Kinetic order displacement mechanism

The concerted displacement mechanism implies both kinetic and stereochemical consequences. The reaction will exhibit second-order kinetics, first-order in both reactant... 

As a general statement, the reaction rate in each direction follows second-order kinetics for all the rhenium compounds studied. Moreover, the rate constants depend on the identities of L and Ly. Both findings argue for an associative (displacement) mechanism, which is also supported by the large and negative values of AS, that often reach —120 J K-1 mol-1 (39). 

The distinction between the SnI and jSat2 mechanisms is not necessarily always a sharp one, and if the attacking group Y can facilitate the departure of X, an intennediate case may occur. Such intermediate cases seem to arise in reactions in which the nucleophilic reagent is a solvent molecule, when, unfortunately, kinetic order with respect to solvent is almost impossible to clarify. It is important to note that where the attacking group Y is an ion such as a halide, X"", or Oil or RO the displacement reaction usually follows fairly clean second-order mixed kinetics. The confusion that arises when Y is a solvent molecule is readily understood when we consider that the mechanism of ionization will involve very strong ion-solvent interactions. In fact ionization is not possible without such interactions. 

There have been a number of studies of the reaction of diazoacetic ester in aprotic solvents, mainly with carboxylic acids (Bronsted and Bell, 1931 Hartman et al., 1946 and references cited). However, the information available hardly justifies conclusions about the mechanism. Addition of relatively basic phenols causes an acceleration in rate which can be interpreted in terms of nucleophilic catalysis of a rate-determining displacement of nitrogen, but the kinetic order in acid varies between one and two. Formally, a mixed order would result if proton loss from the diazonium ion was effected by carboxylate ions alone, while the less discriminating displacement of nitrogen involved competition between anions and unionized molecules. However, there are examples of high or mixed orders in other acid-catalysed reactions (Bronsted and Bell, 1931 Bell, 1941 1959) and in all probability large medium effects play a role. 

Despite earlier claims of a carbene mechanism , more recent kinetic studies indicate that the conversion of diphenylmethyl chloride into tetraphenylethyl-ene by r-butoxide in dimethyl sulphoxide follows the displacement mechanism. The reaction is third-order, second in substrate and first in the medium basicity and a-hydrogen exchange in the substrate is a rapid process ". 

A concerted reaction must be stereospecific. The mechanism described by Fig. 5.2 requires inversion of configuration. An alternative direct displacement mechanism involving front-side attack would also exhibit second-order kinetics and respond similarly to structural and medium effects, but would require retention of configuration as the stereochemical course. As we shall see, the available data support fully the mechanism involving back-side displacement. Additionally, SCF-MO calculations of the hypothetical transition states for hydride displacement of hydride from methane indicate that the inversion geometry is 14.9 kcal/mol more favorable than the retention geometry ... 

The Observation of Second Order Kinetics to Support a Multistep Displacement Mechanism for a tamin Analog... 

The consequences of the concerted displacement mechanism include both kinetic and stereochemical implications. The kinetics of the bimolecular process must be consistent with a second-order rate expression, first order in substrate and first order in nucleophile. For... 

The points that we have emphasized in this brief overview of the S l and 8 2 mechanisms are kinetics and stereochemistry. These features of a reaction provide important evidence for ascertaining whether a particular nucleophilic substitution follows an ionization or a direct displacement pathway. There are limitations to the generalization that reactions exhibiting first-order kinetics react by the Sj l mechanism and those exhibiting second-order kinetics react by the 8 2 mechanism. Many nucleophilic substitutions are carried out under conditions in which the nucleophile is present in large excess. When this is the case, the concentration of the nucleophile is essentially constant during die reaction and the observed kinetics become pseudo-first-order. This is true, for example, when the solvent is the nucleophile (solvolysis). In this case, the kinetics of the reaction provide no evidence as to whether the 8 1 or 8 2 mechanism operates. 

Ni3C decomposition is included in this class on the basis of Doremieux s conclusion [669] that the slow step is the combination of carbon atoms on reactant surfaces. The reaction (543—613 K) obeyed first-order [eqn. (15)] kinetics. The rate was not significantly different in nitrogen and, unlike the hydrides and nitrides, the mobile lattice constituent was not volatilized but deposited as amorphous carbon. The mechanism suggested is that carbon diffuses from within the structure to a surface where combination occurs. When carbon concentration within the crystal has been decreased sufficiently, nuclei of nickel metal are formed and thereafter reaction proceeds through boundary displacement. 

Bohle and co-workers (133) have demonstrated that varying the electronic and stereochemical properties of porphyrin substituents can strongly influence the rates of NO labilization (Eq. (11)). For example, the displacement of NO from Fe(TPP)(NO) by pyridine is many orders of magnitude slower than from Fe(OBTPP)(NO) (OBTPP = octabromo-tetraphenylporphyrin). An analysis of the kinetics of the latter reaction indicated saturation in [L], and the mechanism was suggested to involve reversible formation of Fe(OBTPP)(L)(NO) followed by NO dissociation (Eq. (50)). Clearly changes in porphyrin properties can lead to enhanced reactivity toward NO loss. 

It was found that CO exchange in (diphosphine)Rh(CO)2H complexes proceeds via the dissociative pathway [60], The decay of the carbonyl resonances of the (diphosphine)Rh(13CO)2H complexes indeed followed simple first-order kinetics. The experiments with ligand 20 at different 12CO partial pressure show that the rate of CO displacement is independent of the CO pressure. Furthermore, the rate is also independent of the (diphosphine)Rh(13CO)2H complex concentration, as demonstrated by the experiments with ligand 18. It can therefore be concluded that CO dissociation for these complexes obeys a first-order rate-law and proceeds by a purely dissociative mechanism. 

The two limiting cases of nucleophilic displacement reactions are designated as SnI or Sn2 to indicate those mechanisms that respectively display overall first-order or second-order kinetics. These mechanisms are illustrated by the classical case of the reaction of hydroxide ion with chloromethane, and they differ with respect to the timing of the bond-breaking step relative to the bondmaking step. 

It is easy to argue that the behavior of the [Co en2 Cl2]+ isomers is not surprising. The correlation of aquation rates of complexes of the type, [Co en2 A Cl]n+ with the electron displacement properties of the nonparticipating ligand, A, has led to the belief that ligands able to donate a second pair of electrons to the metal can thereby stabilize the 5-coordinate intermediate and hence promote a unimolecular reaction (2, 18, 24). Chlorine is such a ligand, Cl—Co- -Cl, and the essentially first-order kinetic form could be used as evidence for a unimolecular mechanism, once the ion association pre-equilibrium effects for the displacement of chloride under the electron-displacing influence of the other chlorine atom have been taken into account. 

This approach applies only when we are certain that the substrate is mainly in the form of the free ion at the lowest anion concentrations. This is true in the chloride exchange of cw-[Co en2 Cl2]+ in methanol and we can safely conclude that the mechanism is unimolecular (8, 9. 10, 11, 26, 27). This condition did not exist when we studied the displacement of water in trans-[Co en2N02H20]+2 by anions where, because of the large ion association constants, none of the substrate was in the free ion form under reaction conditions. However, in the reaction between trans-[Co en2N02Br]+ and thiocyanate in sulfolane, the substrate was mainly in the free ion form. The observed second-order kinetic form was fully consistent with assigning a bimolecular mechanism to the rearrangement of the ion pair. 

The reactions of 0-naphthol and 4-methoxyphenol with acetyl, propionyl, butyryl, 0-chloropropionyl and chloracetyl chlorides in acetonitrile produce some striking kinetic results109. The behaviour of acetyl, propionyl and n-butyryl chlorides fit reasonably well into the pattern for acetyl chloride in nitromethane and acetyl bromide in acetonitrile. However, with chloracetyl chloride the mechanism is essentially a synchronous displacement of covalently bound chlorine by the phenol and this process is powerfully catalysed by added salt with bond breaking being kinetically dominant. When no added salt is present the rate of hydrolysis of chloracetyl chloride is ca. 8000 times slower than that of acetyl chloride. Although, normally, in second-order acylation reactions, substituents with the greatest electron demand have been found to have the fastest rates, the reverse is true in this system. Satchell proposes that a route such as... 

In 1933 the two still widely accepted mechanisms for nucleophilic displacement reactions were proposed by Hughes, Ingold, and Patel.4 They found that decomposition of quartenary ammonium salts, R4N+Y, to give R3N and RY exhibited two different kinds of kinetic behavior depending on the ammonium salt used. For example, when methyl alcohol was formed from trimethyl-n-decylammonium hydroxide (Equation 4.3), the rate of formation of methyl alcohol was found to be second-order, first-order each in trimethyl-n-decylam-monium cation and in hydroxide ion as in Equation 4.4. On the other hand, the rate of formation of diphenylmethanol from benzhydryltrimethylammonium... 

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