Iodides, second-order rate constants

SAQ8.16 Consider the following data concerning the reaction between triethylamine and methyl iodide at 20°C in an inert solvent of CCI4. The initial concentrations of [CH3l]o and [N(CH3)3]0 are the same. Draw a suitable graph to demonstrate that the reaction is second order, and hence determine the value of the second-order rate constant k2. 

Herriott and Picker (1975) have studied the reaction between sodium thiophenoxide and 1-bromobutane in benzene-water catalysed by various quaternary ammonium salts and by the dicyclohexyl-18-crown-6 isomers ([20] + [21]). The catalytic activities, as judged from the second-order rate constants, span a range of 104. The best catalyst appeared to be dicyclohexyl- 18-crown-6, directly followed by tetrabutylphosphonium chloride and tetrabutylammonium iodide. 

The second-order rate constant for the reaction of iodide and iodine is 6.2XlO M s in water at 298 K while the first-order rate constant for the dissociation of triiodide is 8.5 X 10 s at 298 The dissociation constant, Kq, for this reaction is on the order of 10 M for the dissociation of triiodide, which is comparable in magnitude to Kq for the R2Se-l2 complexes (5 X 10 M). Consequently, added iodide should compete for iodine as effectively as the diorganochalcogenides and limit the available concentration of I4 in situ. Thus, equation (9) must be followed to a greater extent in the presence of added iodide. 

Second-order rate constant for hydrolysis of ethyl iodide by H2O ( ) used as a surrogate for kH2o,sN for DMIH. Rationale for this approach comes from an analogy with the hydrolysis of n-alkyl bromides, since kn20 N for MeBr, EtBr, n-PrBr and n-HexBr all agree within a factor of two at 25°C (see Table V). 

The kinetics of iodination of indole and some of its derivatives in aqueous ethanol have been studied over a wide range of iodide ion concentration (4 x 10-4 to 10-1 M) and at different pH values (6.50-7.86).107-109 The observed second-order rate constants are inversely... 

The second-order rate constants for reactions of Co(I)(BDHC) with alkyl halides were determined spectrophotometrically at 400 nm (17). These rate constants are listed in Table VII along with those for Co(I)(corrinoid)(vitamin Bi2s) in methanol at 25°C (35). These data indicate that the SN2 mechanism is operative in the reaction of Co(I)(BDHC) the iodides are more reactive with the cobalt complex than the bromides, and the rate decreases with increasing bulkiness of the alkyl donor. The steric effect is more pronounced for Co(I)(BDHC) than for vitamin B12s, which is confirmed by the rate ratios for... 

Presumably less nucleophilically assisted solvolyses could show higher a-deuterium isotope effects, and there is a linear relationship between the magnitude of nucleophilic solvent assistance (Table 2) and the a-deuterium isotope effect for solvolyses of 2-propyl sulpho-nates (Fig. 7). Another measure of nucleophilic assistance is the ratio k2 (OH )/, where k2 is the second-order rate constant for nucleophilic attack by OH and kx is the first-order rate constant for reaction with the solvent water, and a linear correlation was obtained by plotting the ratio versus the experimentally observed isotope effects for methyl and trideuteriomethyl sulphonates, chlorides, bromides and iodides (Hartman and Robertson, 1960). Using fractionation factors the latter correlation may also be explained by a leaving group effect on initial state vibrational frequencies (Hartshorn and Shiner, 1972), but there seems to be no sound evidence to support the view that Sn2 reactions must give a-deuterium isotope effects of 1-06 or less. 

The catalytic cycle for the thiolperoxidase and haloperoxidase-like activity of diorganoselenides and tellurides is summarized in Fig. 25. Stopped-flow spectroscopy has been used to elicit mechanistic details of the cycle.59,64,82,84 Following oxidation to the selenoxide or telluroxide, the catalytic cycle for thiolperoxidase-like activity is shown in Fig. 21. The details of the haloperoxidase-like cycle are not as well defined. Using dihydroxytellurane 52 as a substrate, the addition of 0.5 M sodium iodide in pH 6.8 buffer gave a fast reaction with a second-order rate constant > 100 M-1 s-1 followed by a slower, second-order process with a rate constant of 22.5 0.3 M-1 s-1.84 The two processes could not be resolved by using different wavelengths, which would have allowed a better measurement of the rate constant for the initial process. If we assume that the first, faster process is... 

After this work, a further elegant experiment was carried out by Hughes et al.,2 with the measurement of the second-order rate constant for a concerted nucleophilic substitution reaction, and this was done in two ways. The substrate was one enantiomer of 2-iodooctane (7) and the nucleophile was radioactive iodide anion, 1, in propanone (acetone). The nature of the reaction is outlined in Scheme 7.4. Firstly, the rate constant was determined polarimetrically to give a rate constant ka then the rate constant for exchange of I was determined this is represented by kex. The ratio of these rate constants within experimental error was 2 1. 

To calculate the second-order rate constants for the formation and decomposition of hydrogen iodide in the gas phase. 

Since a requirement of the SnI mechanism is the reaction of the carbonium ion with nucleophiles in a fast step subsequent to rate-determining ionization, it would be desirable to have independent data relative to the trapping of carbonium ions by nucleophiles. Triarylmethyl cations react with nucleophiles at rates sufficiently slow to be studied by conventional means, and have provided much information concerning the effectiveness of various nucleophilic species. This will be discussed in the next section of this chapter. The simpler carbonium ions are more reactive, and special techniques must be employed to determine their extremely rapid rates of reaction with nucleophiles. Benzyl cation has been generated by pulse radiolysis in 1,2-dichloroethane, and the absolute rate constants for its reaction with methanol, ethanol, bromide, and iodide ion measured.The second-order rate constants for this group of nucleophiles fall in the range 10 -10 sec ... 

Table 12.9 Effect of solvent on the second-order rates constants for the quatemisation of triethylamine with ethyl iodide [12a, c]...

Common quenchers are oxygen, xenon, hydrogen peroxide, iodide, and amines. The constant fe is a second-order rate constant for a diffusion-controlled process. It is related to the translation diffusion coefficients of the fluorescence molecule and the quencher and hence is related to the radius of encounter. 

BANK AND BANK Atiion and Radical Reactions 35 Table V. The Log of the Ratio of Second-Order Rate Constants for Chlorides and Iodides vs. Bromides with Various Reducing Reagents. ... 

The use of this model leads to derived rate constants which exceed the diffusion-controlled limit. Further n.m.r. studies and a reconsideration of earlier published experimental data lead to a new proposal of an associative mechanism for iodide exchange, in which iodide attacks at one end of the tri-iodide. It is possible, from the observed kinetic pattern, that the I so generated has a sufficient lifetime to be considered an intermediate rather than a transition state. A transfer diffusion investigation of the same reaction also culminates in the proposal of an associative mechanism, with a linear transition state. Allowing for the non-spherical nature of the tri-iodide, it is possible to calculate a diffusion-controlled rate constant, which turns out to be the same as the experimentally determined (by this method or from n.m.r.) second-order rate constant. Some calculations on the transition state have been made in connection with this transfer diffusion study of the iodide-tri-iodide exchange reaction. Further study of the iodide-thiocyanate reaction has resulted in an estimate of the association constant for the initial rapid association of the reactants to give the intermediate charge-transfer complex la.SCN-. ... 

The correlation of reaction rates with dielectric properties is a well-established approach to the diagnosis of mechanism. Most recent examples of this deal with mixed aqueous solvents (see below), but logarithms of second-order rate constants for oxidative addition of methyl iodide or of oxygen to rraw-[IrCl(CO)(PPh3)2] have been found to correlate with the dielectric constant function (D —l)/(2i) + 1). However, the correlation of these rate constants with the empirical Ex values for the respective solvents, mentioned above, is better. 

Iodide was found to favor the dimeric form and citrate favored the tetramer. Kinetics of the appearance of enzyme activity induced by addition of PLP was found to be second order with respect to enzyme concentration and did not depend on PLP concentration. The second-order rate constant was 5 X 10 M" sec It is of interest to note that this value is of the same order of magnitude as those reported for reactivation of several dehydrogenases (see Table 11.1) (Jaenicke et al, 1979). Therefore reassociation of dimers is the rate limiting step in the regain of enzymatic activity indicating that only the tetramer is functional. 

In equations (10) and (11), the initial concentrations of tetraethyltin and mercuric iodide are denoted by A and B respectively, the concentration of ethylmercuric iodide at time t is denoted by X, and the concentration of Et3Sn+ (and also of Hgl3 ) at any time t is denoted by Y. The equilibrium constant for reaction (9) is K, and k2 is the second-order rate coefficient for the electrophilic substitution (8). Equations (10) and (11) can be solved by the method of numerical analysis17 and values of k2 were thus obtained (values of K were determined16 by direct experiments). It was shown that k2 remained constant over a ten-fold range of initial concentration of tetraethyltin and of mercuric iodide. Values of the second-order rate coefficient and of the associated activation parameters are given in Table 5. Reaction (8) is thus characterised by a very negative activation entropy... 

The reaction of superoxotitanium(IV) with a number of substrates has been monitored by stopped-flow techniques/ In 1 M perchloric acid, the oxidation of iodide and bromide proceeded with second-order ratde constants of 1.1 x 10 M s and 2M s respectively. It is proposed that the reduction of superoxotitanium(IV) proceeds by a one-electron mechanism. Based on proton dependences, the species TiO " is more reactive than the protonated form Ti02(0H)2. The chromium chelate, bis(2-ethyl-2-hydroxybutyrato)oxochro-mate(V), is reduced by iodide, generating a Cr(IV) intermediate. The reaction is considered to proceed through formation of an iodine atom (T) for which both Cr(V) and Cr(IV) compete. In aqueous solution, [Co(EDTA)] forms a tight ion pair with I . Upon irradiation of this ion pair at 313 nm, reduction of [Co(EDTA)] to [Co(EDTA)] occurs with oxidation of 1 to IJ. The results may be interpreted on the basis of a mechanism in which [Co(EDTA)] and V are the primary photoproducts where the latter subsequently disproportionate to I3 and 1 . The kinetics and mechanism of the oxidation of 1 by a number of tetraaza macrocyclic complexes of Ni(III) have been reported. Variations in rate constants and reaction pathways are attributable to structural differences in the macrocyclic ligands. Of interest is the fact that with some of the Ni(III) complexes, spectrophotometric evidence has been obtained for an inner-sphere process with characterization of the transient [Ni(III) L(I)] intermediates. Iodide has also been used as a reductant for a nickel(III) complex of R-2-methyl-1,4,7-triazacylononane. In contrast to the square-planar macrocycles, the octahedral... 

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