Rate law, for nucleophilic substitution

The Rate Law for Nucleophilic Substitution in a Square Planar Complex... 

Rate law, for nucleophilic substitution, 540-543 Ray, P. C., 556 Rfiy-Dutt twist. 556 Rayleigh, Lord, 825 Reaction rates influenced by acid and base. 553-555... 

Platinum(n).—General The normal rate law for nucleophilic substitution at platinum(n) complexes, as at other square-planar rf complexes, is... 

What does the rate law for the substitution mechanism of Figure 2.11 look like The rate of formation of the substitution product Nu—R in the second step can be written as Equation 2.5 because this step represents an elementary reaction. Flere, as in Figure 2.11, and assoc designate the rate constants for the heterolysis and the nucleophilic reaction, respectively. 

The situation is quite different with tetrahedral complexes of Ni(0), Pd(0) and Pt(0). We might anticipate that an associative mechanism would be deterred, because of strong mutual repulsion of the entering nucleophile and the filled d orbitals of the d system. Thus a first-order rate law for substitution in Ni(0) carbonyls, and M (P(OC2H5)3)4M = Ni, (Sec. 1.4.1) Pd and Pt, as well as a positive volume of activation ( + 8 cm mol ) for the reaction of Ni(CO)4 with P(OEt)3 in heptane support an associative mechanism. 

Kinetic studies37,152 have established that the rate laws for the racemization and the nucleophile-assisted substitution reactions are very similar ... 

As we have seen, a study of the kinetics of a reaction is one of the first steps undertaken when one is investigating the mechanism of a reaction. If we were to investigate the kinetics of the reaction of ferf-butyl bromide with water, we would find that doubling the concentration of the alkyl halide doubles the rate of the reaction. We would also find that changing the concentration of the nucleophile has no effect on the rate of the reaction. Knowing that the rate of this nucleophilic substitution reaction depends only on the concentration of the alkyl halide, we can write the following rate law for the reaction ... 

This rate law, of the form normal for nucleophilic substitution at platinum(ii) [cf. equation (1) above], implies parallel spontaneous and chloride-promoted reaction pathways. The observed dependence of rates on pH was rationalized in terms of ligand protonation equilibria, Chelate ring-opening is an important feature in the displacement of bidentate ligands, discussed in the following section. 

The first clue to what s happening in Figure 7.52 comes from an examination of the rate law for the reaction. It turns out that the formation of tert-h ity alcohol in this reaction is a first-order process in which the concentration of added nucleophile is irrelevant to the rate. The rate law leads to the name Substitution, Nucleophilic, unimolecular, or SnI reaction, where k is the rate constant, a fundamental constant of the reaction. 

Another pathway for nucleophilic substitution reactions also exists. This process, which proceeds in two steps, is the mechanism. It is experimentally distinguished from the Sj j2 mechanism in part by a different rate law. In the slow, rate-determining step of the reaction, the bond between the carbon atom and the leaving group breaks to produce a carbocation and a leaving group. In the second, fast step, the carbocation reacts with the nucleophile to form the product. The two-step process is shown below. 

Prompted by earlier results which indicated that the rate law for substitution of Cl in square-planar rran -[Pt(PEt3)2(R)Cl] (R = phenyl, jp-tolyl, or mesityl) complexes included an associative as well as the normal dissociative path only in the case of substitution by strong biphilic ligands (e.g. CN, SeCN ), Ricevuto et al. have re-examined the reaction with weakly nucleophilic pyridine in methanol ... 

It has long been known that substitution at the anion of Zeise s salt, [Pt(CH=CH2)Cl3], is, thanks to the high trans effect of the coordinated ethene, very fast. Recent developments in low-temperature stopped-flow apparatus have now permitted the study of the kinetics of substitution at Zeise s and other [Pt(alkene)Cl3] anions in methanol solution. These substitutions obey the customary two-term rate law (i.e. with kohs = ki+ /s3[nucleophile]), with large negative AS values for the k2 term as expected for Sn2 processes (196). 

The usual kinetic law for S/v Ar reactions is the second-order kinetic law, as required for a bimolecular process. This is generally the case where anionic or neutral nucleophiles react in usual polar solvents (methanol, DMSO, formamide and so on). When nucleophilic aromatic substitutions between nitrohalogenobenzenes (mainly 2,4-dinitrohalogenobenzenes) and neutral nucleophiles (amines) are carried out in poorly polar solvents (benzene, hexane, carbon tetrachloride etc.) anomalous kinetic behaviour may be observed263. Under pseudo-monomolecular experimental conditions (in the presence of large excess of nucleophile with respect to the substrate) each run follows a first-order kinetic law, but the rate constants (kQbs in s 1 ruol 1 dm3) were not independent of the initial concentration value of the used amine. In apolar solvents the most usual kinetic feature is the increase of the kabs value on increasing the [amine]o values [amine]o indicates the initial concentration value of the amine. 

Because the reactions we consider in this section are single-step and therefore elementary reactions, the rate law specified in Section 2.3 as Equation 2.1 is obtained for the rate of formation of the substitution product Nu—R in Figure 2.4. It says that these reactions are bimolecular substitutions. They are consequently referred to as SN2 reactions. The bimolecu-larity makes it possible to distinguish between this type of substitution and SN1 reactions, which we will examine in Section 2.5 nucleophile concentration affects the rate of an SN2 reaction, but not an SN1 reaction. 

Although certain mechanisms for a reaction can be eliminated on the basis of experimental evidence, it is never possible to prove that the reaction follows a particular mechanism. It can only be demonstrated that all the experimental facts are consistent with that mechanism. One piece of experimental information that is of primary importance is the rate law that the reaction follows. The rate law predicted by a possible mechanism must be consistent with the rate law determined in the laboratory. If the two are not consistent, that mechanism can be ruled out. In the case of these nucleophilic substitution reactions, experimental studies have shown that two different rate laws are followed, depending on the substrate (R—L), the nucleophile, and the reaction conditions. This means that there must be two different mechanisms for the reaction. Let s look at each. 

It thus appears that substitution of OH- in the cobalt complex follows a different mechanism than the dissociative pathway usually observed for substitution reactions of such complexes. Therefore, either OH- behaves differently than other nucleophiles or another way of explaining the observed rate law must be sought. 

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