due to a decrease in AH,'..For both reactions the decrease in k-, as the percentage of DMSO increases is due to a large decrease in AS-,$ compensating for a decrease in AH-,$. An interesting feature of the results given in Table 3 is that for the reaction of TND with both di- and tri- ethylamine the overall enthalpy change, AH, is more exothermic in mixed solvent than in 100yoDMSO. This can be explained on the basis that there is a greater degree of solvation of products in the mixed solvent than in lOOyo DMSO, with the greater solvation arising because of hydrogen bonding between methanol in the mixed solvent and nitro-groups carrying a partial negative charge in the product specie^.^^^^^ Thus, the greater release of energy arises because hydrogen bonding stabilises the system relative to the situation where hydrogen bonding does not occur.The fact that the overall entropy change, AS, for the two reactions are much more negative in mixed solvent than in lOOyo -2.0 N 4 c 0 0 0 -1.53 -1 .o II I I I I I 50 60 70 80 90 100 'lo OMSO FIGURE Plots of log k, against DMSO in solvent for re- 3 action of TND with (a) diethylamine at 303.5 (*) and 314 K(m),and (b)triethylamineat 304.5 (O), and 325 I (A)314 (a), DMSO is consistent with the foregoing in that there will be a lesser degree of freedom for product molecules and a more ordered system produced in the mixed solvent. It is noteworthy that in 67 DMSO the overall entropy change is less negative for the reaction with diethylamine than for that with triethylamine. The most likely explanation is that in the former reaction system hydrogen bonding occurs between the amine hydrogen of diethylamine and the methanol in the mixed solvent 25 and, as a consequence, diethylamine is solvated to a greater extent than triethylamine.This would lead to a smaller decrease in entropy on foriiiing products from the TND-diethylamine system than from the TND-triet hylamine system. The large negative values for the entropy of activation, ASI, for the forward step in the reaction of TND with both of the amines in the different solvent systems indicates that the activated complex is more highly ordered than the reactants.It is of interest that the values for the entropy of activation given in Table 3 are similar to the value of -134 J K-l mol-l determined for the reaction of n-butylamine with ethyl 2,4-dinitro-1- naphthyl ether in DMSO to form a zwitterionic Meisen- heimer complex by addition at C-1.18 The value of ASII for the reaction of TND with diethyl- amine in 100 DMSO is more negative than the cor- responding value for the reaction with triethylamine. Since solvation of the reactants in the two systems will be similar in lOOyoDMSO, this result implies that. there J.C.S. Perkin I1 is a greater restriction imposed on the activated complex in the former situation. This could arise by the form- ation of the complex (X) which contains an eight-membered ring in which bonding occurs to the hydrogen atom of diethylamine.This conclusion supports the postulate previously put forward of involvement of an eight-membered cyclic transition state in hydrogen abstraction from 2,4,6-trinitrotoluene by sodium iso- propoxide.26 The entropy of activation, AS-lx, for the reverse reaction has positive values for the reactions carried out in mixed solvent and negative values for the reactions carried out in lOOyo DMSO (see Table 4). This is consistent with the expected large increase in entropy when during the course of the reverse reaction in mixed solvent, the activated complex is formed from the two highly solvated charged species TND- and BH+. Comparison of the results presented in Table 4 for the reaction of TND with triethylamine in DMSO at 313.4 K in the presence of triethylammonium chloride with the corresponding results at 314.0 K in the absence of tri- ethylammonium chloride (Table 2) shows that the effect of the added salt is to decrease the value of the reverse rate constant, K-l, from 1.130 to 0.076 dm3 mol-l s-l and to increase the value of the equilibrium constant from 0.12 to 1.67.These results parallel the observations of previous workers6 who noted that in the formation Et *. H' 'HI (X) of a Meisenheimer complex by addition of methoxide ion to substituted dinitro-anisoles the value of K increased markedly with increase in concentration of sodium methoxide owing to an apparent decrease in K-, values.Both sets of results are explainable in terms of the charged product, benzyl-type ion or Meisenheimer complex, being stabilised by ion association. EXPERIMENTAL 1,5-Diniethyl-2,4,8-trinitronaphthalene(TND) was pre-pared and purified as previously de~cribed.~' Reagent grade di- and tri-ethylamine were distilled using a Fischer Spaltrohr 60 theoretical plate column, the middle fractions being selected. Dimethyl sulphoxide (reagent grade), 2H,dimethyl sulphoxide, AnalaR methanol, and 2Hp-methanol were dried over molecular sieve prior to use. Triethylammoniuni chloride was prepared by addition of concentrated hydrochloric acid to stirred triethylamine at 0 "C ; the crystals formed on neutralisation were filtered off, twice recrystallised from absolute ethanol, and dried in vacuo.90-MHz lH Continuous-wave n.m.r. spectra were recorded at 31 "C on a Perkin-Elmer R32 spectrometer, whilst 90-MHz lH Fourier-transform n.m.r. spectra were recorded on the same instrument fitted with a Nicolet TT4 Fourier transform unit. 220-MHz lH continuous-wave n.m.r. spectra were recorded at PCMU, Harwell. Chemical shifts were measured relative to internal tetramethylsilane as reference. Dissolution of TND (0.021 g) in 2HH,dime- thy1 sulphoxide (0.5 cm3) followed by addition of triethyl- amine (0.01 cm3) and recording of the 220 MHz spectrum within 5 min gave the following spectrum for the benzyl type anion: z 1.35 (1 H, s), 2.54 and 2.72 (2 H, ABq, JA~ 8 Hz), 4.09 (1 H, s), 5.12 (1 H, s), and 7.86 (3 H, s).U.v.-visible absorption spectra were recorded on a Perkin-Elmer SP 1700 spectrophotometer using 10 mm matched silica cells in a thermostatted cell compartment maintained at fO.l "C. Reaction mixtures were prepared by mixing thermostatted solutions of TND with the appro- priate amine in a cell capped with a PTFE stopper. To check that evaporation of the amine did not occur, the PTFE stopper was replaced by a silica-seal rubber cap in some runs and the amine injected through the cap by syringe. Identi-cal results were obtained by each method. Reversal of the reaction was achieved by letting the reaction reach the equilibrium position, removing one-third of the solution from the cell, replacing with the same volume of methanol at the same temperature, the cell being rapidly shaken and replaced in the spectrophotometer.Reversal oi the re-action was also achieved by adding 0.5 cm3 of triethyl- ammonium chloride (3.07 x 10-2~)in DMSO to 2.5 cm3 of an equilibrated mixture of TND (5.12 x 10-5~)and tri- ethylamine (1.023 x lOP3~)in DMSO. Treatment of Experimental Data.-In cases where pseudo- first-order kinetics applied to the forward reaction and second-order kinetics to the reverse reaction of the Scheme values for the pseudo-first-order rate constant, kobsr were obtained from plots according to equation (1) where C, represents the initial concentration of TND, and C- ancl Ce-represent the respective concentrations of TND-at time t and at equilibrium.The reaction between TND and excess of diethylamine in DMSO proceeded virtually to completion and for this system equation (1) could be used in the simplified form (2). -ln(C,--C-) = tiohst + constant (2) The concentration of TND- during a run was determined by monitoring the absorbance, (Abs), at 560 nin and sub- stituting the absorbance values into the Beer-Lambert relationship C-= (Abs)/J. The molar absorptivity, E, of the TND- anion was determined from Benesi-Hildebrand type plots, according to either equations (3) or (4), with the latter equation being used in the majority of cases. The equilibrium constant, K, was also determined from these plots. (3) Base C,Based 1 (4)(Abs), (Abs)e2 + JK Since the reaction of TND with excess of diethylamine in DMSO proceeded to virtual completion under the conditions used for kinetic runs, the molar a.bsorptivity, E, could be obtained directly from the expression C, = (Abs),/d.How-ever the equilibrium constant, K, had to be determined using lower base concentrations than those used in the kinetic runs. Typically a ratio of Base/Co of ca. 10 used, and the values of K were obtained by substitution of the measured value of (Abs), into equation (3) or (4). In a number of runs, reaction mixtures in DMSO were allowed to reach equilibrium and sufficient methanol was then added to give a final solvent composition containing 67 DMSO. Addition of the methanol resulted in a reversal of the forward reaction of the Scheme, and the resultant decrease in absorbance at 560 nni was followed with time.The kinetic expression applicable in these cases is given in equation (5) where Cn is the concentration Ce-(2C, -ce-)k-lt (5) of TND in the original DMSO solvent multiplied by a dilution factor of 2/3, C,- and C- are the respective con- centrations of TND-immediately after the addition of methanol and after time t and C,- is the concentration of TND-at the final equilibrium position. Values of h-, were determined by plotting the left-hand side of equation (5) against t. The reaction of TND with excess triethylamine in DMSO in the presence of triethylammonium chloride was investi- gated in a number of runs and was found to obey pseudo- first-order forward and first-order reverse kinetics.Plots based on equation (6) were used to derive values for the pseudo-first-order rate constant from which values for the rate constant Iz, were determined using the expression A,,,,, = k,Rase. The values of E ancl K for these systems were determined from Benesi-Hildebrand type plots according to equxttion (7) where BH+ represents the con- (7) centration of triethylammmium chloride. Addition of triethylammonium chloride to a previously equilibrated mixture of TND and triethylamine in DMSO causes reverssl of the forward reaction of the Scheme and a consequent decrease in the absorbance at 560 nni due to the TND-anion. The kinetic espression (8) applies to the approach to the new equilibrium position where C, is the -In c,-'---c,-= BH+l -1 ce-I,,, (8)c, -c,-initial concentration of TND corrected fur dilution caused by addition of the solution of triethylammonium chloride in DMSO, C,-is the concentration of TND- at the first equilibrium position, and C- and C,-are the respective concentrations of TND-at time t after the addition of triethylammonium chloride and at the final equilibrium position.The second-order rate constant, kl,for the reverse reaction was determined by monitoring the decrease in absorbance at 560 nm with time after the addition of 138 triethylammonium chloride and plotting the results according to the following modified form of equation (8) -ln(C -CJ = aklt + b (9) where a and b are constants.We thank Dr. B. C. Webb, Rocket Propulsion Establish- ment, Ministry of Defence, for helpful discussions. 9/098 Received, 22nd January, 19791 REFERENCES R. Foster and C. A. Fyfe, Rev. 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