Absolute reactivity

Compound Absolute reactivity (g Oj/g compound) Relative reactivity... 

Here we plan to devote further attention to reaction intermediates. The methods used to verify the intervention of an intermediate include trapping. That is, the intermediate can be diverted from its normal course by a substance deliberately added. A new product may be isolated as a result, which may aid in the identification of the intermediate. One can also apply competition kinetics to construct a scale of relative reactivity, wherein a particular intermediate reacts with a set of substrates. Certain calibration reactions, such as free radical clocks, can be used as well to provide absolute reactivities. 

The absolute reactivity of vinalstannanes was evaluated by copolymerizing them in bulk with ethylene at 160 °C and 1400 kg/cm2 in the presence of dibuthyl peroxide. In the system triethyl-vinylstannane/ethylene, as in other systems, the reactivity of both vinylstannane and its radical is found to decrease due to conjugation within the molecule. The mean effective copolymerization constants are rt 0 and r2 = 3.5 + 1.0 52). 

In this article we have summarized the use of both photochemical and more classical thermal kinetics techniques to deduce the nature of intermediates in the ambient temperature, fluid solution chemistry of several triruthenium clusters. In some cases the photochemically generated intermediates appear to be the same as those proposed to be formed along thermal reaction coordinates, while in other cases unique pathways are the results of electronic excitation. The use of pulse photolysis methodology allows direct observation, and the measurement of the reaction dynamics of such transients and provides quantitative evaluation of the absolute reactivities of these species. In some cases, detailed complementary information regarding... 

Ritchie was the first to directly measure the absolute reactivity of cations toward solvent and added nucleophiles. The cations were highly stabilized examples, triarylmethyl cations bearing stabilizing substituents such as 30 and 31, xanthylium ions (e.g., 32) and tropylium ions (e.g., 33). The feature (and requirement) of these cations was that they had a lifetime in water such that kinetics could be followed by conventional or stopped-flow spectroscopy whereby one solution containing the pre-formed cation was added to a second solution. The time required to mix these solutions was the important factor and limited measurements to cations with lifetimes longer than several milliseconds. The lifetimes in water for 30-33 are provided below. Lifetime is defined as the reciprocal of the first-order rate constant for the decay of the cation in solvent. 

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. 

Five Standard Frontier Orbital Treatments of Reactivity Absolute Reactivity... 

Use of triphenylmethyl and cycloheptatrienyl cations as initiators for cationic polymerization provides a convenient method for estimating the absolute reactivity of free ions and ion pairs as propagating intermediates. Mechanisms for the polymerization of vinyl alkyl ethers, N-vinylcarbazole, and tetrahydrofuran, initiated by these reagents, are discussed in detail. Free ions are shown to be much more reactive than ion pairs in most cases, but for hydride abstraction from THF, triphenylmethyl cation is less reactive than its ion pair with hexachlorantimonate ion. Propagation rate coefficients (kP/) for free ion polymerization of isobutyl vinyl ether and N-vinylcarbazole have been determined in CH2Cl2, and for the latter monomer the value of kp is 10s times greater than that for the corresponding free radical polymerization. 

Use of stable organic cations to initiate cationic polymerization allows characterization of free ions and ion pairs as intermediates and in some cases facilitates measurement of their respective absolute reactivities. Further work is in progress to extend the range of catalysts and monomers which may react in this way and therefore to extend our knowledge of the absolute reactivity in cationic polymerization. 

In order to compare the absolute reactivities of active species and monomers it is, however, necessary to react a series of monomers with a model active center and to react a sertes of active species with one chosen monomer. This, in principle, can be conducted in copolymerization studies, which are not included in this review. Nevertheless, we may cite here at least one work conclusively demonstrating that the rate constants of the additkm of various monomers to a given active species do not change too much with monomer reactivity, most of the variations stemming from the diversity of the active species . ... 

It appears that the reaction mechanism and the intermediates involved in the solid-state polymerization of diacetylenes are reasonably well understood. However, experimental results obtained with special monomers should not be generalized. It is not possible to design a monomer with desired properties. Inspection of Table 1 shows that on the basis of the crystallographic data and the monomer packing the absolute reactivity and the polymerization kinetics caimot be quantitatively predicted, e.g. it is not possible, to date, to explain why certain diacetylenes can be polymerized thermally whereas others with equal packing are thermally inactive. A more realistic kinetic model should include the various energy transport processes and the complex side group motions which are connected to the reaction. 

A special case arises if fej, = 21 s d k 2 22> the absolute reactivities of the two types of chain-end are identical. In that case. 

Recently, Higashimura [7] has reviewed the data on elementary rate coefficients (fej, fep, fet and fej) in cationic polymerization of vinyl monomers. Information available on initiation and termination reactions is extremely limited, and virtually no attempt [50] has been made to elucidate, either qualitatively or quantitatively the role of free ions and ion pairs in these processes. Numerical data on the separate contributions to propagation by free ions and ion pairs is slowly becoming available, though in a less ordered fashion than in the case of anionic systems. It seems likely that the most fruitful approach to the problem of absolute reactivity, in initiation processes at least, will be an examination of reactions of non-polymerizable monomer models, where electronic factors... 

Several quantitative values of absolute reactivities of alkylmagnesium halides and of dialkylmagnesium compounds toward ketones have been published. The values listed in Table 6 are quoted from Ref. 25b. In general, the symmetric organomagnesium compounds react faster than the corresponding Grignard reagent. 

Absolute Reactivity.—Significant interest has continued to be shown in the question of absolute reactivity in cationic propagations, and in particular in quantifying the contribution from free ion and ion-paired species. 

The recent data for kp(+) for A-ethyl-3-vinylcarbazole allows a further interesting correlation of absolute reactivity with structure. The cation derived from this monomer is a substituted p-aminobenzyl type and as such its reactivity might be expected to lie between that of the propagating cation from N-vinylcarbazole and those from isobutylvinyl ether and/7-methoxystyrene. Indeed this seems to be so as the value obtained, 2 x lO s S slots neatly between the figures of 5 x 10 s" and 5 x 10 M s as predicted. 

Absolute Reactivity.—The question of absolute reactivity in carboanionic propagations was admirably reviewed in 1974 by Szwarc. Since then there have been further developments in the quantification of reactivity with increasing interest in acrylate, vinyl pyridine, and diene monomers. 

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