Internal ion pair

Br than was the yield of the />ara-ester, 94. The differential effects of Br on the yield of these two products led to consideration of the mechanism of Scheme 42, in which three ion pair intermediates are invoked. Variation in the yields of the para-products as a function of [H ] and [Br ] led to estimates of the lifetimes for 95c of ca. 21 ps and for 96c of 0.25-0.5 ns. The estimated lifetimes suggest that both 92 and 95 should be thought of as tight ion pairs while 96 appears to be a solvent-separated ion pair. Internal return of 96 to the para-ester, 94, amounts to about 20% of the fate of 96 in the absence of Br, according to the fitting procedure, so this is apparently not an insignificant path for this ion pair. Curiously, 96 has about the same lifetime one would expect for the free ion, and it is not clear what role, if any, the free ion plays in this reaction. The authors did not include a preassociation path which would have accounted for some of the Br derived products. 

The kinetics of solvolysis of optically active cis- and /raAZ5-5-methyl-2-cyclohexenyl chlorides provides additional insight into the ion-pair internal return mechanism of allylic isomerization. Solvolysis of both of these chlorides in ethanol and acetic acid is accompanied by loss of optical activity without cis-trans isomerization of the unreacted chloride [(12), Y = C1]. The fact that rate of loss of optical activity exceeds rate of formation of solvolysis products provides evidence for internal return from carbonium chloride ion pairs, unaccompanied by geometrical isomerization. This conclusion is supported by the observation that the ratio of racemization rate to solvolysis rate is insensitive to added ionic chloride. 

In protic solvents, the allylic carbonium ion formed by acid-catalyzed alkyl carbon-oxygen bond fission can recombine either with the carboxylic acid molecule or with a solvent molecule. The electrostatic attraction between the carbonium and carboxylate ions, which is a major factor in isomerization of allylic esters by ion-pair internal return during solvolysis, is absent in the acid-catalyzed reaction. The more numerous, usually more nucleophilic, solvent molecules in the solvation shell of the carbonium ion should compete effectively with the departed carboxylic acid molecule and solvolysis rather than isomerization should be the predominant reaction. For example, in the presence of 0.05 M perchloric acid, solvolyses of cis- and //- //7.s-5-methyl-2-cyclohexenyl p-nitrobenzoates are not only very much faster than in the absence of the acid, but polarimetric and titrimetric rates of solvolysis of optically-active esters were identical within experimental error. For these esters, the acid-catalyzed solvolysis was not accompanied by a detectable amount of isomerization. Braude reported, on the basis of indirect evidence, that isomerization accompanies acid-catalyzed hydrolysis of a-ethynyl-y-methylallyl acetate in aqueous dioxane. It was shown that, under some experimental conditions, the spectrophotometrically determined rate of appearance of the rearranged 1 -yne-3-ene chromophore exceeds the titrimetrically determined rate of hydrolysis,... 

Cavalli R, Morel S, Gasco M, Chetoni P. Preparation and evaluation in vitro of colloidal lipospheres containing pilocarpine as ion pair. International Journal of Pharmaceutics. 1995 117(2) 243-246. 

Studies of the stereochemical course of rmcleophilic substitution reactions are a powerful tool for investigation of the mechanisms of these reactions. Bimolecular direct displacement reactions by the limSj.j2 meohanism are expected to result in 100% inversion of configuration. The stereochemical outcome of the lirnSj l ionization mechanism is less predictable because it depends on whether reaction occurs via one of the ion-pair intermediates or through a completely dissociated ion. Borderline mechanisms may also show variable stereochemistry, depending upon the lifetime of the intermediates and the extent of internal return. It is important to dissect the overall stereochemical outcome into the various steps of such reactions. 

The kinetic method of determining relative acidity suffers from one serious complication, however. This complication has to do with the fate of the ion pair that is formed immediately on removal of the proton. If the ion pair separates and difiuses into the solution rapidly, so that each deprotonation results in exchange, the exchange rate is an accurate measure of the rate of deprotonation. Under many conditions of solvent and base, however, an ion pair may return to reactants at a rate exceeding protonation of the carbanion by the solvent. This phenomenon is called internal return ... 

The species R X is called an internal, contact, or intimate ion pair, and R (s),X (s), sometimes symbolized R X , is an external or solvent-separated ion pair. 

We have previously discussed the possibilities of racemization or inversion of the product RS of a solvolysis reaction. However, the formation of an ion pair followed by internal return can also affect the stereochemistry of the substrate molecule RX. Cases have been found where internal return racemizes an original optically active RX, an example being solvolysis in aqueous acetone of a-p-anisylethyl p-nitrobenzoate, while in other cases partial or complete retention is found, for example, solvolysis in aqueous acetone of p-chloro benzhydryl p-nitrobenzoate. the pathway RX R+X some cases where internal return involves racemization, it has been shown that such racemization is faster than solvolysis. For example, optically active p-chlorobenzhydryl chloride racemizes 30 times faster than it solvolyzes in acetic acid. ... 

The product ratios were identical within experimental error from pure 179a and 179b. The products were shown to be stable under the reaction conditions. A small amount of cis tram reactant isomerization, 179a =5 179b, presumably via ion pairs and internal return, was observed this factor, however, was ruled... 

Okada et al. have found that internal mohilities are strongly related to the separating motion of unlike ion pairs defined by the self-exchange velocity, which can be easily calculated from MD simulation ... 

Part 24 Determination of maleic acid and maleic anhydride in food simulants Ion pair HPLC of maleic acid with cetyl trimethyl ammonium chloride and UV detection (245 nm) with citraconic acid as internal standard... 

Radon daughter ions and the ionization caused by the decay chains of radon and thoron in indoor air play important roles both from the contribution made by the daughter product positive ions to internal dose and from the effects of ion-pair production on the indoor atmospheric electrical parameters. 

Mozumder and Magee, 1967), the success of the geminate pair model in kinetics (Warman et al., 1969 Rzad et al, 1970) and in free-ion yield determination (Hummel and Allen, 1966 Freeman, 1963a, b) has been rationalized on the basis that in multi-ion pair spurs all but the final e-ion pair would quickly neutralize due to intense internal coulombic interaction. Therefore, emphasis is laid in this section on the geminate pair with two caveats ... 

Hardy effect.248-249 The internal return part of the ionization equilibrium is particularly hard to detect since it is almost completely independent of the concentration of anything in the bulk of the solution outside of the solvent cage. The extent of internal return will depend on the reactivity of the cage walls and their resistance to the escape of either ion. Unless internal return has been eliminated by the use of an extremely reactive cage wall, the measured rate is not that of the ionization but the lesser rate of ion pair dissociation. In the case of the acetolysis of a, a-dimethylallyl chloride (XXXIX), internal return is detectable by virtue of the fact that the chloride ion can return to either of two allylic carbon atoms.248... 

A y-lactone was formed in excellent yield by the nucleophilic cyclization of a carboxylic acid onto an alkene radical cation generated from a (i-nilrophosphale under tin hydride conditions (Scheme 21) [139]. Related experiments employing the acetate group and an internal carboxylate nucleophile failed, emphasizing the very rapid collapse of the alkene radical cation/acetate ion pair [127]. 

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