Solvolysis, definition

A plot against Hammett s cr-constants of the logarithms of the rate constants for the solvolysis of a series of Mz-substituted dimethylphenylcarbinyl chlorides, in which compounds direct resonance interaction with the substituent is not possible, yielded a reasonably straight line and gave a value for the reaction constant (p) of — 4 54. Using this value of the reaction constant, and with the data for the rates of solvolysis, a new set of substituent parameters (cr+) was defined. The procedure described above for the definition of cr+, was adopted for... 

Probably the most important development of the past decade was the introduction by Brown and co-workers of a set of substituent constants,ct+, derived from the solvolysis of cumyl chlorides and presumably applicable to reaction series in which a delocalization of a positive charge from the reaction site into the aromatic nucleus is important in the transition state or, in other words, where the importance of resonance structures placing a positive charge on the substituent - -M effect) changes substantially between the initial and transition (or final) states. These ct+-values have found wide application, not only in the particular side-chain reactions for which they were designed, but equally in electrophilic nuclear substitution reactions. Although such a scale was first proposed by Pearson et al. under the label of and by Deno et Brown s systematic work made the scale definitive. 

Besides the work done on solvolysis of 2-norbomyl compounds, the 2-norbomyl cation has also been extensively studied at low temperatures there is much evidence that under these conditions the ion is definitely nonclassical. Olah and co-workers prepared the 2-norbomyl cation in... 

In the course of the tempestuous development of organophosphorus chemistry, interest has only recently been focused on compounds of formally quinquevalent phosphorus having coordination number 3, such as 1, 2, or 3, although one of the other species of this kind has long been postulated as reactive intermediate of solvolysis of phosphorylation reactions. Definite evidence of even proof of the existence of such coordinatively unsaturated phosphorus compounds, however, has been obtained only recently in mechanistic studies, by trapping reactions with suitable cycloaddends, or actually by direct isolation. 

The observation of hidden reactions during solvolysis, through the use of chiral or isotopically labeled substrates has created considerable excitement in communities interested in the mechanisms of nonenzymatic and enzyme catalyzed reactions. These hidden reactions reveal something interesting about reaction mechanisms. However, chemists and biochemists are still working on the problem of extracting simple and definitive conclusions from analysis of data for these isomerization reactions. 

There is an ongoing controversy about whether there is any stabilization of the transition state for nucleophilic substitution at tertiary aliphatic carbon from interaction with nucleophilic solvent." ° This controversy has developed with the increasing sophistication of experiments to characterize solvent effects on the rate constants for solvolysis reactions. Grunwald and Winstein determined rate constants for solvolysis of tert-butyl chloride in a wide variety of solvents and used these data to define the solvent ionizing parameter T (Eq. 3). They next found that rate constants for solvolysis of primary and secondary aliphatic carbon show a smaller sensitivity (m) to changes in Y than those for the parent solvolysis reaction of tert-butyl chloride (for which m = 1 by definition). A second term was added ( N) to account for the effect of changes in solvent nucleophilicity on obsd that result from transition state stabilization by a nucleophilic interaction between solvent and substrate. It was first assumed that there is no significant stabilization of the transition state for solvolysis of tert-butyl chloride from such a nucleophilic interaction. However, a close examination of extensive rate data revealed, in some cases, a correlation between rate constants for solvolysis of fert-butyl derivatives and solvent nucleophicity. " ... 

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). 

For solvolysis of 2-ary 1-2-chloropropanes, r is 1.0 by definition, and this value appears to be approximately correct for pyrolysis of 1-arylethyl acetates. Although the magnitude of the charge is smaller in the latter reaction, resonance stabilization of this charge by the aromatic is proportionally greater because of the absence of the solvent (63T937). Nevertheless, for some very polarizable heterocycles, differences in reactivity be-... 

Reaction according to equation (22) is also distinguishable from that involving vinyl cations but it must be carefully taken into account before reaching a definite conclusion on the solvolysis mechanism (Hanack, 1970). In answer to a paper by Schubert and Barfnecht (1970) which is very critical of the current mechanistic interpretation of some solvolytic reactions of vinyl compounds, Rappoport et al., (1970) gave 20 distinctive criteria for mechanisms of equations (21) and (22), in particular the strong rate dependence upon proton concentration in the reaction media. 

The electrophilic substituent constants, given in Table 1, were defined by a set of apparent substituent constants, i.e. (1/p) log(A /Aro), derived from the solvolysis rates of a,a-dimethylbenzyl(a-cumyl) chlorides [2] (Scheme 1) in 90% aqueous acetone at 25°C. For the definition, the reaction constant p = -4.54, based exclusively on meta and ir-electron withdrawing (tt-EW) para substituents, was applied. 

Besides the work done on solvolysis of 2-norbomyl compounds, the 2-norbornyl cation has also been extensively smdied at low temperatures there is much evidence that under these conditions the ion is definitely nonclassical. Olah and co-workers have prepared the 2-norbomyl cation in stable solutions at temperamres below 150°C in SbFs—SO2 and FSO3H SbF5 S02, where the stmcmre is static and hydride shifts are absent Studies by proton and NMR, as well as by laser Raman spectra and X-ray electron spectroscopy, led to the conclusion that under these conditions the ion is nonclassical. A similar result has been reported for the 2-norbomyl cation in the sohd state where at 77 and even 5 K, NMR spectra gave no evidence of the freezing out of a single classical ion. ... 

The solvolysis of isopropyl p-toluenesulphonate in 50% aqueous acetone shows a value of ACp JAS which is very close to that observed for established reactions and the same mechanism can therefore be assumed. However, a different conclusion is required when the solvent is 85% acetone, a poorer ionising medium, since AC I AS is now a little smaller than would have been expected (see Table 10). These observations and their interpretation are fully consistent with solvolysis near the mechanistic border-line. Completely Sj l solvolysis should therefore also occur in water but the validity of this conclusion cannot be definitely established since AC appears to be independent of mechanism in the solvolysis of sulphonates (see Section IVC4). However, for the hydrolysis of isopropyl methanesulphonate and benzenesulphonate in water is about 10 caldeg. greater than for the Sjj2 reactions of the primary compounds, a difference often found between S l and Sif2 solvolysis, but isopropyl -toluenesulphonate is clearly anomalous (see Table 6). 

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