Kinetic studies of nucleophilic substitution

They also found that the Erofeev equation described the observed kinetics much better than other simple kinetic equations. Yufit and Zinovyev [212] compared the kinetic study of nucleophilic substitution under PTC conditions in liquid-liquid and solid-liquid systems. They observed the effect of initial exponential burst (IB) on the kinetic curve in the reaction with solid salts for the Sn2 reaction of 2-octylmesylate with potassium halides under PTC conditions. In their study, they assumed that the active sites on which the reaction occured were present on the solid surface through the formation of complexes of salts, catalysts, and substrate [212-215]. They also concluded that the phenomenon of IB was characterized by the first-order dependence on the initial stage of conversion and by zero-order dependence up to high conversion. Therefore, the kinetic equation for the reaction becomes a sum of linear and exponential terms with correlated parameters A and B... 

Kinetic studies of nucleophilic substitution reactions of traw5-[NiBr(QBr5)L2] (L is PPh2Et or PPh2Pr) by SCN", NJ, NO2, and I" reveal the operation of the usual two-term rate law at these sterically hindered complexes/ ... 

Modarai B, Ghandehari MH, Massoumi H, Shafiee A, I.alezari 1, Badali A (1974) Kinetic studies of nucleophilic substitution of various halothiadiazoles with methoxide ion. J Heterocycl Chem 11 343-345... 

Most of the kinetic measures of solvent effects have been developed for the study of nucleophilic substitution (Sn) at saturated carbon, solvolytic reactions in particular. It may, therefore, be helpful to give a brief review of aliphatic nucleophilic substitution. Two mechanistic routes have been clearly identified. One of these is shown by... 

Kinetic and mechanistic studies of nucleophilic substitution at metal(IV) centers are fairly rare (263). Platinum(IV) has the substitution-inert low-spin d configuration, and presumably undergoes nucleophilic substitution by an associative mechanism thanks to its high charge and large size. However there are actually very few data, probably thanks to the tendency for platinum(IV) to oxidize ligands. Substitution kinetics at metal(IV) centers may be more conveniently studied for complexes of the type ML2X2, where M — e.g., Sn, Ti, V, or... 

Kinetic studies of the substitution reaction of 2-chloro-l-methylpyridinium iodide with phenoxides are consistent with the SnAt mechanism, with rate-determining nucleophilic attack.38 The effects of a variety of ring substituents on the reactivities of 2-fluoro- and 2-chloro-pyridines in reactions with sodium ethoxide in ethanol have been examined. The results were discussed in terms of the combination of steric, inductive, and repulsive interactions.39 Substitution in 2,4,6-trihalopyridines normally occurs preferentially at the 4-position. However, the presence of a trialkylsilyl group at the 3-position has been shown to suppress reaction at adjacent positions, allowing substitution at the 6-position.40 Methods have been reported for the introduction and removal of fluorine atoms for polyfluoropyridines. Additional fluorine atoms were introduced by metallation, chlorination, and then fluorodechlorination, while selective removal of fluorine was achieved by reduction with either metals or complex hydrides or alternatively by substitution by hydrazine followed by dehydrogena-tion-dediazotization.41... 

It is important to note that the terms SnI and Sn2 do not merely identify the kinetic results observed in studies of nucleophilic substitution. Rather, these terms are intended to characterize the mechanisms of those reactions. It is sometimes thought that the 1 and 2 refer to kinetics, but Ingold wrote that the numerical indication in the symbolic label, no less than in the verbal name, refers to the molecularity of the reaction, and not to its kinetic order.To put it another way, those terms designate nonobservable mechanistic and not observable kinetic properties. 

The kinetic order of nucleophilic substitutions can be studied by measuring the effect on rate of varying the concentrations of haloalkane and nucleophile. Those reactions whose rate is dependent only on the concentration of haloalkane are classified as Sj l those reactions whose rate is dependent on the concentration of both haloalkane and nucleophile are classified as S 2. 

Kinetics of isomerisation in methanol solution of cis- to trans- [Co(en)2Cl2] is described in J.Anter.Chem.Soc.SO 95S)7SS. For the study of nucleophilic substitution of Cl in the trans-isomer by other anions in methanol see J.C/iem.5 oc.Ai/ro/t rrofts.(1973)2514. 

The reactivity of halogens in pyridazine N- oxides towards nucleophilic substitution is in the order 5 > 3 > 6 > 4. This is supported by kinetic studies of the reaction between the corresponding chloropyridazine 1-oxides and piperidine. In general, the chlorine atoms in pyridazine A-oxides undergo replacement with alkoxy, aryloxy, piperidino, hydrazino, azido, hydroxylamino, mercapto, alkylmercapto, methylsulfonyl and other groups. 

A kinetic study of the nucleophilic substitution of Cl in [FeCp(PhCl)]+ BF by these different N-, O-, and S-anionic nucleophiles was performed using the titration of displaced Cl- by AgN03. The reactivity order increases according to the sequence ... 

In basic aqueous media, a kinetic study of the reaction between stannate(II) ions and alkyl halide shows that mono- and disubstituted organotin compounds are formed (Eq. 6.12a).27 The monosubstituted organotin compound is obtained after a nucleophilic substitution catalyzed by a complexation between the tin(II) and the halide atom. The disubstituted compound results from an electrophilic substitution coupled with a redox reaction on a complex between the monosubstituted organotin compound and the stannate(II) ion. Stannate(IV) ions prevent the synthesis of the disubstituted compound by complexation. Similarly, when allyl bromide and tin were stirred in D2O at 60° C, allyltin(II) bromide was formed first. This was followed by further reaction with another molecule of allyl bromide to give diallyltin(IV) dibromide (Eq. 6.12b).28... 

After 20 years pharmacological interest in nitrofurans is as strong as ever, and still prompts studies including nucleophilic substitutions. Kinetic studies of the replacement of halogen by dimethylamine in 5-halo-2-nitrofurans yield second order rate constants and disclose spectroscopic (IR, UV, and... 

Kinetic studies of the unnatural 6-a -epimer of ampicillin, fi-ept-ampicillin (154), have revealed an intramolecular process not undergone by ampicillin (or other natural /3-substituted penicillins) At pH 6-9, intramolecular attack of the jS-lactam carbonyl group by the side-chain amino group of (154) yields a stable piperazine-2,5-dione derivative (155). Theoretical calculations show that the intramolecular aminolysis of 6-epi-ampicillin nucleophilic attack occurs from the a-face of the -lactam ring with an activation energy of 14.4kcalmor In other respects, the hydrolysis of the b-a-epimer is unexceptional. 

Hydrolysis and condensation reactions of silanes may be considered in the broad category of nucleophilic substitutions at silicon. The common nomenclature for these reactions is SN.V-Si, where A represents the kinetic order or molecularity, Si indicates that silicon is the reaction center, and SN indicates that the reaction is a nucleophilic substitution. Nucleophilic reactions at silicon have been reviewed thoroughly and have been the subject of fundamental studies by several laboratories over the last three decades [33]. The literature is not as voluminous as the literature on the corresponding reactions at carbon. A general mechanistic view of these reactions has, however, emerged. There are many parallels to carbon-centered reaction mechanisms. One distinction from carbon-centered reactions is clearly apparent. Silicon is able to form relatively stable higher coordinated (pentavalent) intermediates carbon is not [33]. 

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