INTRODUCTION
The simple Grunwald-Winstein equation (1) has been applied to a mechanistic study involving the kinetics of solvolyses reaction in a wide range of solvents. In this equation (1) the sensitivity (m) to change in one parameter, the solvent ionizing power (YX) and it can be determined by studying the rates of solvolyses of a standard substrate (m = 1).1 However, Fainberg and Winstein found
log (k/ko) = mYx + c (1)
that the plots for some substrates showed a scattering of plots in binary solvent systems due to the solvent nucleophilicity.2 Nucleophilic solvent assistance can be defined as electron donation from solvent to the developing positive dipole of a reacting C-X bond, and electrophilic solvent assistance can be defined as electron acceptance by the solvent from the leaving group, Scheme 1.
Scheme 1. Nucleophilic and electrophilic assistances.
A powerful tool in the consideration of the reaction mechanism for solvolyses reactions involves the use of the Grunwald-Winstein equation, especially in its extended Grunwald-Winstein equation expressed equation (2).3 In equation (2), k and ko represent the rate constants of solvolysis in a given solvent and in the standard solvent (80% ethanol), l represents the sensitivity to changes in the solvent nucleophilicity values (NT),3 m represents the sensitivity to changes in the solvent ionizing power for a chloride-ion leaving group (YCl),3 and c represents constant (residual) quantity.
log (k/ko) = lNT + mYCl + c (2)
The nucleophilic substitution reactions of benzenesulfonyl chloride (2) have been extensively studied.4 In this reaction, the chloride (Cl) atom bonded to the sulfur (S) atom of the sulfonyl group is displaced by the nucleophile. It has been described5 as controversial and indeed, both SN16 and SN27 mechanism have been proposed. In SN2 mechanism, the nucleophile approaches and by donation of its electron pair begins to form a bond to sulfur while the sulfur-leaving group (Cl) bond is breaking (Scheme 2). The reaction is completed in a single step and both nucleophile and substrate take part in the transition state. Thus, the rate is second-order.7
Scheme 2. A limiting SN2 mechanism.
On the other hand, a limited SN1 mechanism was the solvolysis of benzenesulfonyl chloride (2) where substrate can be ionized without nucleophilic assistance, Scheme 3.
Scheme 3. A limiting SN1 mechanism.
The purpose of this study is to gain further understanding of the mechanism of sulfonyl transfer, we carried out a kinetic investigation of the solvolyses of 4-(acetylamino)-1-naphthalenesulfonyl chloride (C12H10ClNO3S, 1) in a variety of pure and binary solvents at 35.0℃, Scheme 4. The reaction mechanism was investigated by applying the rate constant value to the extended Grunwald-Winstein equation (2). Solvent kinetic isotope effect (SKIE) was discussed to obtain additional information about the reaction mechanism.
Scheme 4.
RESULTS AND DISCUSSION
The rate constants of solvolyses of 1 have been determined in the twenty-four kinds of solvents at 35.0℃. The solvents consisted of methanol and binary mixtures of water with ethanol, methanol, acetone, and 2,2,2-trifluroethanol (TFE). Both ethanol and water have high nucleophilicity and acidity, but water (ε = 80 at 25.0℃) has a greater ionizing power than ethanol (ε = 24.3 at 25.0℃).4,8 Therefore, as the ethanol content increases in the ethanol-water mixed solvent, the nucleophilicity remains almost unchanged, the ionization power greatly decreases. The methanol-water mixed solvent also shows a similar tendency as the ethanol-water mixed solvent. In contrast, both water and TFE (ε = 26.7 at 25.0℃) have large ionizing power, but water has a much greater nucleophilicity than TFE.4,8 Therefore, an increase in TFE in a TFE-water mixed solvent does not significantly affect the ionization power, but greatly reduces nucleophilicity, and as the content of water with strong nucleophilicity increases, the reaction rate increases shown in Fig. 1. These results can be explained that the solvolyses of 1 is dominated by a bimolecular reaction mechanism (SN2), Table 1.
Figure 1. Plot of NT against YCl in 10 solvents.
Table 1. Rate constants for the solvolyses of 4-(acetylamino)-1-naphthalenesulfonyl chloride (1)(a) in a variety of pure and mixed solvents at 35.0℃, and the NT and the YCl values for the solvents
(a)Unless otherwise indicated, a 10-3mol solution of the substrate in the indicated solvent, containing 0.1% CH3CN.
(b)On a volume-volume content at 25.0℃, and the other component is water.
(c)Values from ref. 9.
(d)Values of k [= 6.40(±0.04)×10-5s-1] in deuterated methanol (CH3OD), corresponding to kCH3OH/kCH3OD value of 1.75±0.03 (with associated standard error).10
(e)Solvent prepared on a weight-weight basis at 25.0℃, and the other component is water.
The simple Grunwald-Winstein equation, which represents the correlation between reaction rate and ionization power of the solvent, was applied in this study. As a result, the correlation coefficient of 1 was very dispersed. Therefore, rates were applied to the extend-Grunwald-Winstein equation to analyze the nucleophilicity and ionization power of the solvents. We have found that eqn. 2 can be applied successfully (R = 0.941) to the rate constants of solvolyses of 1 over the full range of solvents commonly employed in this type of study. The l value was 0.76±0.07 and the m value was 0.37±0.03, the standard error of the estimate was 0.03, Fig. 2.
Figure 2. Plot of log(k/ko) for 4-(acetylamino)-1-naphthalenesulfonyl chloride (1) in 24 solvents at 35.0℃ against (0.76NT + 0.37YCl).
The sensitivity values, l and m, are reported in Table 2, along with the corresponding parameters obtained in the analyses of previously studied substrates, where they can be compared with literature values for related substrates.
Table 2. Extended Grunwald-Winstein equation correlations of the kinetics of solvolytic displacement of chloride
(a)Number of solvents. (b)Correlation coefficient. (c)This work
When the solvolyses reaction proceeds as an SN1 reaction, the l value is ≈ 0 and the m value is ≈ 1, whereas in the SN2 reaction, the l value is ≈ 1 and the m value is ≈ 0.5.14 As a result, the solvolysis reaction of 1 can be expected to proceed through the SN2 mechanism. Since the l value is greater than the m value, it can be expected that bond formation proceeds more than bond destruction in the transition state.14
We compare l value (=0.76) for the solvolyses of 1 with reported of solvolyses of 9-fluorenyl chloroformate (l = 0.95),13 dimethoxybenzenesulfonyl chloride (l = 0.93),9f N,N-dimethyl sulfamoyl chloride (l = 0.92)12 and benzenesulfonyl chloride (l = 1.10)12 which are believed to be normal SN2 mechanism in Table 2. The l value of 0.76 for the solvolyses of 1 is a smaller than proceed through a SN2 mechanism (l = 0.92~1.10).14 This l value (l = 0.76) is similar to those previously reported for the solvolyses of benzesulfonyl chloride (2) as proposed SN2 mechanism with some SN1 reaction pathway. Therefore, this similarity suggest that the solvolyses of 1 is an SN2 mechanism involving an attack by solvent at sulfur atom in substrate with a little of the character of SN1 mechanism.12
The l/m values from the extended Grunwald-Wainstein equation could be a useful mechanistic criteria, l/m values of 1.4 to 2.5 for a SN2 mechanism. On the other hand, if the value of l/m is less than 1, it is known to proceed as an SN1 mechanism, Table 2.9 For solvolyses of 1, the l/m value was 2.1 which is consistent with the proposed SN2 mechanism. The value of l (0.76) is greater than the value of m (0.37), it is indicated to proceed by a bimolecular pathway, reflecting nucleophilic assistance from a solvent nucleophile.9
Kinetic solvent isotope effects (KSIEs) were obtained, and reaction rate measurements for solvents 100% CH3OH and 100% CH3OD were performed under the same experimental conditions. The measured reaction rate constant values were 11.2×10-5s-1 for CH3OH and 6.40× 10-5 s-1 for CH3OD. As a result, the reaction rate ratio was 1.75. KISEs were used to predict the reaction mechanism.15 Generally, in the reaction via the SN1 reaction mechanism, KISE is 1, and in the case of the SN2 reaction mechanism in which the solvent acts as a general base catalyst, the KISE value is greater than 2.16 According to previous studies, the solvolysis reaction of phenyl chloroformate proceeds through a typical bimolecular reaction route and the solvent isotope effect is 2.4.17 Also, for chlorodiphenylethane, which proceeds through a typical single molecule reaction path, it was 1.1.18 The lower KSIE value (1.75) for 1 than normal SN2 mechanism is as expected for the dissociative SN2 mechanism with some SN1 mechanism.
EXPERIMENTAL
Ethanol, methanol, acetone and 2,2,2-trifluroethanol (TFE). Solvents were GR grade. Water was used after the third distillation. The substrate, 4-(acetylamino)-1-naphthalenesulfonyl chloride (1, 98%), was used as received. The substrate did not react with the pure acetonitrile within the stock solution. The rate constants (kobs) of solvolyses of 1 was measured by the electrical conductivity method by using the primary proportion of the increase in electrical conductivity due to the change in the concentration of hydrogen ions and halides produced during the reaction in proportion to the change in the concentration of the product and the rate constants were obtained by the curve fitting method.2 The reaction proceeds to a pseudo-first order reaction in which the concentration of the substrate is about 10-3 mol, and the concentration of the nucleophile is much greater than the concentration of the substrate. The electrical conductivity device was made and used as an analog-to-digital changer (A/D converter) using an extended computer connection. A platinum conductivity cell was manufactured and used, and the capacity of the conductivity cell was 5 mL. The uncertainty in the kobs values was estimated to be less than ±3% from replicate runs.
CONCLUSION
The rate constant values for the solvolysis reaction of 1 in pure and various two-component mixed solvents showed that the rate constant value increased as the water content increased in both nucleophilic and electrophilic solvents. The solvolysis reaction rate constant values of 1 showed a very severe dispersion phenomenon when applied to the Grunwald-Winstein equation, whereas when applied to the extended Grunwald-Winstein equation, they showed a relatively good correlation. The solvolysis rate constants of 1 are well correlated with the extended Grunwald-Winstein equation, using the NT solvent nucleophilicity scale and the YCl solvent ionizing power scale, with sensitivity values of 0.76 and 0.37 for l and m, respectively and the kinetic solvent isotope effect was 1.75. Based on these results, we suggest that the solvolysis of 1 has dissociative SN2 mechanism with a little of the character of SN1 reaction. The l/m values from the extended Grunwald-Winstein equation could be a useful mechanistic criteria. For solvolysis of 1, the l/m value was 2.1 which is consistent with the proposed SN2 mechanism.
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