Solvent Effects—Grunwald-Winstein Plots

The reference reaction chosen was the SnI reaction of f-BuCl (Eq. 8.37). This reaction was chosen because it was assumed that very little if any Sn2 pathway occurred. The reference solvent was 80% ethanol / 20% water, both of which could act as a nucleophile to add to the incipient t-butyl carbocation. Hence, the LFER given in Eq. 8.38 was defined. Here, fc, Buo, soj is the rate of the reaction in the solvent being analyzed, and fc, Buci, so% Etou is the rate of the reaction in 80% ethanol. Thus, Y reflects the ability of the new solvent to influence the rate of the reference reaction compared to 80 20 ethanokwater. Table 8.4 lists a variety of Y values. 

Our goal is not to study just the SnI reaction of t-BuCl, but instead to apply the LFER to many different reactions. Hence, an LFER with a sensitivity factor m for any new reaction under study is used (Eq. 8.39). Here, k evi is the rate constant for the new reaction in various solvents. The reference solvent is still 80% ethanol. Although the majority of the studies using this equation have been SnI and Sn2 reactions (see examples in Chapter 11), it is in principle applicable to any reaction. 

Since the Y parameter was based upon a reaction that has little nucleophilic assistance, those reactions that have m values near 1 reflect nearly full ionization in the rate-determining step. For those reactions that have an m value less than 1, the reaction is not as sensitive to the ionizing ability of the solvent as is f-BuCl. This means less charge has been created in the transition state, which is most often accomplished by some degree of nucleophilic assistance (Chapter 11 discusses the shades of grey between pure SnI and pure Sn2 reaction mechanisms). Hence, a reaction with some Sn2 character will have a reduced extent of charge development in the transition state and therefore an m value less than unity. More- 

Values of Solvent Ionizing Ability as Defined by LFERs Eqs. 8.38,8.39 and 8.41  

Mixed solvent systems (all expressed as percent co-solvent in water) 

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Kinetic studies of the solvolysis of phenylmethanesulfonyl chloride (PhCH2S02Cl) in a wide range of solvent mixtures at 308 K were reported. Since an extended Grunwald-Winstein plot was linear and a kinetic solvent isotope effect of 2.34 was seen for MeOH/MeOD, a dissociative S 2 mechanism was indicated. ... 

The cr-substituent effects and solvent effects (extended Grunwald-Winstein equation) for solvolyses of / -X-substituted benzoyl chlorides (X = OMe, Me, H, and Cl) in 97% wt/wt hexafluoroisopropanol/water was explained by two competing pathways. Plots of log k versus a were consistent with a cationic path and explained by an 5 n2-5n1 spectrum of mechanism. Electron-donating groups favoured the reaction and values of p increased in the order AcOH < HCO2H < TEE < HFIP. A benzoyl cation intermediate was trapped in hexafluoroisopropanol. Ab initio calculations of heterolytic bond dissociation energies of various chloro- and fluoro-substituted and other benzoyl chlorides were correlated with log k for solvolyses... 

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