Substitution rates

In order to compare S l substitution rates in a range of alkyl halides experimental con ditions are chosen m which competing substitution by the 8 2 route is very slow One such set of conditions is solvolysis m aqueous formic acid (HCO2H)... 

Oxidation—Reduction. Redox or oxidation—reduction reactions are often governed by the hard—soft base rule. For example, a metal in a low oxidation state (relatively soft) can be oxidized more easily if surrounded by hard ligands or a hard solvent. Metals tend toward hard-acid behavior on oxidation. Redox rates are often limited by substitution rates of the reactant so that direct electron transfer can occur (16). If substitution is very slow, an outer sphere or tunneling reaction may occur. One-electron transfers are normally favored over multielectron processes, especially when three or more species must aggregate prior to reaction. However, oxidative addition... 

Photochemistry. Substitution rates of many complexes are enhanced by irradiation of the low energy d—d transitions, such as t2g — in... 

Evidently S, is a measure of intramolecular selectivity because it involves a ratio, the contribution of the benzene substitution rate disappears, and the selectivity factor expresses the selectivity of the reagent X in Eq. (7-83) for the para position relative to the meta position. Each individual partial rate factor, on the other hand, is expressive of an inteimolecular selectivity thus p is a measure of the selectivity of the reagent for the para position in CgHsY relative to benzene. It was observed that Eq. (7-85), where Cmc is a constant, is satisfied for a large number of electrophilic substitutions of toluene. 

Experiments have shown that sulfuric acid enhances the rate of substitution of alcohols sufficiently to make this a practical reaction, but substitution of amines is not practical under these conditions, and substitution rates for fluorides and chlorides are not significantly affected by H2SO4. Why ... 

IV, C, 1, d). Second, for both classes of aromatic compounds such values show a surprisingly small dependence on the nature of the attacking reagent, probably indicating the predominant role of the reorganization of the substrate toward a new state represented by structure 63 or 65. FinaUy, it may not be fortuitous that a correspondence is found between structural effects on substitution rates and on ionization constants (Section IV,C, l,a). Bond-making would in fact be the essential analogy between these phenomena [Eqs. (16) and (17)], and... 

Since niobates and tantalates belong to the octahedral ferroelectric family, fluorine-oxygen substitution has a particular importance in managing ferroelectric properties. Thus, the variation in the Curie temperature of such compounds with the fluorine-oxygen substitution rate depends strongly on the crystalline network, the ferroelectric type and the mutual orientation of the spontaneous polarization vector, metal displacement direction and covalent bond orientation [47]. Hence, complex tantalum and niobium fluoride compounds seem to have potential also as new materials for modem electronic and optical applications. 

It was concluded that the variations in rate are due to variations in activation enthalpy rather than entropy, and since the rates of substitution rates at the para positions of toluene and /-butylbenzene varied by only 4 % for a change in reactivity of 6,430, it was concluded that the Baker-Nathan reactivity order does not arise from a solvent effect (c/. Table 57). 

Since the rate was independent of acidity even over the range where H0 and pH differ, and the concentration of free amine is inversely proportional to the acidity function it follows that the rate of substitution is proportional to h0. If the substitution rate was proportional to [H30+] then a decrease in rate by a factor of 17 should be observed on changing [H+] from 0.05 to 6.0. This was not observed and the discrepancy is not a salt effect since chloride ion had no effect. Thus the rate of proton transfer from the medium depends on the acidity function, yet the mechanism of the reaction (confirmed by the isotope effect studies) is A-SE2, so that again correlation of rate with acidity function is not a satisfactory criterion of the A-l mechanism. 

It is pertinent, then, to seek a dependence of substitution rates on (/) leaving group, (ii) solvent, Hi) steric crowding, iv) charge, v) nature of non-labile substituents including stereochemistry, consistent with this picture of the activation mode. If these tests generally support d modes it will be desirable to examine rate laws closely to attempt a distinction between D and 7j stoichiometric pathways. 

According to this equilibrium argument, the matched S-carbonate B-C-S should give a better branch to linear (B/L) ratio and enantiomeric excess if the nucleophilic substitution rate prior to Jt-allyl Mo conversion from complex A to B is increased, (see Table 2.7) For example, when the reaction was run at a higher concentration, [Malonate]0 0.6M rather than the typical -0.07 M, the ee of the product increases to 97% from 92%. 

Fig. 2.7. Characteristic rate constants (s 1) for substitution of inner-sphere H20 of various aqua ions. Note The substitution rates of water in complexes ML(H20)m will also depend on the symmetry of the complex (adapted from Frey, C.M. and Stuehr, J. (1974). Kinetics of metal ion interactions with nucleotides and base free phosphates in H. Sigel (ed.), Metal ions in biological systems (Vol. 1). Marcel Dekker, New York, p. 69).

In the case of complexes such as (21) and (23) which have an extended planar ligand, a significantly higher proportion of interstrand cross-links in DNA is formed in comparison to either cis- or trans-platin.172 The steric effects of these planar ligands result in the formation of structurally unique 1,2-interstrand cross-links like those formed by cisplatin, a unique example of how steric effects may alter a nonactive lesion into an active one (Figure 13).173,174 Model studies predicted this outcome by preparation of the monofunctional models trans-[PtCl(9-ethylguanine) (NH3)(quinoline)] and comparison of substitution rates of the Pt—Cl bond by G or C mononucleotides.175 176 Interestingly, the iminoether compound (25) appears to form predominantly monofunctional adducts with DNA.177... 

Although the terms labile and inert have been in use for more than 50 years, they are only qualitative descriptions of substitution rates. A more appropriate way to describe the rates has been given by Gray and Langford (1968), which categorizes metal ions according to the rate of exchange of coordinated water with water in the bulk solvent. The four classes of metal ions are shown in Table 20.1. 

We have recently extended our interest to the analogous halfsandwich osmium-arene complexes and are exploring the chemical and biological properties of [Os(r 6-arene)(XY)Z]ra 1 complexes (Fig. 25) (105). Both the aqueous chemistry and the biological activity of osmium complexes have been little studied. Third-row transition metals are usually considered to be more inert than those of the first and second rows. Similar to the five orders of magnitude decrease in substitution rates of Pt(II) complexes compared to Pd(II), the [Os(ri6-arene)(L)X]"+ complexes were expected to display rather different kinetics than their Ru(II)-arene analogs. A few other reports on the anticancer activity of osmium-arene complexes have also appeared recently (106-108). 

Mercury(II) reacts with organochromium complexes by electrophilic substitution. Rate constants have been reported for Hg2+ attack at a series of alkylchromium complexes with the macrocyclic ligand 1,4,8,11-tetrazacyclotetradecane, CrR(H20)([14]aneN4). The Hammett relationship established for a series of meta and para substituted benzyl analogues is consistent with attack of the Hg2+ at a-carbon (89). 

Ten years ago Rorabacher (13) observed the substitution rate constants for aquonickel(II) ion with different amines (Table II). There is a decrease in the rate constants by a factor of 14 in going from ammonia to dimethylamine. If nickel-(II) substitution reactions are dissociative, then why is the effect this large Is this a steric effect with some associative contribution or is it an outer-sphere effect There has been surprisingly little investigation of the nature of the entering ligand so far as its bulk or its nucleophilicity is concerned even for what have been generally considered as simple substitution reactions. 

Axial substitution rate constants have been determined by Haines and McAuley (25) for anions reacting with nickel(III)-cyclam and some of the constants are given in Table III. The rate constants for the replacement of an axial water molecule by... 

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