Solvolytic rate constants

Region Arene Oxides of Phenanthrene. Comparative Solvolytic Rate Constants of K-Re-gion and Non-K-Region Arene Oxides , J. Am. Chem. Soc. 1976, 98, 2965 - 2973. 

Solvolytic rate constants of tertiary cyclopropylcarbinyl / -nitrobenzoates 129 and 130 revealed that each cyclopropyl group causes a rate enhancement of the order of 102—103... 

Figure 10. Correlation of solvolytic rate constants for trans-2-bromocyclohexyl p-bromobenzenesulphonate [43] at 50° and Y (data from Winstein et al., 1951).

Figure 11. Correlation of solvolytic rate constants for 1-adamantyl bromide (45, X = Br) at 25° and Y. (From Raber et al., 1970, and reproduced by permission of the American Chemical Society.)...

Figure 15. Correlation of solvolytic rate constants for bicydo[2,2,2] octyl tosylate [49] at 25° and YqTs (data from Morten, 1975).

Swain et al. (1953b) noted that a qualitative relationship exists between the stability of a carbocation and its selectivity. For example, the selectivity of a number of carbocations in aqueous solution and in the presence of azide ion was enhanced with increasing carbocation stability the ratio An /Aw, where An and Aw are the specific rate constants with azide ion and water respectively, was found to increase from 3-9 for the t-butyl cation to 170 for the diphenylmethyl cation, to 240 for the 4,4 -dimethyldiphenylmethyl cation, and to 280,000 for the highly stabilized triphenylmethyl cation. Sneen et al. (1966a) observed that this relationship could be quantified. It was found that a plot of log (An /Aw ) against log A (where A is the solvolytic rate constant) for a number of alkyl chlorides gave a linear correlation. Sneen made the first attempt to utilize such a relationship as a mechanistic tool. The selectivity of... 

Certain limitations were noted however. First, the plot did not predict the zero value of selectivity for highly unstable ions, expected from the reactivity- selectivity principle. Secondly, as discussed on p. 77, solvolytic rate constants are only a very approximate measure of substrate reactivity. Furthermore, the possibility exists of solvent sorting, in which the concentration of potential nucleophiles around the reaction intermediate differs from that in the bulk solution. Such a phenomenon would, of course, seriously diminish the significance of selectivity as a measure of reactivity, and, while such a possibility... 

The solvolysis of alkyl chlorides and aralkyl p-nitrobenzoates in phenol, an ambident nucleophile, gave C- and O-alkylated phenols. The logarithm of the ratio of product concentrations was found to be a linear function of the logarithm of the solvolytic rate constant for the particular substrate. 

Traditionally, relative stabilities of carbocations have been derived from the comparison of the rates of solvolysis reactions following the SN1 mechanism, for which the designation Dm + An has recently been proposed [36], The comparison of solvolytic rate constants for substrates of a large structural variety is hampered by the fact that the published solvolysis rates refer to different solvents, different temperatures, and precursors with different leaving groups. Dau-Schmidt has, therefore, converted solvolysis rates of a manifold of alkyl chlorides and bromides to standard conditions, i.e., soiv of RC1 in 100% EtOH at 25° C (Scheme 6) [37]. Although from a theoretical point of view, ethanol is not an ideal solvent for observing unassisted SN 1-type reactions (nucleophilic solvent participation), it has been selected as the reference solvent because most available experimental data have been collected in solvents of comparable nucleophilicity, a fact which made conversions to 100% ethanol relatively unproblematic [38],... 

TABLE 6. Charge transfer wavelengths of PhR with TCNE in CH2CI2 and solvolytic rate constants for RCH2X"... 

As seen in Table 6 there is a correspondence between the solvolytic rate constants and the charge transfer frequencies. It is notable that the more highly substituted cyclopropyl rings show enhancements in both the rate constants and the CT frequencies. 

The preparation of the l-aryl-2,2,2-trifluoroethyl tosylates I—III and the mesylate IV was described (11-16) as well as the solvolytic rate constants of these derivatives in a range of solvents (11-16). Solvolytic and polarimetric rate constants were also obtained for optically active 1-phenyl-2,2,2-tri-fluoroethyl triflate Va and solvolytic rate constants for the a-deuterated analogue Vb (11). Solvolytic and polarimetric rate constants were also measured for optically active Vla-c (17). 

For comparative purposes, the solvolytic rate constants for I-VI at 25 °C in hexafluoroisopropyl alcohol (HFIP) are given in Table I, together with the m values, which measure the dependence of the rates on the solvent ionizing power parameter OTs by the equation log (k/k0) = raY0Ts (3, 6). 

The solvent may also drastically affect the anchimerically assisted fraction in nucleophilically assisted ionization reactions. As the solvent becomes more nucleophilic, there is greater solvent participation, if possible, at the expense of neighboring group assistance. The data in Table 4 are illustrative of this effect. It is obvious that in both formic acid and acetic acid there is considerable MeO-5 and MeO-6 nucleophilic participation. In ethanol, however, the ratio kjk is diminished as solvent attack (fcj becomes more effective. These effects may be quantitatively assessed. Winstein et alP used a Taft treatment for the system RCH2OBS in estimating the kj portion of the solvolytic rate constant [Eq. (14)]. Values for K/K where /c, is the titrimetrically observed rate constant, were determined. 

Charton used his LDR and related equations in the correlation analysis of solvolytic rate constants for 2-exo- and 2-cndb-norbomyl derivatives [(7) and (8), respectively], substituted in positions 1, 4, 5-exo, 6-exo, 6-endo, or 1-anti A delocalized (resonance) electronic effect was found for all but the 4-substituted derivatives. It was suggested that the transition state resembles a classical carbocation rather than the nonclas-sical norbornyl carbocation. Similar treatments for adamantyl and bicyclo[2.2.2]octyl systems found much less indication of a delocalized effect. 

---