Isotope effects in solvolysis

In addition to these probes, theoretical calculations are being made in efforts to decide upon the inherently most stable stmcture of the ion. These will be discussed along with the conclusions which have been derived from the study of isotope effects in solvolysis and substituent effects in stabilizing media. 

The magnitude of kinetic isotope effects in solvolysis may be distorted by possible multiple reaction pathways such as competing elimination, participation, ion pairing, rearrangements and other factors which might complicate unequivocal interpretation. Equilibrium isotope effect measurements in... 

That the situation may be even more complex than it seems is suggested by some recent work of Pocker (98a) on the solvolysis of variously deuterated benzhydryl chlorides, also in 80% aqueous acetone.. Comparing the rates of solvolysis, racemization, and radio-chloride exchange, he concludes that solvolysis follows the prior formation of a rather loose ion-pair. Moreover, the isotope effect on solvolysis due to a-deuteration (k /ko = 1.15) is the same as that on the more rapid racemization, suggesting that the isotope effect in solvolysis can nearly be identified with that on an ionization pre-equilibrium. In addition, nuclear deuteration, according both to Pocker and to Klein... 

In TFE the secondary isotope effect for solvolysis ( h/ d3)s is found to have the value 1.46 and that for Na,Np-rearrangement (kH/kD3)r has the value 1.42 (Szele and Zollinger, 1981). The two isotope effects are similar to the largest values observed in nucleophilic aliphatic substitutions following the DN + AN mechanism (Shiner,... 

A number of kinetic /J-deuterium isotope effects in the solvolytic generation of vinyl cations have been measured. Stang and co-workers (193) observed a kn /kp = 1.43 in the solvolysis of 237 in 80% aqueous ethanol at 25° C. This effect is considerably larger than the corresponding j8-deuterium... 

Stang and Hargrove (197) have examined the effect of substituents on the kinetic deuterium isotope effects in the solvolysis of 240 in 80% aqueous ethanol at 50°. The results are shown in Table XVI. The results indicate... 

An excellent review of the use of secondary a and isotope effects in the norbomyl system has recently been written by Scheppele (1972). We shall briefly summarize a few of the factors and conclusions arrived at because of their relevance to the interpretation of other solvolysis data, but the interested reader should turn to that article and the original sources quoted for a more extensive discussion. 

It has been recognized that the solvent isotope effect on solvolysis reactions is a disappointing piece of information (Laughton and Robertson, 1969a Schowen, 1972), since, regardless of mechanism, the substitution of DzO for H20 produces very much the same rate ratio. Some typical results are collected together in Table 19. The puzzling feature is that one would expect Sn2 reactions to have more pronounced solvent isotope effects than SN1... 

Deuterium isotope effects in the solvolysis of 5a-cholestan-3a-yl and -3/3-yl brosylates are interpreted as evidence for a rate-determining ionization step, with ring A in a fairly rigid chair conformation. Product analyses, however, suggest a conformational change towards half-chair in the transition state leading from the carbocation to products of inverted configuration. There is also evidence of a... 

Deuterium isotope effect in the solvolysis of 2-cyano-2-propyl... 

Kinetic 14C isotope effect in the solvolysis of 1,1,1-trifluoro-2-phenyl-2-propy 1-3-uC-p-toluenesulphonate (216)... 

These data, as well as the equality of a-isotope effects in the acetolysis and ethanolysis are considered by the authors to clearly testify to the formation of an intermediate nonclassical norbomyl ion in these reactions. The value of kj,/kp (25 °C) for the endo isomer is independent of the solvent and equal to 1.20 which is typical for the solvolysis of simple secondary sulphonates. This argument favours the absence of steric hindrance to ionization of 2-endo brosylate 23. 

The authors have failed to establish whether hyperconjugation or steric strain relief is decisive for the effects observed with 6,7-dinitro brosylate 26S however, the aryl-group participation is the main reason for the low value of the isotopic effect upon solvolysis of the unsubstituted 2-exo-brosylate 264 (cf Contrary to the previous case in endo epimers the isotopic effects are rather large, sinre the latter solvolyzed without n-participation of the aromatic ring and without effective participation of the solvent in the transition state. Isotopic effects point to weak participation of the solvent in the transition state leading to a classical ion with... 

Supportive of the suggestion that ionization is not a major pathway in the Claisen rearrangement of the parent compound is the fact that the SDKIEs in aqueous solution are comparable to those in the gas phase and in m-xylene. Furthermore, attempts to solvolyze 1,1-dideuterioaUyl mesylate in aqueous methanol resulted in no ionization to an allyl cation instead, the direct displacement product was formed exclusively. Finally, determinations of a solvent kinetic isotope effect in deuterium oxide resulted in values around unity 10%. ° In the solvolysis reaction of tert-butyl chloride the value is 40% at room temperature. " It is possible to cause allylvinyl ethers to ionize by providing cation stabilizing substituents and Lewis acids or Lewis acidic solvents. ... 

Table 2. Secondary Isotope Effects in the Solvolysis of Methoxy-Substituted Alkyl p-Bromobenzenesulfonates...

Why is there a difference in the isotope effects in the solvolysis reactions of the following alkenyl tosylates Explain why the second example has the larger isotope effect. 

The low values obtained for the secondary deuterium kinetic isotope effects in the comparative solvolyses of (141) and (142) in formic acid (kn/ko = 1.154 at 29.9 °C) and in aqueous ethanol kji/ko = 1.116 at 64.4 °C) are regarded as evidence for extensive bridging (143) in the solvolytic transition state the deuterium atom at the C-1 bridgehead in (142) obviates the scrambling problem.As the contribution from the P-deuterium is unknown, then assuming the effects of a- and -deuterium are cumulative the tt-kinetic isotope effect is further decreased from the value expected for reaction occurring without nucleophilic assistance. The products of solvolysis of ent-methyl-12p-p-tolylsulphonyloxybeyeran-19-oate, which is a related bicyclo [3,2,1 ] -octan-2-yl system, have been discussed in terms of intermediate carbocation stabilities and lifetimes in the solvent employed. ... 

---