Nucleophilic constant structures

For nucleophilic displacement reactions, some relationship might be expected between the ability of an anion A to act as nucleophile and the dissociation constant of the acid HA the best nucleophiles should be the anions of the weaker acids. For an uncharged nucleophile B there should be a relationship with the dissociation constant of the conjugate acid BH. Nucleophilicity is often treated in terms of Bronsted-type relationships, but deviations and sometimes almost complete breakdown are found when wide ranges of substrates and nucleophiles are considered. Nucleophilicity is a more complex function of structure than acid-base behavior. This situation has led to the development for such reactions of a special system of correlation analysis, which involves scales of nucleophilicity constants. The first of these, the n scale, was proposed by Swain and Scott in 1953 and was based on methyl bromide as a standard substrate for an LFER. Other scales have subsequently been developed and applied in both simple and multiple regression. 

Hydrolysis of an enamine yields a carbonyl compound and a secondary amine. Only a few rate constants are mentioned in the literature. The rate of hydrolysis of l-(jS-styryl)piperidine and l-(l-hexenyl)piperidine have been determined in 95% ethanol at 20°C 13). The values for the first-order rate constants are 4 x 10 sec and approximately 10 sec , respectively. Apart from steric effects the difference in rate may be interpreted in terms of resonance stabilization by the phenyl group on the vinyl amine structure, thus lowering the nucleophilic reactivity of the /3-carbon atom of that enamine. 

The fundamental understanding of the diazonio group in arenediazonium salts, and of its reactivity, electronic structure, and influence on the reactivity of other substituents attached to the arenediazonium system depends mainly on the application of quantitative structure-reactivity relationships to kinetic and equilibrium measurements. These were made with a series of 3- and 4-substituted benzenediazonium salts on the basis of the Hammett equation (Scheme 7-1). We need to discuss the mechanism of addition of a nucleophile to the P-nitrogen atom of an arenediazonium ion, and to answer the question, raised several times in Chapters 5 and 6, why the ratio of (Z)- to ( -additions is so different — from almost 100 1 to 1 100 — depending on the type of nucleophile involved and on the reaction conditions. However, before we do that in Section 7.4, it is necessary to give a short general review of the Hammett equation and to discuss the substituent constants of the diazonio group. 

In summary we think that, on a superficial basis, a comparison of the effects of different nucleophilic species added covalently at the (3-nitrogen atom of an arenedi-azonium ion yields results that are almost trivial. Of more interest are unexpected results such as those of Exner and Lakomy for the substituent -N = CHC6H5. A possible explanation for the latter results emerged when the twisted structure of the substituent became known. We emphasize, however, that definitive explanations on the basis of Hammett or related substituent constants are not found very frequently. 

In this chapter we review published results of studies of the kinetics and products of stepwise nucleophilic substitution and elimination reactions of alkyl derivatives, and we present a small amount of unpublished data from our laboratory. Our review of the literature is selective rather than comprehensive, and focuses on work that provides interesting insight into the factors that control the rate constant ratio ks/kp for partitioning of carbocations, and that provides an understanding of how the absolute rate constants ks and kp that constitute this ratio change with changing carbocation structure. 

In summary, there now exists a body of data for the reactions of carbocations where the values of kjkp span a range of > 106-fold (Table 1). This requires that variations in the substituents at a cationic center result in a >8 kcal mol-1 differential stabilization of the transition states for nucleophile addition and proton transfer which have not yet been fully rationalized. We discuss in this review the explanations for the large changes in the rate constant ratio for partitioning of carbocations between reaction with Bronsted and Lewis bases that sometimes result from apparently small changes in carbocation structure. 

To what extent are the variations in the rate constant ratio /cs//cpobserved for changing structure of aliphatic and benzylic carbocations the result of changes in the Marcus intrinsic barriers Ap and As for the deprotonation and solvent addition reactions It is not generally known whether there are significant differences in the intrinsic barriers for the nucleophile addition and proton transfer reactions of carbocations. 

Steric effects on the nucleophile, aniline, were clearly evident. Rate constants for bimolecular attack of 2,6-dimethyl- 70a, 2,6-diethyl- 70b, and 3,5-dimethylaniline 70c at 308 K indicate that the ort/zo-substituted anilines react more than an order of magnitude slower at the same temperature (Table 7). Structure 70c must be able approach the reactive nitrogen more closely.42,43 A comparison of the rate constants for reaction of aniline 72c, /V-methyl- 71a and /V-phenylaniline 71b provides further evidence of steric effects although the very small rate constant for the diphenylamine could also be accounted for by reduced nucleophilicity on account of lone pair resonance into the additional phenyl ring. 

The polymers used in this study were prepared by a nucleophilic activated aromatic substitution reaction of a bisphenate and dihalo diphenyl sulfone ( ). The reaction was carried out in an aprotic dipolar solvent (NMP) at 170°C in the presence of potassium carbonate (Scheme 1) (5,6). The polymers were purified by repeated precipitation into methanol/water, followed by drying to constant weight. The bisphenols used were bisphenol-A (Bis-A), hydroquinone (Hq) and biphenol (Bp). Thus, the aliphatic character of Bis-A could be removed while retaining a similar aromatic content and structure. The use of biphenol allows an investigation of the possible effect of extended conjugation on the radiation degradation. 

The ethylene bromonium and 1-bromoethyl cations and their neutral and anionic counterparts have been the subject of a tandem mass spectrometric study of dissociation and gas-phase redox reactions. IR and Raman studies of the bioactive bromonium cation (19), as its hydrogensulfate salt, agree with the results of an X-ray structure determination, and theoretical calculations are also in agreement, except for the details of the NO2 groups. The azaallenium ion (22) is an intermediate in the photolysis of (20) (21) and (22) could both be seen. Flash photolysis of (23) leads to (24), (25), and (26), all of which could be trapped by nucleophiles (27) was not an intermediate. NMR lineshape analysis of the spectmm of (28) leads to reaction rate constants of formation for both the intimate ion pair (29) and the solvent-separated ion pair (30). ... 

Gas-phase nucleophilic substitution reactions of Y-benzyl chlorides and X-phenoxide or X-thiophenoxide nucleophiles have been investigated by using the PM3 semiempirical MO method. The structure of the transition state was examined. The values of the gas-phase Hammett constants px and py are much greater than for the solution reactions, but a theoretical cross-interaction constant pxy (ca —0.60 for both phenoxides and thiophenoxides) agrees well with an experimental value of —0.62 for the thiophenoxide reactions in MeOH at 20 °C. Other work by the same group has involved theoretical studies of competitive gas-phase 5 n2 and E2 reactions of NCCH2CH2CI with HO and An ab initio method at the 6-31-l-G level was... 

Nucleophilic substitution on methyl / -nitrobenzenesulfonate in CH2CI2 has been studied with a series of chloride salts with different structures and solvations BU4NCI, PPNCl [bis(triphenylphosphoranylidene)ammonium chloride], KCl complexed by 18-crown-6 or Kryptofix 2,2,2, and for comparison PPNBr. ° Rate constants and activation parameters are in accordance with an S 2 mechanism. The results were treated by the Acree equation. There are two reaction paths the first, involving the chloride ion, has the same rate for all the salts, whereas the second slower path, involving the ion pair, has a rate related to the dissociation constant of the salt. 

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