Inhibition, common ion

The addition of water to a free carbocation intermediate of solvolysis can be distinguished from addition to an ion-pair intermediate by an examination of common ion inhibition of solvolysis. Common leaving group inhibition of solvolysis is observed when the leaving group ion (X ) acts, by mass action, to convert the free carbocation (R , Scheme 5A) to substrate (R-X). This results in a decrease in the steady-state concentration of R that leads directly to a decrease in the velocity of solvolysis. Some fraction of the solvolysis reaction products form by direct addition of solvent to the carbocation-anion pair intermediate. The external... 

Azide ion is a modest leaving group in An + Dn nucleophilic substitution reactions, and at the same time a potent nucleophile for addition to the carbocation reaction intermediate. Consequently, ring-substituted benzaldehyde g m-diazides (X-2-N3) undergo solvolysis in water in reactions that are subject to strong common-ion inhibition by added azide ion from reversible trapping of an o -azido carbocation intermediate (X-2 ) by diffusion controlled addition of azide anion (Scheme... 

Recognizing this, Richard and Jencks, proposed using azide ion as a clock for obtaining absolute reactivities of less stable cations. The basic assumption is that azide ion is reacting at the diffusion limit with the cation. Taking 5 x 10 M s as the second-order rate constant for this reaction, measurement of the selectivity fcaz Nu for the competition between azide ion and a second nucleophile then provides the absolute rate constant since feaz is known. The clock approach has now been applied to a number of cations, with measurements of selectivities by both competition kinetics and common ion inhibition. Other nucleophiles have been employed as the clock. The laser flash photolysis (LFP) experiments to be discussed later have verified the azide clock assumption. Cations with lifetimes in water less than about 100 ps do react with azide ion with a rate constant in the range 5-10x10 M- s-, " which means that rate constants obtained by a clock method can be viewed with reasonable confidence. 

Clausius-Mosotti equation, 389 Closed systems, 10 Coalescence, 169 Cohesive energy density, 412 Collision, bimolecular, 188 Collision theoiy, 188 Common-ion effect. 428 Common-ion inhibition, 183 Compensation effect, 369 Competitive reactions, 59 Complex... 

T. L. Amyes and W. P. Jencks, Lifetimes of oxocarbenium ions in aqueous solution from common ion inhibition of the solvolysis of a-azido ethers by added azide ion,./. Am. Chem. Soc., Ill (1989) 7888-7900. 

Generation of R " from the covalent azide itself and common ion inhibition of this process by added azide enabled the product analysis to be dispensed with completely.Applying the steady-state approximation, d[R "]/dt=0, to the reaction scheme of eqn (3.2) results in the rate law given by eqn (3.3). Inversion of this expression gives eqn (3.4), which describes a linear plot, from which can be calculated from slope and intercept. 

In 1989, Amyes and Jencks reported a study of azide common ion inhibition for solvolysis of a series of ot-azido ethers (Fig. I).23 The rate constants A Hoh for addition of water to the oxocarbenium ions were determined with an assumed diffusion controlled second-order rate constant /caz of 5.0 x 109 M-1 s 1 for attack on the oxocarbenium ion by azide and the observed rate suppression by varying concentrations of added azide, as determined from Equation (1). With Hoh in hand, the lifetime of the oxocarbenium ion was taken as hoh... 

The nitrosonium ion complex of 2,3-dimethylbut-2-ene is the dynamic r-complex (26), according to and NMR studies and ab initio calculations. The kinetics of the decomposition of two cyclic a-acetoxynitrosamines have been examined. The intermediacy of (27) and (28) in the mechanism is inferred from the thermodynamic parameters found, observed common-ion inhibition, and trapping experiments with azide ion the products in water are the corresponding alcohols. There is a 100-fold difference in reaction rate for the two ring sizes. Ai-Methylmorpholine Ai-oxide undergoes an autocatalytic decomposition in the presence of Mannich intermediates such as (29). The ambident reactivities of carbenium salts with a thiocarbonyl group at the j8-position, such as (30), have been investigated. ... 

If (A i[X ]/A 2[Y ]) is not much smaller than unity, then as the substitution reaction proceeds, the increase in [X ] will increase the denominator of Eq. (8-65), slowing the reaction and causing deviation from simple first-order kinetics. This mass-law or common-ion effect is characteristic of an S l process, although, as already seen, it is not a necessary condition. The common-ion effect (also called external return) occurs only with the common ion and must be distinguished from a general kinetic salt effect, which will operate with any ion. An example is provided by the hydrolysis of triphenylmethyl chloride (trityl chloride) the addition of 0.01 M NaCl decreased the rate by fourfold. The solvolysis rate of diphenylmethyl chloride in 80% aqueous acetone was decreased by LiCl but increased by LiBr. ° The 5 2 mechanism will also yield first-order kinetics in a solvolysis reaction, but it should not be susceptible to a common-ion rate inhibition. 

There is no detectable common bromide ion inhibition of the reaction of tert-butyl bromide in 90% acetone in water, " " or common chloride ion inhibition of the reaction of 5-Cl in 50 50 (v/v) water/trifluoroethanol or... 

Pt alloy monolayer catalysts exhibited even more active ORR behavior compared to Pt monolayer catalysts. To understand this phenomenon computational DFT studies were carried out. The hypothesis to be tested was that, for instance, Ru metal atoms in the Pt—Ru monolayer are OH-covered and could inhibit the adsorption of additional OH on neighboring surface sites (adsorbate-adsorbate repulsion effect). A very similar hypothesis was put forward about three years earlier by Paulus et al. [105] who postulated that Co surface atoms might exhibit a so-called common-ion effect, that is, they could repel like species from neighboring sites. A combined computational-experimental study finally confirmed this hypothesis [123] If oxophilic atoms such as Ru or Os were incorporated into the Pt monolayer catalysts, the formation of adjacent surface OH was delayed, if not inhibited. Oxo-phobic atoms, such as Au, displayed the opposite effect, would not inhibit Pt—OH formation, and were found to be detrimental to the overall ORR activity. 

The increase in concentration of the acetate ions will drive the reaction to the left, which will further inhibit the dissociation of acetic acid. Adding hydrochloric acid will have the same effect because it will increase the concentration of protons, which will also drive the reaction to the left. Sodium acetate and hydrochloric acid have two features that allow them both to cause the common-ion effect to occur. First, they are both strong electrolytes, and second they each have an ion in common with the acetic acid equilibrium. These are the key ingredients that cause the common-ion effect. 

The common ion effect is also important in solutions of polyprotic acids. The production of protons by the first dissociation step greatly inhibits the succeeding dissociation steps, which also produce protons, the common ion in this case. We will see later in this chapter that the common ion effect is also important in dealing with the solubility of salts. 

These calculations predict that calcareous soils should have pH values near 8.3 however, the actual pH of these soils in the field is often well below 8.3, in the range of 7.5 to 7.8. This may be a consequence of the higher CO2 pressure in soil air compared with the atmosphere, or it may reflect the presence of carbonate solids other than pure crystalline calcite (Mg carbonates, for example). Calcareous soil solutions are commonly supersaturated with respect to calcite, perhaps because dissolved molecules and ions inhibit crystallization. 

It is suggested that the anodic dissolution will be inhibited if the adsorbed anion and the reaction intermediate are stable and hardly dissolve in aqueous solution. On the contrary, if the reaction intermediate is relatively unstable and readily dissolves into aqueous solution, the anion will function as an electrocatalyst accelerating the metal dissolution rate. It is now common knowledge that hydroxide ions, OH, catalyze the anodic dissolution of metallic iron and nickel in acid solution [81,82]. It is also known that chloride ions inhibit the anodic dissolution of iron in acidic solution [83]. No clear-cut understanding is however seen in literature on why hydroxide ions catalyze but chloride ions inhibit the anodic dissolution of iron, even though the two kinds of anions are in the same group of hard base. We assume that the hardness level in the Lewis base of adsorbed anions will be one of the most effective factors that determine the catalytic activity of the adsorbates. Further clarification on the catalytic characteristics will require a quantum chemical approach to the adsorption of these anions on the metal surface. 

Reduction of the dye is inhibited by divalent eations in the Rhizobium system. Common ions, calcium and magnesium, inhibit the reduetion of the dye. The toxicity of the ions is shown in Table 15.2.2.5. Mereury and eadmium, generally thought to be the most toxie minerals were the most toxie with this assay. Caleium and magnesium are also toxic. Water and soil samples typieally eontain ealeium and magnesium so in order to analyze water and soil samples for toxic organic chemicals, a method to eliminate this inhibition by metal ions was sought. 

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