External ion pair return

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

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). 

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 ... 

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 ... 

Interestingly, the order of the fractions of ion pairs returned from the 17 X ion pairs (0.47, 0.38, and 0.24 for X = Br , Cl , and OMs , respectively), which reflects the intramolecular nucleophilicity in the ion pair, is parallel to the a values (74). The order of return from ion-paired X or from external X-seems therefore to be governed by similar factors. 

The overall consumption rate of the covalent precursor (ks) is determined by HPLC and/or titration measurements this correlates with monomer consumption in propagation. The rate of racemization of optically active 1-phenylethyl chloride (ka) is determined by polarimetric measurements, Racemization is usually faster than solvolysis, confirming that activation is reversible and that internal return may occur before the carbenium ion reacts with an external nucleophile, Racemization requires not only that the C—Cl bond of the covalent precursor is broken, but that the lifetime of the ion pair is long enough for the flat carbenium ion to rotate, such that both sides of the carbenium ion are completely equivalent as shown in Eq. 18. 

In the alcoholysis of 1-phenylethanediazotate (149), reaction with the external nucleophile affords 1-alkoxy-l-phenylethane (7 JO) (44-64%) whereas return of the hydroxide ion gives 1-phenylethanol (151) (18-29%). When optically active (149) is employed, the ether (150) is formed with predominant inversion and the alcohol (151) with high retention of configuration142,143. The data on solvent and salt effects suggest that both products arise from an ion pair. Similar results have been... 

The solvolysis and isomerization of a,a-dimethylallyl chloride in aqueous 75% ethanol is more complex In this solvent, the rate of isomerization at 25°C is doubled by the addition of molar LiCl. In the presence of 0.68 M Li Cl, about 63% of the chlorine incorporated into the primary chloride during rearrangement came from radioactive chloride in solution. Thus, under these conditions isomerization involves external return of chloride from solution as well as internal return from a tight ion pair intermediate. a-Phenylallyl chloride also undergoes simultaneous solvolysis and allylic... 

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