Solvolysis Grunwald-Winstein equation

The extended (two-term) Grunwald-Winstein equation has been applied to the solvolyses of ethyl chloroformate (117) and ethyl chlorothioformate (118). For each substrate, there is evidence for two competing reaction channels. " Solvolysis... 

The specific rates of solvolysis of benzyl p-toluenesulfonate and nine benzylic-ring-substituted derivatives (324) have been satisfactorily correlated using Aij and Tots scales within the extended Grunwald-Winstein equation. The reactions of Z-phenylethyl X-benzenesulfonates (325) with Y-pyridines (326) in acetonitrile at 60 °C have been studied at high pressures. The results indicated that the mechanism of the reaction moves from a dissociative 5)vr2 to an early-type concerted 5)vr2 with increasing pressure. 

The application of correlation analysis of solvent effects to mechanistic studies of solvolysis has been reviewed by Takeuchi in Japanese.110 The article mainly covers die behaviour of tertiary chloro compounds. Tins author s research group has continued experimental studies in this area.111-113 Rates of solvolysis of 2-chloro-2,4-trimethylpentane have been measured in 17 solvents and analysed through the extended Grunwald-Winstein equation, which includes a term for nucleophilic participation.111... 

Contrary to earlier suggestions of an SnI pathway for solvolyses of A,A-dimethyl-sulfamoyl chloride (41), the results of an extended Grunwald-Winstein equation treatment of the specific rates of solvolysis of (41) in 32 solvents pointed to an Sn2 pathway. Results of a similar treatment of its rates of solvolysis in a range of solvents supported an SN2 pathway for 2-propanesulfonyl chloride (42).59... 

In Eq. (7-17), k and are the speeifie rate eonstants of SnI solvolysis of the 5-methyl-dibenzothiophenium ion in a given solvent and in a standard solvent (Et0H/H20, 80 20 cL/L At = 0), respeetively. At values are known for 37 pure and binary solvents and span a range from At = -5.26 [(CF3)2CH0H/H20, 97 3 cg/g] to +0.37 (EtOH) [305, 308]. The At solvent nueleophilieity seale ean be used directly to correlate the solvolytic behaviour of other RX+ substrates with a neutral nueleofuge, and also in appHcations of the extended Grunwald-Winstein equation (7-15). Many examples of the use of Nj values in the correlation of specific rates of solvolysis of a variety of RX and RX" " substrates are given in reference [305]. 

A detailed study of the specific rates of solvolysis of N,N,N, N -tetra-methyldiamidophosphorochloridate (80) (TMDAPC) with analysis in terms of the extended Grunwald-Winstein equation has been reported (Scheme 19). The stereochemistry of nucleophilic attack at tetracoordinate phosphorus was also discussed." The initial reaction of bis (2,4-dinitrophenyl) phosphate (BDNPP) (81) with hydroxylamine involves release of 1 mol 2,4-dinitrophen-oxide ion and formation of a phosphorylated hydroxylamine (82), which reacts readily with further NH2OH, giving the monoester (83). The intermediate (82) also breaks down by two other independent reactions one involves intramolecular displacement of aryloxide ion (83) and the other involves migration of the 2,4-dinitrophenyl group from O to N and formation of phosphorylated 2,4-dinitrophenylhydroxylamine (84) (Scheme 20)." ... 

The extent to which a solvent stabilises ions could be measured by comparing the equilibrium constant for a standard dissociation with the value for that in a standard solvent. The Grunwald-Winstein equation (Equation 48) uses as its standard the solvolysis of /-butyl chloride (Equation 49) and assumes that the transition state has almost complete carbenium ion character and that the formation of the ion-pair is rate determining. The rate constant would therefore measure the energy of formation of the free carbenium ion. Equation (48) defines the solvent parameter, K, where is the solvolysis rate constant of /-butyl chloride in the solvent (S) and the standard solvent (ss) is 80% EtOH/H2O. Other solvolyses can be used as standards and those of 1-adamantyl species has largely supplanted the original one for reasons given later. 

The success of the single parameter Grunwald-Winstein equation is largely due to the limited range of solvent change such as variation of composition of mixtures. When different solvent types are employed extra terms are needed in the equation to fit the data. Solvolysis reactions in aliphatic nucleophilic substitution involve nucleophilic attack of the solvent and it is unlikely that solvents of different structure would have similar nucleophilicities excepting those in a series of mixed solvents such as ethanol-water (Chapter 2). The simplest treatment involves dividing the solvent action into nucleophilic and electrophilic components as shown in Equation (32). 

The solvolysis of a-D-glucosyl fluoride in aqueous methanol solutions follows the Grunwald-Winstein equation ... 

The initial objective of our work was to quantify solvent effects (particularly solvent nucleophilicity) by adapting the Grunwald-Winstein equation (2) (5). In equation 2, k is the rate of solvolysis of a substrate (RX) in any solvent relative to 80% v/v ethanol-water (k0) and Y is the solvent ionizing power defined by m = 1.000 for solvolyses of tert-butyl chloride at 25 °C. In this chapter, a discussion of equation 2 and similar free-energy relationships is presented. At the time our work began (1969), in collaboration with Schleyer, mechanisms of solvolytic reactions were close to a high in controversy (6-8). More recent mechanistic developments (9-13) are not reviewed in detail here, but increased recognition of the importance of nucleophilic solvent assistance should be noted. 

DFT studies of the hydrolysis of acetyl and chloroacetyl chloride and of variously substituted benzoyl chlorides supported an S 2 mechanism. An extended Grunwald-Winstein equation correlation for the specific rates of solvolysis of 3,4,5-trimethoxybenzoyl chloride gave sensitivities towards changes in solvent nucleophilic-ity (/-value) of 0.29 and solvent ionizing power (m-value) of 0.54. The low m-value allowed specific rates to be determined in highly ionizing fiuoroalcohol/H20 mixtures. A parallel correlation of the specific rates of solvolysis of 2,4,6-trichlorobenzoyl chloride revealed that solvolyses in 100% and 90% ethanol or methanol did not appreciably follow the ionization pathway indicated for solvolyses in the other solvents and it was proposed that, despite the two or//to-substituents, the addition-elimination pathway had become dominant. ... 

Rates of solvolysis of cinnamoyl chloride and its 4-chloro and 4-nitro derivatives were analysed using the simple and extended Grunwald-Winstein equations. Studies of solvent effects in the solvolysis of isopropenyl, isobutyl,and propargyl chloroformate " and of 1- and 2-naphthyl chloroformate were reported. 

Solvolysis rate constants of 5-nitro-2-furoyl chloride in 27 different solvents were well correlated with the extended Grunwald-Winstein equation, with sensitivity values of 1.20 0.05 and 0.37 0.02 for I and m, respectively. These data, the range of values found for A// and AS, and the solvent kinetic isotope effect (SKIE, MeOH MeOD) of 2.65 were in accord with an 5 2 mechanism. ... 

The specific rates of solvolysis of the acid anhydride, acetyl p-toluenesulfonate (30) were measured by a rapid-response conductivity technique at temperatures in the range 263-218 K. For 13 solvents at 233.4 K, an extended Grunwald-Winstein equation correlation led to sensitivities to changes in solvent nucleophilicity of 0.65 and to changes in solvent ionizing power of 0.61. In 89.1 % acetone at 253 K, the comparison with acetyl bromide solvolysis led to a koj /k ratio of 1.4. In methanol and methanol-d at 233.4 K, the solvent deuterium isotope effect A MeOH MeOD was 0.99. These results are consistent with an 5 1 reaction with appreciable nucleophilic solvation or an 5 2 reaction with a loose TS. ... 

The specific rates of solvolysis of methyl chloroformate, MeOCOCl, are very well correlated by the extended Grunwald-Winstein equation over a wide range of solvents the pathway is believed to be predominantly addition-elimination, except that a positive deviation for solvolysis in 90% l,l,l,3,3,3-hexafiuoropropan-2-ol suggests an 80% contribution from an ionization mechanism. " ... 

The specific rates of solvolysis of phenyl chlorothionoformate (PhOCSCl) were found to be remarkably similar to those reported previously for phenyl chlorothioformate (PhSCOCl). When analysed using the extended Grunwald-Winstein equation over the usual range of solvent types, these solvolyses showed essentially identical divisions... 

The specific rates of solvolysis of benzenesulfonyl chloride and a series of 4-and 2,4,6-substituted derivatives (Scheme 25) have been very well correlated using the extended Grunwald-Winstein equation, with incorporation of A/r solvent nucle-ophilicity and Tq solvent ionizing power values. In two instances, it was shown that almost identical correlations were obtained after the incorporation of methyl groups into the 2- and 6-positions of the benzene ring. No evidence was found for a dissociative (5n1) mechanism or for a duality of mechanism. All of the results could be rationalised in terms of a concerted bimolecular displacement (5n2) mechanism, involving attack by solvent at sulfur. ... 

The rate constants for the solvolysis of chloromethyl ethyl ether, chloromethyl octyl ether, and chloromethyl methyl sulfide have been determined in several pure and binary solvents. Application of the extended Grunwald-Winstein equation, logffe/fe, ) = /Nj + mY + c, gave appreciable T values (0.55-0.71) for the three substrates indicating that there is significant nucleophilic solvation of the developing carbenium ion in the transition states of these reactions. The kQ lkp = 1.2 x 10 found for the hydrolysis of chloromethyl methyl ether in water is virtually identical to that observed for the uni-molecular solvolyses of t-butyl chloride and trityl halides confirming the unimolecular mechanism for these reactions. 

Equation (50) correlates rate constants for a number of solvolysis reactions and Figure 13 illustrates the Grunwald-Winstein plot for the solvolysis of z-butyl chloride against a K value from 1-adamantyl chloride where w = 0.78. 

Figure 11 Extended Grunwald-Winstein plot for the solvolysis of isopropyl tosylate , plot of the data against , improved plot when bilinear equation is...

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