Rate constants displacement reactions

Bruice and Lapinski (1958) reported that logarithms of second-order rate constants for reaction of p-substituted phenoxide ions with p-nitrophenyl acetate were a linear function of the p/sfa-value of the phenol with a slope of 0-8. Phenolate ions cannot displace... 

Polarography is valuable not only for studies of reactions which take place in the bulk of the solution, but also for the determination of both equilibrium and rate constants of fast reactions that occur in the vicinity of the electrode. Nevertheless, the study of kinetics is practically restricted to the study of reversible reactions, whereas in bulk reactions irreversible processes can also be followed. The study of fast reactions is in principle a perturbation method the system is displaced from equilibrium by electrolysis and the re-establishment of equilibrium is followed. Methodologically, the approach is also different for rapidly established equilibria the shift of the half-wave potential is followed to obtain approximate information on the value of the equilibrium constant. The rate constants of reactions in the vicinity of the electrode surface can be determined for such reactions in which the re-establishment of the equilibria is fast and comparable with the drop-time (3 s) but not for extremely fast reactions. For the calculation, it is important to measure the value of the limiting current ( ) under conditions when the reestablishment of the equilibrium is not extremely fast, and to measure the diffusion current (id) under conditions when the chemical reaction is extremely fast finally, it is important to have access to a value of the equilibrium constant measured by an independent method. 

It is of interest to examine the general case when a nucleofuge is displaced from a carbonyl group by a nucleophile with a labile hydrogen, HNu, which becomes much more acidic upon formation of the tetrahedral intermediate (Scheme 11.10). Catalysis by the general base B will be observed when the intermediate 4 breaks down to reactants faster than it transfers a proton to water. The rate constant for formation of 5, which may or may not represent the overall rate constant of reaction, is given by kn [B] K, where K is the equilibrium constant for formation of 4 and kK is the rate constant of proton transfer from 4 to the catalyst B. 

In addition to the use of spatially-resolved concentration measurements for the determination of rate constants for reactions of ground state atoms, the discharge-flow method has been extensively applied to kinetic and spectroscopic studies of chemiluminescent phenomena. In these cases, the flow parameters in the flow tube are of no great importance, as time resolution is not obtained from axial displacements consequently, the total pressures and flow rates, and tube diameters may be varied over wide limits, since it is unnecessary to ensure adherence to the conditions for plug flow. 

If the substrate is sticky (i.e., dissociates more slowly than it reacts to give products), the plQ values will not be seen in the correct position on the profile, but will be displaced outward (i.e., to lower pH when protonation decreases activity, and to higher pH when protonation increases V/K). The amount of displacement will be Iog(i + k /kz), where is the net rate constant for reaction of the collision complex to yield products, and is for dissociation. With a sticky substrate, the displacement can be a pH unit or more, although values of 0.5-1.0 pH unit are more common. 

Solvent for Displacement Reactions. As the most polar of the common aprotic solvents, DMSO is a favored solvent for displacement reactions because of its high dielectric constant and because anions are less solvated in it (87). Rates for these reactions are sometimes a thousand times faster in DMSO than in alcohols. Suitable nucleophiles include acetyUde ion, alkoxide ion, hydroxide ion, azide ion, carbanions, carboxylate ions, cyanide ion, hahde ions, mercaptide ions, phenoxide ions, nitrite ions, and thiocyanate ions (31). Rates of displacement by amides or amines are also greater in DMSO than in alcohol or aqueous solutions. Dimethyl sulfoxide is used as the reaction solvent in the manufacture of high performance, polyaryl ether polymers by reaction of bis(4,4 -chlorophenyl) sulfone with the disodium salts of dihydroxyphenols, eg, bisphenol A or 4,4 -sulfonylbisphenol (88). These and related reactions are made more economical by efficient recycling of DMSO (89). Nucleophilic displacement of activated aromatic nitro groups with aryloxy anion in DMSO is a versatile and useful reaction for the synthesis of aromatic ethers and polyethers (90). 

The intermediate reaction complexes (after formation with rate constant, fc,), can undergo unimolecular dissociation ( , ) back to the original reactants, collisional stabilization (ks) via a third body, and intermolecular reaction (kT) to form stable products HC0j(H20)m with the concomitant displacement of water molecules. The experimentally measured rate constant, kexp, can be related to the rate constants of the elementary steps by the following equation, through the use of a steady-state approximation on 0H (H20)nC02 ... 

A number of other spectroscopies provide information that is related to molecular structure, such as coordination symmetry, electronic splitting, and/or the nature and number of chemical functional groups in the species. This information can be used to develop models for the molecular structure of the system under study, and ultimately to determine the forces acting on the atoms in a molecule for any arbitrary displacement of the nuclei. According to the energy of the particles used for excitation (photons, electrons, neutrons, etc.), different parts of a molecule will interact, and different structural information will be obtained. Depending on the relaxation process, each method has a characteristic time scale over which the structural information is averaged. Especially for NMR, the relaxation rate may often be slower than the rate constant of a reaction under study. 

The rate constant for the k term equals that for reaction of [Ca(parv)] with cydta, consistent with rate-determining dissociation of [Ca(parv)] in both cases the k2 term may be assigned to an associative (adjunctive) process (497). This mechanism parallels that of parallel associative and dissociative pathways established for displacement of edta from Ca(edta)2 by Ttr+ (cf. Section II.D.3 (334)). 

The nature and distribution of the products of an olefin oligomerization reaction will depend, inter alia, on the relative rate constants of the insertion step (ki)vs. the displacement step (fcd) [Eq. (11)] ... 

The nucleophilic displacement reactions of organolithium compounds with alkyl halides are second order insofar as the rates have been measured, but there are unexplained examples of autocatalysis and non-reproducable rate constants. The product of the reaction in the case of the methylallyl chlorides is the same mixture regardless of... 

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). 

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