Ion pair return

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. 

The ion-pair return phenomenon can also be demonstrated by comparing the rate of loss of enantiomeric purity of reactant with the rate of product formation. For a number of systems, including 1-aiylethyl tosylates, ftie rate of decrease of optical rotation is greater than the rate of product formation. This indicates the existence of an intermediate that can re-form racemic reactant. The solvent-separated ion pair is the most likely intermediate in the Winstein scheme to pl this role. 

Solvolysis of either 178a or 178b in 80% aqueous ethanol in the presence of p-toluenethiolate or benzylthiolate gives a 1 1 mixture of cis and trans products. A 1 1 ds trans product ratio also was observed in acetic acid in the presence of either sodium acetate or silver acetate. The 1 1 product ratio was observed as early as 10% reaction and remained constant throughout the reaction. The acetolysis is accompanied by extensive ion-pair return, which causes cis trans, 178a 178b, isomerization. However, this isomerization... 

TIGHT ION PAIRS LOOSE ION PAIRS SOLVENT-SHARED ION PAIR SOLVENT-SEPARATED ION PAIR Ion pair return,... 

EXTERNAL ION-PAIR RETURN HIDDEN RETURN Ion permeation across membrane, PERMEABILITY CONSTANT PERMEABILITY COEFFICIENT ION PUMPS... 

Excess of rate of racemization over rate of product formation supports the idea of ion pairs that can return to substrate nevertheless, as it is possible that some, or even most, of the ion pairs return without racemizing (Scheme 4), considerable doubt remains about kt, the rate of formation of the ion pairs. The difficulty is that there is no way to detect the event represented by kt if it is followed immediately by k t. We shall know that something has happened only when kt is followed by kT or by kv. If k t competes with these processes, some ionization will go undetected. 

Scheme 5 depicts Winstein s complete solvolysis mechanism.29 Ion-pair return can be from the intimate ion pair (ion-pair return or internal return), from the external ion pair (external ion-pair return), or from the free ions (external ion return). The term external return refers to the sum of external ion-pair return and external ion return. The special salt effect operates by diversion of the external.ion pair, probably through the mechanism shown in Equation 519, so that it can no longer... 

In certain favorable cases of rearranging systems, the occurrence of internal return distinct from external ion-pair return can be demonstrated without recourse to optical rotation or isotopic labeling experiments. See S. Winstein and A. H. Fainberg, J. Amer. Chem. Soc., 80, 459 (1958) S. Winstein, P. E. Klinedinst, Jr., and E. Clippinger, J. Amer. Chem. Soc., 83, 4986 (1961). 

Internal Ion Pair Return—collapse of a contact ion pair to covalent material, sometimes referred to as hidden return if the covalent material is formed without scrambling of isotopic label or without partial racemization ... 

Section 3, p. 36). Also, the theoretical definition does not specify the stereochemistry of the products, which are very rarely (if ever) found to be the 50/50 mixture of inverted and retained configurations predicted by classical theory (Doering and Zeiss, 1953 Okamoto et al., 1975a, b Bone et al., 1975). We shall use the term Sn 1 to refer to mechanisms in which k j is rate determining and the term classical SN 1 to refer to the mechanism (possibly defunct) in which rate-determining formation of a free carbocation is envisaged. However, kx (Fig. 2) is only rate determining if k2 > k l when internal ion pair return is important (k2 < k x) a more complex mechanistic notation is required (e.g. for solvolysis of t-butyl chloride in trifluoroethanol, p. 36). 

Whilst these results cast considerable doubt on Shiner s evidence for appreciable internal ion pair return, it should be emphasized that Shiner s interpretation is also based on a-deuterium isotope effects, which are discussed and criticized below. 

There is also relatively direct evidence for internal ion pair return from 1 sO-scrambling studies [eqn (3)] during solvolyses of simple... 

Other evidence for ion pair return is less relevant to solvolyses of simple secondary substrates. It is known that during solvolyses of certain optically active substrates racemization occurs more rapidly than solvolysis, but it is not known whether the ion-pair intermediates undergoing racemization are the same as those undergoing solvolysis (Hammett, 1970a). Such behaviour is usually observed in solvolyses where neighbouring group participation occurs and the intermediates are probably more stable than those from simple secondary solvolyses. As the stabilities of the intermediates increase, there appears to be a general trend towards formation of more dissociated species,4 and thus the relevance of these results is questionable. 

The remarkable agreement in the above results surprised Bunton and Del Pesco (1969) because, as they pointed out, no allowance was made for the temperatures of the two processes, and for the presence of solvent and ion-pair return processes. From extensive studies of structurally related methyl norbornyl chlorides (e.g. 19), they concluded... 

The reaction was thought to involve an ion-pair return and two possible mechanisms were envisaged, with either step-wise or synchronous fission of the C-O and C-S bonds. Currently available data favour the latter type of mechanism. 

Typical product-forming behavior for solvolyses of cyclopropylidenemethyl derivatives is that below. The l-bromo-2-methylcyclobutene results from ion-pair return. 

Even for reactions in which the Sij2 contribution to ionization is negligible, one does not have a means of estimating from solvolysis rates, the solvent activity coefficients for the transition state corresponding to ionization of RX. Although Vrx easily found from Henry s Law constants, and equation (16) does produce an activity coefficient for some transition state, this may not be a simple transition state corresponding to ionization of RX. Solvolysis rates may be smaller than ionization rates of many compounds, in certain solvents, because of ion-pair return, a phenomenon which has been firmly established by the investigations of Winstein et al. (1965). No matter whether Ag is a titrimetric rate constant 7... 

An important feature of these reactions is that acyloxy migration occurs intramolecularly. Moderate (30-70% ee) induction of asymmetry at the a-carbon is observed. In fact, Pummerer reactions in which chiral transfer occurs are almost exclusively restricted to this category. The very tight nature of the a-thiocarbenium-acetate ion pair and very rapid recombination (ion pair return) are considered to be two... 

If the ions are paired as a loose ion pair and form the covalent chemical species via a tight ion pair, this is called external ion-pair return ... 

When the covalent molecule RZ is re-formed without direct evidence of prior partial racemization or without other direct evidence of prior formation of a tight ion pair (e.g., without partial racemization if the group R is suitably chiral), the internal ion-pair return is sometimes called a hidden return. 

External (unimolecular) ion-pair return should be distinguished from external (bimolecular) ion return, the (reversible) process whereby dissociated ions are converted into loose ion pairs ... 

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