Grunwald-Winstein analysis

Grunwald-Winstein Analysis.—Although the dielectric constant of the medium in the vicinity of the reaction centre undoubtedly plays an important role in determining substitution rates, there are other properties of the solvent, such as its nucleophilicity, which are also of some importance in this connection. This was recognized by Grunwald and Winstein two and a half decades ago, and prompted them to formulate their rate-constant expression 

Close inspection of m T plots for aquation of metal ammine- or amine-halide complexes reveals that there is some pattern in the (relatively small) scatter of points about the best straight line. Usually rate constants for acetone-water mixtures lie on the opposite side of the line from those for alcohol-water mixtures the rate constants for aquation in acetone-water mixtures vary slightly less with solvent Y values than expected. It is possible to accommodate this pattern by the use of a more sophisticated equation, such as that [equation (1)] established for organic reactions.  

Here the IN term permits allowance to be made for the nucleophilicity (iV) of the solvent. The kinetics of aquation of the /r j 5-[Co(l,3-diaminopropane)2Cl2]+ cation in mixtures of water with methanol, ethanol, or acetone can be analysed successfully according to equation (1) values of m are 0.36, 0.34, and 0.17, respectively. It has also been found that earlier results - on similar complexes, e.g. 

Nishizawa, H. Kido, I, Kinoshita, Y. Soma, and K. Saito, Bull. Chem. Soc. Japan, 1976,49, 819. 

Grunwald-Winstein analyses of kinetic results have proved less successful for the two-stage mercury(ii)-catalysed aquation of the [RuCl(NH3)6] + cation, for associative attack of cyanide at the [Fe(bipy)a] cation, and for aquation of the [Fe(phen)a] + cation. Logarithms of rates of isomerization of trans-[Cr(ox) -(0112)2] correlate better with logarithms of rates of base hydrolysis of ethylene chlorohydrin than with solvent Y values (i.e. than with logarithms of rates of t-butyl chloride solvolysis).  

Minniti, and S. Lanza, Inorg. Chim. Acta, 1976,18, LI 5. 

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In summary it may be concluded that while the Grunwald-Winstein analysis m values may be useful in delineating the broad mechanistic features of octahedral complexes, attempts to probe the precise degree of associative and dissociative character in the reaction are not feasible. 

A disturbing feature of the Grunwald-Winstein analysis is that quite often different values of m are obtained for the same complex with different solvent pairs. This has been attributed to variation in the nucleophilicity, N, of the organic component of the solvent and a more complex relationship (10) has been proposed16 to account for the variation of rates in different solvents ... 

The analysis seems to work only if halides are the leaving group, as attempts to extend the Grunwald—Winstein analysis to thiocyanate as the leaving group in a series of complexes of Cr resulted in quite scattered values of m (-0.01 to 0.4). The lower value of m obtained for Cr compared with Co can be taken to indicate more associative character during the aquation of Cr than in Co, ... 

Graham s ammonium theory, 4 Gravimetry, 524-535 Group overlap integrals angular overlap model, 226 Grunwald-Winstein analysis, 504... 

Rates of cis->trans isomerization for [PtClR(PEt3)a] in methanol and propan-2-ol are consistent, from a qualitative Grunwald-Winstein analysis, with rate-determining chloride loss and generation of a three-co-ordinate intermediate. The solvent must also play a vital role in cis trans isomerizations involving five- or six-co-ordinate intermediates, as for example for [Pd(N3)2(PPh3)a]. ... 

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