Electrophilic aromatic substitution. Part 21. Rate constants for formation of nitronium ion in aqueous sulphuric, perchloric, and methanesulphonic acids

摘要

1979 133 Electrophilic Aromatic Substitution. Part 21.l Rate Constants for Form- ation of Nitronium Ion in Aqueous Sulphuric, Perchloric, and Methane- sulphonic Acids By Roy B. Moodie.' Kenneth Schofield, and Peter G. Taylor, Department of Chemistry, The University, Exeter EX4 4QD Studies of the kinetics of nitration of anisole in 66-80 H2S04,64-70 HCIO,, and 88-97 CH3S03H, and of toluene in 74-79 H,S04 are reported. Concentrations of the aromatic compound were high enough for the re- action to be less than first-order in them. Rate constants for formation of the nitronium ion are reported, and rate constants for the reverse reaction estimated. It is suggested that the reaction is a one-step process. ITis well known from studies of the kinetics of nitration of aromatic compounds in various media,2 that the order of the reaction in the aromatic compound can change from unity to zero both with an increasing concentration and reactivity of it, and decreasing concentration of water.This is rationalised by the Scheme (in which k HNO, NO,+ k -1 kr AR + NO,+ --jproducts SC-IEME solvent species are not included) and the rate equation (1) derived using the steady-state approximation with respect to the concentration of nitronium ion (AR = aromatic substrate). -dHNO,I =*I = ~,krHN0,1ARl (1)dt dt k-, + krARI This paper reports the measurement of k,, the rate constant for formation of nitronium ion from solute nitric acid, in aqueous sulphuric, perchloric, and methane- sulphonic acids.Values of k, for aqueous sulphuric acid (74.7--81.5) have previously been reported by Chapman and Stra~han.~ Our results are not in quanti- tative agreement with theirs. With sufficiently low concentrations of aromatic substrate the reaction is first order in both nitric acid and aromatic substrate see equation (l). This is the situation which normally obtains in these media, even with aromatic compounds sufficiently reactive to react upon encounter with the nitronium ion,4 because with most such substrates low solubilities make low concen- trations unavoidable. Under such circumstances only k, equation (2) can be determined. It is convenient to combine equations (1) and (2) to give (3). --dHNO, --dAR klk2HNOJAR (3)dt --= dt k, + k,ARI Provided that the concentration of the aromatic substrate can be made high enough for k,AR not to be negligible compared to k, in the denominator on the right of equation (3), k, can be determined. The substrate we chose for this study was anisole, because its reactivity5 and solubility are sufficiently high for the purpose.Toluene, the substrate used by Chapman and Stra~han,~ was also used for the measurement of k, in aqueous sulphuric acid. Products from the nitration of both substrates have previously been rep~rted.~?~ EXPERIMENTAL Materials.-Sulphuric acid (98) and perchloric acid (72) were AnalaR reagents. Concentrations of diluted acids were determined as previously described.4* Com-mercial methanesulphonic acid (99) was fractionally distilled under reduced pressure.A sample (500 cm3) typically gave a forerun (20 cm3) after which material was collected for use (250 cm3, b.p. 140 "C at 0.15 mmHg). Titrimetric assay with standard base of some acid prepared in this way gave a purity of 99.8. For dilution use was made of inverted U tubes, into the arms of which the required amounts of acid and water respectively were weighed; the apparatus was stoppered and cooled before the contents were mixed. That consistent concentrations were achieved by this method was checked by measuring the rate constant for nitration of toluene dissolved in each sample. Nitric acid was purified as previously described.8 Anisole and toluene were purified by distillation. Solubilities.-Excess of aromatic substrate was vigorously stirred with aqueous sulphuric acid for 15 min at 25 "C.The acid layer was separated and centrifuged for 5 min. A sample was taken from the middle of the solution and the concentration of the aromatic substrate determined by comparison of the absorbance after appropriate dilution with the asorbance-concentration plot determined pre- viously. Kinetics.-For the faster reactions a Nortech SF 3A stopped-flow spectrophotometer combined with a Datalab DL If01 transient recorder was employed to monitor the variations of transmittance with time. For the slower reactions a Pye-Unicam SP 1800 spectrophotometer was used, as previously de~cribed.~ RESULTS Solubilities of Anisole and Toluene in Aqueous Sulphuric Acid.-These results are given in Table 1.Values for the more concentrated acid are less accurate because of concurrent sulphonation, although the extent of this was small 10 during the time taken to make the measurement. The solubility of toluene at the lower acidity is in appro- ximate agreement with literature data.l09 l1 However, we were not able to achieve reproducible results in the kinetic studies when the concentration of toluene was, as in the previous work,, at or close to this solubility limit. It is our view that true homogeneity in such ' solutions ' is IP 500I-/ 1 I I I I 2000 GO00 6000 8000 1I IARI FIGURE Plot of l/kobs.us. -1/AR for the nitration of anisole1 in 69.7 H,SO,.(For explanation of error bar see footnote b of Table 2) difficult or impossible to achieve. Mechanical shaking of toluene, enough to make a solution of concentration TABLE1 Solubilities in aqueous sulphuric acid at 25 "C Solubility/mol HW4 d m-3 Anisole 73.5 0.039 77.5 0.07 a Toluene 73.5 0.0028 77.5 0.004 a a Approximate figures : some sulphonation of the aromatic during equilibration being unavoidable. See text concerning the significance of this figure. 2 x 10-3 mol dm-,, with 78 H,SO,, for 15 min led to a turbid solution. Although this turbidity was apparently due to bubbles it was very slow to clear, unlike that in a solution of nitric acid in sulphuric acid shaken in the same way.No difficulties in obtaining reproducible kinetics were encountered with solutions of anisole, or of toluene at substantially lower concentrations, as used in the kinetic studies reported below. Solubilities in aqueous perchloric and methanesulphonic acids were not determined, but solubilities of organic compounds in these media are generally higher than in aqueous sulphuric acid.12 With anisole at the concentrations used we had no doubt about the homogeneity of the solutions. Kinetic Studies.-With neither substrate and in none of the media was it possible to make the concentration of aromatic substrate large enough for the inequality k,AR 9 k, to hold. Accordingly K, was determined by one of the following methods. (a) With AR HNO,, the reaction follows a first-order course with rate constant hobs..Equation (4) follows from equation (3): K, was deduced, either directly from 1 ----1 +-1 (')hob k2AR1 hi J.C.S. Perkin I1 equation (4) using a value of K, determined independently, or from the intercept of a plot of ( l/Kot,s.) 'us. -(l/AR). Such a plot is shown in Figure 1. (b) With HNO, $ AR equation (6) derived by inte-gration of equation (4), was used as previously to deter- mine K, and K,. This method is more accurate than that , of using the initial part of the absorbance-time curve to determine K, and the latter part to determine K,. y is -=t -1 lny ARIO (q 1 -y K,HNO, 1 -y K,HNO, Y = WI/ARJ~ = (A~-~)/(A~-A,) defined as shown and A,, A,, and A are the initial and final absorbances and the absorbance at time t respectively.Values of K, and K, are given in Table 2. Whether method (a) or (b) was used to derive these values is obvious from the relative concentrations of nitric acid and aromatic sub- strate. The ratio k,/K, for anisole represents the concen- tration of anisole which would lead to equal partitioning of the nitroniuni ion between regression to nitric acid and forward reaction to products. This ratio changes from ca. 8 x loe3mol dmP3 in 66 H2S04 to ca. 2 x mol dm-3 in 80 H,S04. DISCUSSION The dependence of log k, upon the acidity function H, is an approximately linear one in each of the aqueous acid systems (Figure 2). The agreement between the values of k, obtained using anisole and those obtained using toluene is seen to be satisfactory, though as ex- plained in the results section the former are probably the more reliable.Values of d(1og k,)/d(-H,) are 1.5 (H,SO,), 1.1 (HClO,), and 1.0 (CH,SO,H). For the ionisation of triarylcarbinols (a rather similar reaction), values of d(1og k,)/d(-H,) are in the same range (1.0 to 1.5).13 In considering the mechanism of the ionisation of nitric acid it is convenient to focus attention on the -3L-I i I I I I -5.5 -6.0 -6.5 -7-0 -7.5 10 2FIGURE Plot of log (kJs0)vs.-the acidity function H,. 0= sulphuric acid, x = perchloric acid, 0= methanesulphonicacid. Full circles relate to the use of toluene as scavenging aromatic, in all other cases anisole was used.(For explanation of error bar see footnote b of Table 2) reverse step, the hydration of the nitronium ion. This may be done by combining present data with observed second-order rate constants 4s7 for the nitration of benzene, k2bz. Equation (6) follows from equation (2). TABLE2 Rate constants k, and k, (see text) in aqueous acids at 25 ldquo;C 103AR 103HN0, kl k2I Acid mol dm-, mol dme3 S-1 mol-l dm3 s-l Toluene in aqueous sulphuric acid 74.5 0.20-1.03 0.047 0.094 118 76.2 0.25-0.67 0.040 0.29 680 77.3 0.74 0.14 0.66 1860 a 0.41 0.077 0.52 1860 a 78.8 0.70 0.12 1.20 6 600 a 0.35 0.065 1.28 6 600 a Anisole in aqueous sulphuric acid 66.4 0.1 6.2 0.30 66.4 6.2 0.I 0.0023 0.30 a 67.2 5.8 0.1 0.0020 0.48 68.3 0.1 6.7 1.11 68.3 6.7 0.1 0.0045 1.11 a 69.2 0.1 6.6 1.78 69.2 5.9 0.1 0.0087 1.78 a 69.7 0.14-1.52 0.026 0.0045-0.0084 3.6 69.8 0.03 0.1P-7.2 3.4 71.6 9.9 0.60 0.026 8.5 a 73.9 9.9 0.48 0.088 63 74.5 9.9 0.60 0.15 100 74.8 1.39 21 0.14 148 74.8 1.39 7.5 0.16 120 74.8 1.39 0.1 0.14 130 a 76.2 1.43 10.7 0.32 560 76.2 1.43 16.4 0.33 650 77.5 1.43 13.5 0.60 1200 78.5 0.04 0.26 3 200 78.5 9.9 0.60 0.78 3 200 a 78.5 5.0 0.60 0.79 3 200 a 78.5 1.48 0.26 0.88 3 200 a 79.9 9.9 0.60 1.77 10 000 Anisole in aqueous perchloric acid 64.0 1.46 14.7 0.022 64.0 1.46 0.147 0.021 43 65.2 1.44 12.5 0.056 65.2 1.44 0.125 0.057 115 67.5 1.58 14.3 0.25 2 560 68.3 1.44 15.0 0.33 68.3 1.44 0,150 0.33 4 100 69.3 1.43 15.4 0.56 69.3 1.43 0.154 0.54 10 500 70.1 1.27 0.155 1.16 9 x 104 Anisole in methonesulphonic acid 88.0 6.6 37 0.0033 0.61 89.5 5.1 23 0.0077 1.53 91.0 2.8 36 0.0109 8.9 93.2 1.40 18.8 0.030 47 97.0 1.38 16.7 0.147 1 350 Interpolated values using present data and those from refs.3 and 4. The intercept of the double-reciprocal plot was subject to considerable error because the reaction was largely first-order in aromatic under the conditions. (See error bar in Figures 1, 2, and 3). Value obtained by short extrapolation of the rate profile. In (6) krbz is the specific rate constant for the reaction of benzene with the nitronium ion.Since this reaction is not diffusion ~ontrolled,~ krbz will be independent of viscosity, and can be expected to vary little with acidity. There is independent evidence that krbz is not strongly medium dependent .14 Variation of kl/k,hz then indicates the medium dependence of kl,the rate constant for the hydration of the nitronium ion. A plot of log (kl/k2brdquo;) DS. -log (activity of water) of the data relating to H2S0, (Figure 3) is linear with a slope of 1.95. Thus it appears that two water molecules are needed to hydrate the nitronium ion, and a satisfactory picture of the reaction is (7), in which BH+ and B are hydronium ion and water respectively. The role of the second water molecule B .L I B-H + O-NO, k.B + HO + NO2 (7)I Ik-1 H H in the hydration step is that of a general-base catalyst. There are several other cases in which attack of water on an electrophilic centre in aqueous solution is thought to be subject to general base catalysis by water or other species.15 -3.0 -2.5 2.0 -h N na-3-m L I 1.5--log oo 3FIGURE Plot of log (kl/k2bZ)/mol dm-3 us. -log (activity of water) for sulphuric acid. (For explanation of error bar see foot-note b of Table 2) An apparent difficulty is that in other media (notably the binary solvent systems HNO,-CH,NOZ and HN0,- CH,CO,H) the formation of nitronium ion has been held to occur in two steps as in (8) and (9).16 The evidence HNO, + HNO, 1-H,NO,+ + NO,-(8) H,NO,+ H,O + NO,+ (9) has been summarised; in particular the effect of added nitrate ion on the rate constants for nitrations which are zeroth order in aromatic (depression of the rate constant without change in order) could thus readily be explained. For these media too, equation (7) with B a solvent species, (HNO,, CH,NO,, or CH,CO,H), provides an alternative and satisfactory explanation of the observ- ations.The effect of nitrate ion is to depress the concen- tration of the protonated solvent molecule BH+. If this is protonated nitric acid, the form with the two protons on different oxygen atoms which cannot be the direct precursor of the nitronium ion as in (9)is implied because it is likely to be predominant.(Carboxylic acids are protonated predominantly on carbonyl oxygen.17) The implication of equation (7) is that the alternative form, with both protons on the same oxygen, is too unstable to exist. The linear dependence of k1 upon the concentration of water in trifluoroacetic acid is accommodated by equation (8) with B being a solvent molecule. In summary, equation (S), with BH+ and B identified as in Table 3, gives a satisfactory and coherent explan- TABLE 3 k 6-H + O-NO,+ B + HO + NO,+ k-1I IH H Nitration medium BH+ B Aqueous H,SO,, HClO,, and CH,S0,H H,O+ HZO Trifluoroacetic acid CF,C(OH),+ CF,CO,H HNO,-CH,COZH (HO),NO+ HNO, or or CH,C( OH),+ CH,CO,H HN03-CH3N02 (HO),NO+ HNO, or or CH,NO,+H CH,NO, ation of all the observations.The reactivity of water relative to that of benzene towards the nitronium ion, deduced previously,g now loses some significance because it depends upon the nature and concentration of B in the solvent in ques- tion. The absolute magnitude of the rate constant K-, can be estimated as follows: benzene reacts with nitronium ion at 1/36th the rate of encounter in 68.3 H,SO,, in which medium the bimolecular rate constant for en-counter is estimated to be4 ca. 6 x los mol-l dm3 s--,. Use of equation (6) and Figure 3 then gives a value for R-, of 2 x lo6 s-l in this medium. By extrapolation, J.C.S. Perkin II values of k; of 5 x lo8 s-l in water, and 2 x lo4 s-l in 80 H,SO, can be deduced.One of us (R. B. M.) thanks the Royal Society for a loan for the purchase of the stopped-flow spectrophotometer. We also thank the S.R.C. for support (P.G. T.). 8/599 Received, 31st March, 19781 REFERENCES Part 20; H. W. Gibbs, R. B. Moodie, and K. Schofield,J.C.S. Perkin 11,in the press. a J. G. Hoggett, R. B. Moodie, J. R. Penton, and K. Schofield, ' Nitration and Aromatic Reactivity,' Cambridge University Press, Cambridge, 197 1. J. W. Chapman and A. N. Strachan, J.C.S. Chem. Comm., 1974, 293. R. G. Coombes, R. B. Moodie, and K. Schofield,J. Chem. SOC. (B),1968, 800. J. W. Barnett, R. B. Moodie, K. Schofield, J. B. Weston, R. G. Coombes, J. G. Golding, and G. D. Tobin, J.C.S. Perkin II, 1977, 248.J. W. Barnett, R. B. Moodie, K. Schofield, and J. B. Weston, J.C.S. Perkin 11,1975, 648. R. B. Moodie, K. Schofield, and P. N. Thomas, J.C.S. Perkin II,1978, 318. J. G. Hoggett, R. B. Moodie, and K. Schofield,J. Chem. SOC. (B),1969, 1. @ R. B. Moodie, K. Schofield, and G. D. Tobin, J.C.S.Perkin 11,1977, 1688. lo ' Mechanistic Aspects in Aromatic Sulfonation and Desulfon- ation,' H. Cerfontain, Interscience, New York, 1968; P. G. Taylor,unpublished work. l1 A. N. Strachan in L. F. Albright, and C. Hanson (eds.) ' Industrial and Laboratory Nitrations,' A. C. S. Symposium Series 22, Washington, D.C., 1976, p. 215. l2 K. Yates and R. A. McClelland, Progr. Phys. Org. Chem., 1974, 11, 323. l3 J. N. Ride and P. A. H. Wyatt, J.C.S. Perkhz 11,1973, 746. 1, J. H. Ridd, Adv. Phys. Org. Chem., in the press. l5 C. D. Ritchie, J. Amer. Chem. Soc., 1972, 94, 3275; J. N. Ride, P. A. H. Wyatt, and 2. M. Zochowski, J.C.S. Perkin II, 1974, 1188; R. B. Moodie, E. M. Ihsan, and P. J. Battye, un-published work on the hydrolysis of l-methoxycarbonylpyridin-ium ions; R. B. Moodie and P. J. Sanson, unpublished work on the hydrolysis of phenyl isocyanate. l6 G. BenfordandC. K. Ingold, J. Chem. Soc., 1938, 929; E. D. Hughes, C. K. Ingold, and R. I. Reed, J. Chem. Soc., 1950, 2400. l7 R. Stewart and K. Yates, J. Amer. Chem. Soc., 1960. 4059.