Reactivities of Ions

In dimethoxyethane DME, a more powerful solvating agent than THF, solvation by solvent molecules competes with the intramolecular solvation, increasing the reactivity of ion-pairs. Indeed, the propagation constants of Na+ and Cs+ salts of polymethyl methacrylate are higher in that solvent than in THF, although again both salts are nearly equally reactive 39) as shown in Fig. 5. 

Occasionally, the reactivity of ions with differing coordination numbers may be compared. Laser ablation of Mo02 or M0O3 produced the ions Mo+, [MoO]+, and [Mo02]+, which were reacted with alkanes, alkenes, and C6 hydrocarbons (183). None of the ions reacted with methane and Mo+ was generally less reactive than the other ions with the alkanes. Additions of the alkane to the Mo+, [MoO]+, and [Mo02]+ with loss of H2 or 2H2 were the major reaction products. 

A few studies compare the reactivity of ions with the metal in several oxidation states and the ions in the higher oxidation state are often more reactive. 

Of course, high reactivity of ion-radicals dictates some complexity in the registration methods, such as stop-flow spectroscopy or isolation of ion-radicals by capillary electrophoresis. Whereas stop-flow spectroscopy is used more or less frequently, capillary electrophoresis has been involved only in recent times. Choosing the appropriate capillary length, a researcher can isolate and register UV spectra of relatively less-stable ion-radicals. When the interaction of the corresponding... 

One s intuition also leads to the notion that the lifetime of an encounter complex must be of importance in promoting reactivity of ions and solutes. 

It is generally accepted that there is little effect of counterion on reactivity of ion pairs since the ion pairs in cationic polymerization are loose ion pairs. However, there is essentially no experimental data to unequivocally prove this point. There is no study where polymerizations of a monomer using different counterions have been performed under reaction conditions in which the identities and concentrations of propagating species are well established. (Contrary to the situation in cationic polymerization, such experiments have been performed in anionic polymerization and an effect of counterion on propagation is observed see Sec. 5-3e-2.)... 

In addition to the choice of Lewis acid, added common ion salt, and temperature, the fast equilibrium between active and dormant species can be fostered by including additional nucleophiles (separate from the nucleophilic counterion) in the reaction system and by variations in solvent polarity. Nucleophiles act by further driving of the dynamic equilibrium toward the covalent species and/or decreasing the reactivity of ion pairs. Nucleophilic counterions and added nucleophiles work best in nonpolar solvents such as toluene and hexane. Their action in polar solvents is weaker because the polar solvents interact with the nucleophiles and nucleophilic counterions, as well as the ion pairs. Polar solvents such as methylene... 

It seems that the reactivities of ion pairs and free ions cannot be readily compared. It may be imagined that a polarization of the CH2 S bond in the monomer occurs before the ring opening and is induced by the ion pair dipole of the living end. Such a modification should Increase with the magnitude of the ion pair dipole and then with the interionic distance This leads to an Increase of the interaction with the monomer, together with the reaction rate. It is understandable that the modification of the polarization of the CH2-S bond may be quite different when the interaction occurs with free ions. 

More recently Penczek and his collaborators (134) have used l,3-dioxolan-2-ylium salts to initiate the polymerisation of THF in a variety of solvents. In these systems initiation is clean and efficient, and in the case of carbon tetrachloride as solvent, the only significant ionic species present are ion pairs. Rate constants /c for propagation in this solvent were obtained directly from conversion/time data and are a measure of the reactivity of ion pairs. At 25° C (tTHF]0 = 8.0 M) the value of /c (4.0 x 10 2 M-1 sec-1) was independent of the counter-ion employed, in the series AsFg, PFg, and SbFg. 

Alunni, S., Pero, A., Reichenbach, G., Reactivity of ions and ion pairs in the nucleophilic substitution reaction on methyl p-nitrobenzenesulfonate. J. Chem. Soc., Perkin Trans., 21998, 1747-1750. 

Three main topics relevant to the gas-phase chemistry of Ge, Sn and Pb derivatives are discussed in the present chapter (a) the mass spectrometry related to organometallic compounds of group 14 with particular emphasis on the more general aspects (b) the gas-phase ion chemistry comprising the thermochemistry, structure and reactivity of ions and (c) gas-phase reactions involving neutral species. 

Rate constants were calculated by assuming the extinction coefficient of growing macromolecular styryl cation to be c - 10,000 mol lL/cm l. Values of kp might be three times larger, if t 30,000 mol -L-cm 1 b Relative reactivities of ions and ion pairs estimated from common ion effect. 

The ratio of the reactivities of ions and ion pairs (kp+/kp ) are also included in Table 13. They were determined from kinetic studies of the apparent rate constants at either different acid concentrations which vary the extent of dissociation into free ions, or in the presence of tetrabutylam-monium salts with common counteranions such as perchlorates and triflates. This results in ratios of the reactivities of ions and ion pairs of approximately 6 to 24. However, addition of an equimolar amount of salt to triflic acid may lead conjugation of acid with anions [215], with complete deactivation of the system. Therefore, the lower rate constants of propagation for ion pairs may be partially due to removal of the acid from the system. Thus, the values reported in Table 13 can be considered the upper limit of kp+/kp. The true ratio might be lower, with very similar reactivities for ions and ion pairs as in model systems [4]. Miscalculations of the ratio of reactivities of ions and ion pairs has led to unrealistic values of activation parameters calculated for propagation by ions (Ep = 51 kJmol-, ASP = +54 Jmol- K-1) and ion pairs (Ep = 21 kJmol- ASP = -84-mol-,-K l) [17] the latter values are similar to the overall activation parameters for ionic propagation and are quite reasonable. Extrapolation of Kunitake s data to - 80° C shows ion pairs being 30 times more reactive than ions [17], which contradicts the available experimental data [213]. 

The relative reactivity of ions and ion pairs are very different in anionic and cationic polymerizations. In anionic systems, the reactivity of ions (kp ) is similar to that of solvent-separated ion pairs (kp x), but much higher than that of contact ion pairs (kp-r) [12]. For example, the rate constants of propagation of styrene at ambient temperature are kp = 10s moI Lsec 1, kp s 104 mol-1L sec and kp c 101... 

Because the reactivities of ions and ion pairs are similar and only weakly affected by the structure of the counteranions, kp + or kp determined by either stopped-flow studies or y-radiated systems (cf., Section IV. 13) can be used in Eq. (75). The equilibrium constant of ionization can then be estimated from the apparent rate constant of propagation and the rate constant of propagation by carbenium ions [Eq. (77)]. For example, Kf 10-s mol-,L in styrene polymerizations initiated by R-Cl/SnCl4 [148]. Kt for vinyl ether polymerization catalyzed by Lewis acids can also be estimated by using the available rate constant of ionic propagation (kp- = 104 mol Lsec-1 at 0° C) [217], The kinetic data in Ref. 258 yields Kj == 10 3 mol - l L in IBVE polymerizations initiated by HI/I2 in toluene at 0° C and Kf 10-1 mol- -L initiated by HI/ZnI2/acetone can be calculated from Eq. (76). 

The initial decrease of the apparent rate constants is due to the suppression of free ions in the system. Typically a 2- to 5-fold rate reduction is observed which corresponds to 50 to 80% population of free ions in the system (assuming similar reactivities of ions and ion pairs). The addition of larger amounts of salt has two effects. The first one is the entrapment of Lewis acid and reduction of the polymerization rate. The second effect, which increases the concentration of carbocations, is related to the formation of aggregates of anions (e.g., ionic triplets) as found for SnCI4 [39,269]. 

The numerical values indicate, that in polar solvents (CH3N02, C6H5N02) the fractions of free ions are relatively high, thus their reactivity could be determined for several systems and compared with the reactivity of ion pairs, according to the methodology developed by Szwarc [75]. 

De Jong and Van Fossen have presented one of the clearest examples of a differential steric effect on the reactivity of ions and neutrals. 88) The mass... 

A word of caution must be finally given concerning the assumed general validity of the postulate that the looser the ionic association, the hi er their propagating potential. A few exceptions to this rule have been reported in anionic polymerisation and unusually high reactivity of ion pairs in certain specific situations has been discussed by Szwarc To our knowledge, no such reactivity inversion has been published concerning cationic systems. 

Insertion of impurities Superconductivity of bombarded metals Variations of thin film properties Wear, friction and lubrication of materials Wear, friction and lubrication of materials Chemical state of implanted atoms Reactivity of ion-bombarded surfaces Reactivity of ion-bombarded surfaces Ionization phenomena Charge exchange studies... 

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