Transition states solvolysis

The problem of norbomyl cation stabilities vs. solvolysis rate discrepancies in the norbomyl system has been addressed in an important paper.159 The classical and non-classical norbomyl cations do not resemble the 2-endo- and 2-exo -norbomyl solvolysis transition states very closely. The authors conclude that Brown was wrong, but that Winstein was not entirely right either.159 A substituent in the benzene ring has little effect upon the kinetics of the acid-catalysed hydrolysis of 2-exo-norbomyl phenyl ether.160 The FTIR spectra of matrix-isolated 2-methylbenzonorbomen-2-yl cations have been examined at —196 °C the structure can best be represented as (108), rather like a phenonium cation, but at higher temperatures a transition takes place to a structure that is more nearly represented as (109), with some 71-bridging.161 The stereoselectivities of some 7-methyl-7-norbom(en)yl cations have been investigated (110) has a classical structure and reacts in a stereo-random manner, whereas (111) is... 

The identity of the empirical r value for the solvolysis transition state with that for the corresponding gas-phase cation indicates that the structures of the transition states in benzylic solvolyses should be essentially constant and that... 

The characteristic change of the r value in the solvolysis reaction of benzylic precursors and for the corresponding carbocations should provide important information concerning the solvolysis transition state. The r value, reflecting the TT-delocalization within the cationic species, appears to remain essentially the same in solution as in the gas phase, and the charge delocalization in the transition state of the solvolytic ionization should be close to that in the carbocation intermediate. Advanced ab initio molecular orbital calculations can be used to And the underlying relationship between quantum chemical quantities and experimental r values, and the relation between r values and theoretical indices provides a basis for the physical meaning of the r parameter (Nakata ei ai, 1996, 1998, 1999). 

The generally observed identity of the r value for solvolysis reactivity and gas-phase stability AAG(c+> of the corresponding carbocation leads to an important prediction concerning the solvolysis transition state. In a typical (limiting) two-step SnI mechanism with a single dominant transition state, the r values of transition states for the various nucleophile-cation reactions should be essentially controlled by the intrinsic resonance demand of the intermediate cation the substituent effect should be described by a single scale of substituent constants (a) with an r value characteristic of this cation. In a recent laser flash-photolysis study (Das, 1993) on the recombination of stable trityl and benzhydryl cations with nucleophiles and solvents, McClelland et al. (1986, 1989) have treated the substituent effects on solvent-recombination processes by (2). 

In agreement with these considerations, a fairly good correlation between the rate constants for reaction (Eq. 21) and the solvolysis rates of the addition products has been observed (Fig. 13). Taking into account that approximately 90% of carbocationic character is manifest in the solvolysis transition states [175], the slope of the correlation in Figure 13 (slope x 2.3 RT = 0.45) indicates that roughly half of the character of the new carbocations is developed in the transition states of the addition reactions. 

We first applied the solvatochromic equation (SCE) to solvolysis of tert-butyl chloride (t-BuCl) to determine if the method could give a reasonable result for this much-studied reaction (7). Abraham et al. (11) had previously attempted correlation of these rates with the SCE without the cavity term, but as Bentley and Carter (12) have noted, an unsatisfactory result was achieved (7). First, TFE and hexafluoroisopropyl alcohol (HFIP) did not fit the correlation. Second, no rate dependence on solvent nucleophilicity 0 was found, despite other works indicating a weak dependence on this parameter (12, 13). Also, different correlations were observed for hydroxylic and nonhydroxylic solvents Bentley considered this finding to indicate that the dehydrohalogenation transition state (in nonhydroxylic solvents) and the solvolysis transition state (in hydroxylic solvents) were significantly different and thus concluded that the two types of reactions should not be included in the same correlation. 

In 1977 Olah studied the characteristics of the NMR spectra of the same series of ions. Fig. 7 presents a graph of 5C depending on 8C the observed break in the slope is accounted for by an increase in the contribution of the C —C bond a-participation to the charge delocalization with increasing electron demand at C. Of similar character is the plot of 80 vs The character of the latter dependence does not change qualitatively when use is made ofa constants reflecting an increased demand of stable ions in comparison with solvolysis transition states. 

The Y values determined in this way are empirical measures of the solvent s ability to accommodate formation of the dipolar solvolysis transition state. Table 4.5 lists the y values for some alcohol-water mixtures and for some other solvent systems. 

Table III. Comparison of AG values for the t-Butyl Chloride Solvolysis Transition State with Values for Polar Species(34, 35,37)...

Gajewski, J.J., Is the tert butyl chloride solvolysis the most misunderstood reaction in organic chemistry Evidence against nucleophihc solvent participation in the tert butyl chloride transition state and for increased hydrogen bond donation to the 1 adamantyl chloride solvolysis transition state, /. Am. Chem. Soc., 2001, 123(44), 10877-10883. 

The bond between a-carbon and deuterium is not broken in the solvolysis transition state, but the presence of deuterium, bonded to the reaction center, causes the reaction rate decrease. Isotope effects such as in (1.14.20) and (1.14.21) are called secondary kinetic isotope effects, to be clearly distinguished from primary isotope effects. Secondary isotope effects were defined as rate effects caused by isotope substitution on the bond not broken in the rate-determining step. 

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