Intimate ion pair intermediates

There seems to be no reason why the transition state LXVIII should not be more Stable than the transition state LXVI, just as the corresponding diene is more stable than the other diene. Hence it would be expected that the faster reaction would produce the stable product/ LXX, while in fact LXX is formed only under conditions where an equilibrium ensures that the product will be the more stable isomer. The alternative explanation in terms of a covalent, or at most an intimate ion pair, intermediate thus seems more likely for this particular case. 

The reported analyses of the products suggested that the COOCH2CH3 substituent provided anchimeric assistance for the reaction paths 2 and 3. The mechanism was explained in terms of an intramolecular solvation of the bromide ion through an intimate ion-pair intermediate which decomposes in two different directions (equation 74, where X = Br and CH3CH2 replaces CH3). The formation of bromobutyric acid and ethylene (path 3) indicated a normal six-membered transition state for their formation as found in primary ethyl esters pyrolyses in the gas phase. Bromobutyric acid, which is known to be unstable at room temperature, rapidly produced butyrolactone. The consecutive reaction of path 4, under the experimental conditions, was ascribed to a similar mechanism where an intimate ion-pair intermediate is formed through COOH participation (equation 76). 

Evidently, on changing the ethyl ester to methyl ester in bromoacids, the pyrolytic eliminations will be limited only to a debromination process. In this respect, Table 21 describes the results on the pyrolysis of methyl bromoesters under maximum inhibition170 and the data provide further evidence for COOCH3 participation in the elimination from methyl co-bromoesters. By analogy with previous papers16 168,169 the mechanisms of these reactions were also explained in terms of an intimate ion-pair intermediate as for equation... 

Deuteriated neopentyl chloride -a,a-d2was pyrolyzed under maximum inhibition of cyclohexene at 445 °C. 2-Methyl-1-butene-3,3-d2 and 2-methyl-2-butene-3-d were found to be unequivocally the products of a rearrangement process183. Consequently, the reaction mechanism is consistent with involvement of an intimate ion-pair intermediate (equation 94). 

Path 3 is a low-temperature decomposition path which is significantly favored over path 4 (when both paths are possible) in polar solvents. In non-polar solvents, variation in extent of paths 3 and 4 depend on a variety of factors such as the specific base, the solvent, and the anion. The products resulting from this path may also be formed via the rearrangement of an intimate ion-pair intermediate (path 2). It is difficult to separate these extreme cases from product analysis alone. 

Intimate Ion Pair Intermediates in the Solvolysis of Thio Addition Products of NAD(P) Analogs and Their Relevance to the Chemistry of 3-Phosphoglyceraldehyde Dehydrogenase... 

Winstein suggested that two intermediates preceding the dissociated caibocation were required to reconcile data on kinetics, salt effects, and stereochemistry of solvolysis reactions. The process of ionization initially generates a caibocation and counterion in proximity to each other. This species is called an intimate ion pair (or contact ion pair). This species can proceed to a solvent-separated ion pair, in which one or more solvent molecules have inserted between the caibocation and the leaving group but in which the ions have not diffused apart. The free caibocation is formed by diffusion away from the anion, which is called dissociation. 

If it is assumed that ionization would result in complete randomization of the 0 label in the caihoxylate ion, is a measure of the rate of ionization with ion-pair return, and is a measure of the extent of racemization associated with ionization. The fact that the rate of isotope exchange exceeds that of racemization indicates that ion-pair collapse occurs with predominant retention of configuration. When a nucleophile is added to the system (0.14 Af NaN3), k y, is found to be imchanged, but no racemization of reactant is observed. Instead, the intermediate that would return with racemization is captured by azide ion and converted to substitution product with inversion of configuration. This must mean that the intimate ion pair returns to reactant more rapidly than it is captured by azide ion, whereas the solvent-separated ion pair is captured by azide ion faster than it returns to racemic reactant. 

Racemization, however, does not alwiys accompany isotopic scrambling. In the case of 5ec-butyl 4-bromobenzenesulfonate, isotopic scrambling occurs in trifluoroethanol solution witiiout any racemization. Two mechanisms are possible. Scrambling may involve an intimate ion pair in which the sulfonate can rotate with respect to the caibocation without allowing migration to die other face of the caibocation. The alternative is a concerted mechanism, which avoids a caibocation intermediate but violates the prohibition of front-side displacement. ... 

Stabilization of a carbocation intermediate by benzylic conjugation, as in the 1-phenylethyl system shown in entry 8, leads to substitution with diminished stereosped-ficity. A thorough analysis of stereochemical, kinetic, and isotope effect data on solvolysis reactions of 1-phenylethyl chloride has been carried out. The system has been analyzed in terms of the fate of the intimate ion-pair and solvent-separated ion-pair intermediates. From this analysis, it has been estimated that for every 100 molecules of 1-phenylethyl chloride that undergo ionization to an intimate ion pair (in trifluoroethanol), 80 return to starting material of retained configuration, 7 return to inverted starting material, and 13 go on to the solvent-separated ion pair. 

This aminium radical salt in aqueous solution in the form of solvated radical salt is very stable and will not polymerize acrylonitrile even with CeHsCOONa to form the corresponding benzoate. Therefore, we believe that in the nucleophilic displacement, there must be some intermediate step, such as intimate ion pair and cyclic transition state, which will then proceed the deprotonation to form the active aminium radical ion [14], as shown in Scheme 1. The presence of the above aminomethyl radical has also been verified [15] through ultraviolet (UV) analysis of this polymer formed such as PAN or PMMA with the characteristic band as the end group. 

Formation of an intimate ion pair of OH " and aminium radical cation was also proposed for the intermediate step before deprotonation. The presence of the above radical was verified through UV analysis of the polymer formed with the characteristic band on the end group. Through chromatographic analysis of the TBH-DMT reaction products, H2O was detected as the above mechanism proposes after deprotonation. 

The ethylene bromonium and 1-bromoethyl cations and their neutral and anionic counterparts have been the subject of a tandem mass spectrometric study of dissociation and gas-phase redox reactions. IR and Raman studies of the bioactive bromonium cation (19), as its hydrogensulfate salt, agree with the results of an X-ray structure determination, and theoretical calculations are also in agreement, except for the details of the NO2 groups. The azaallenium ion (22) is an intermediate in the photolysis of (20) (21) and (22) could both be seen. Flash photolysis of (23) leads to (24), (25), and (26), all of which could be trapped by nucleophiles (27) was not an intermediate. NMR lineshape analysis of the spectmm of (28) leads to reaction rate constants of formation for both the intimate ion pair (29) and the solvent-separated ion pair (30). ... 

Inspection of Fig. 14 provides a naive, but nonetheless interesting explanation, as to why ion-pair intermediates may form in SN1 processes. One can see that after the transition state is overcome, a minimum on the reaction surface is reached. Since this minimum is part of the ionic configuration it would properly be described as an ion-pair intermediate. Thus the reason why R—X may pass through an intermediate called a contact or intimate ion-pair (Winstein et al., 1965) on the pathway toward full dissociation becomes more readily discernible. 

The photochemical pathway is indicated by the light arrows (Fig. 17) and involves excitation of the charge transfer complex DA up to the D+A surface. The reaction complex then funnels down onto the ground state surface so as to follow the thermal route. Clearly, both pathways involve formation of the ion-pair intermediate, and are intimately related. We see therefore that the CM model constitutes a useful framework for understanding the relationship between ground state and excited state reactions. 

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