Addition of solvent to carbocation-anion pairs

The value of (fcobsd — kc.dc) at a given concentration of bromide anion depends on the association constant for formation of the ion pair from free ions (Aias = fcd/fc d), and on the relative reactivity of the ion-pair and free carbocation toward addition of solvent (k Jks). For example, if Kas is small, then the concentration of the ion-pair intermediate will be low and its reaction will make a small contribution to fcobsd or, if (k jks) is small then the ion pair intermediate will be unreactive compared with the free cation.32 Equation (7) shows the relationship between the experimental parameters, K s and (ks/kls) where fcobsd — Kale = (3 1) X 10 5s 1 (equation (6)) ksolv = 0.049 s-1 [(fcsolv = kik-d/(k-i + fc d)] kBl/ks = 77 M-1 (kBl/ks = kdk-,/ [ks(k-1 + k-d)], see equation (4)) and = kd/k-d for formation of an encounter complex between 1+ and Br-. 

The value of 0.05 M from equation (7) is consistent with values of K s 1 and ( /fcs) 1 for reactions in water. For example, K s = 0.3 M20 gives (k jks) = 0.17 for the relative rate constants for addition of solvent to the carbocation-anion pair and free carbocation. By comparison, the three-fold smaller rate constant for addition of water to an intramolecular trityl carbocation-sulfonate ion pair compared with addition to the analogous substituted trityl carbocation o-sulfonyl methyl ester has been used to estimate a value of (ks/k s) = 0.33.34 

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A Global scheme for solvolysis 2 Clocks for reactions of ion pairs 3 Addition of solvent to carbocation-anion pairs i Protonation of a carbocation-anion pair 11 Isomerization of ion pair reaction intermediates Reorganization of ion pairs in water 13 Internal return of isotopically labeled ion pairs Racemization of ion pairs 22 Concluding remarks 24 Acknowledgements 24 References 24... 

The addition of solvent to a carbocation paired with its leaving group anion (fcjf and Scheme 2). The effect of direct addition of solvent to an ion-pair reaction intermediate is to cause the rate of solvolysis to become faster than for a reaction where products form exclusively by addition of solvent to the unpaired carbocation (ks), and it is possible to detect this as a deviation from a rate law for the latter reaction. 

The addition of solvent to a carbocation paired with its leaving group anion... 

The addition of water to a free carbocation intermediate of solvolysis can be distinguished from addition to an ion-pair intermediate by an examination of common ion inhibition of solvolysis. Common leaving group inhibition of solvolysis is observed when the leaving group ion (X ) acts, by mass action, to convert the free carbocation (R , Scheme 5A) to substrate (R-X). This results in a decrease in the steady-state concentration of R that leads directly to a decrease in the velocity of solvolysis. Some fraction of the solvolysis reaction products form by direct addition of solvent to the carbocation-anion pair intermediate. The external... 

The solvolysis of MeS-4-0(S)CPh in aqueous/trifluoroethanol at 25 °C is zero-order in [N ].42 Fig. 4A shows that formation of MeS-4-N3 occurs at the expense of MeS-4-OSolv but not MeS-4-S(0)CPh. The reaction of MeS-4-(3,5-dinitrobenzoate) under the same conditions proceeds through the liberated carbocation intermediate MeS-4+ which partitions between addition of solvent to form MeS-4-OSolv and azide anion to form MeS-4-Nj.14 Fig. 4B shows that the yields of MeS-4-Nj, calculated as a fraction of the sum of the yields of MeS-4-Nj and MeS-4-OSolv, from the reactions of MeS-4-0(S)CPh and MeS-4-(3,5-dinitrobenzoate) are identical. These data show that essentially 100% of the ion-pair intermediate of reaction of MeS-4-(3,5-dinitrobenzoate) undergoes separation to the free carbocation which is trapped by azide ion, while 14% of the reaction of MeS-4-0(S)CPh proceeds by this pathway and 86% proceeds by the competing isomerization reaction.42... 

The carbocation intermediate must exist in water long enough to allow for racemization of the carbocation-anion or ion dipole pair to occur, so that k < 1 X 10 s for addition of solvent (Scheme 15). In the case of solvolysis of (R)-l-phenylethanol in H2 0, which proceeds by a stepwise mechanism through a 1-phenylethyl carbocation intermediate that is captured by water... 

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