Solvated transition state

The reactions of the thiophene derivatives in both forward and reverse directions are characterized by lower enthalpies and entropies of activation than the reactions of the selenophene analogs. In the forward reactions, enthalpy and entropy changes compensate nearly exactly and result in slightly greater rates of adduct formation for the selenophene derivatives despite the higher enthalpies of activation. The higher entropies of activation for the selenophene derivatives have been attributed to less solvated transition states as compared to the reactions of the thiophene analogs (Table XXVIII). 

Draw the solvated transition state for the reaction of 19 with KI if water is the solvent. 

Consider what happens when you increase the steric hindrance to attack of a nucleophile on a carbonyl by increasing the size of the R group (Eq. 8.66). A larger R group would lead to a more crowded, less solvated transition state, thereby raising the AH. However, the nucleophile is further away from the carbonyl carbon at the transition state, and therefore it and the solvent would be less tightly held at the transition state than the reaction with the smaller R group. These structural issues manifest themselves as a lower loss in entropy and therefore a more favorable entropy of activation. 

Measurements of pressure effectson the kinetics of the potassium t-butoxide-assisted isomerization of em-dimethyl- and em-di-isopropyl-cyclohexenes has enabled activation volumes for proton abstraction in the range —18 to — 26dm mol" to be obtained. These are attributed to the electrostriction of the solvated transition state and partly to the negative activation volume for ionization of the butoxide. 

For analysing equilibrium solvent effects on reaction rates it is connnon to use the thennodynamic fomuilation of TST and to relate observed solvent-mduced changes in the rate coefficient to variations in Gibbs free-energy differences between solvated reactant and transition states with respect to some reference state. Starting from the simple one-dimensional expression for the TST rate coefficient of a unimolecular reaction a— r... 

The electrostatic solvait effects discussed in the preceding paragraphs are not the only possible modes of interaction of solvent with reactants and transition states. Specific structural effects may cause either the reactants or the transition state to be particularly stroi ly solvated. Figure 4.12 shows how such solvation can affect the relative energies of the ground state and transition state and cause rate variations from solvent to solvent. 

Fig. 4.12. Potential energy liagrams showing effect of preferential solvation of transition state (a) and ground state (b) on the activation energy.

Fig. 4.14. Reactant and transition-state solvation in the reaction of ethyl acetate with hydroxide ion. [From P. Haberfield, J. Friedman, and M. F. Pinkson, J. Am. Chem. Soc. 94 71 (1972).]...

In fee absence of fee solvation typical of protic solvents, fee relative nucleophilicity of anions changes. Hard nucleophiles increase in reactivity more than do soft nucleophiles. As a result, fee relative reactivity order changes. In methanol, for example, fee relative reactivity order is N3 > 1 > CN > Br > CP, whereas in DMSO fee order becomes CN > N3 > CP > Br > P. In mefeanol, fee reactivity order is dominated by solvent effects, and fee more weakly solvated N3 and P ions are fee most reactive nucleophiles. The iodide ion is large and very polarizable. The anionic charge on fee azide ion is dispersed by delocalization. When fee effect of solvation is diminished in DMSO, other factors become more important. These include fee strength of fee bond being formed, which would account for fee reversed order of fee halides in fee two series. There is also evidence fiiat S( 2 transition states are better solvated in protic dipolar solvents than in protic solvents. 

Let us now return to the question of solvolysis and how it relates to the stracture under stable-ion conditions. To relate the structural data to solvolysis conditions, the primary issues that must be considered are the extent of solvent participation in the transition state and the nature of solvation of the cationic intermediate. The extent of solvent participation has been probed by comparison of solvolysis characteristics in trifluoroacetic acid with the solvolysis in acetic acid. The exo endo reactivity ratio in trifluoroacetic acid is 1120 1, compared to 280 1 in acetic acid. Whereas the endo isomer shows solvent sensitivity typical of normal secondary tosylates, the exx> isomer reveals a reduced sensitivity. This indicates that the transition state for solvolysis of the exo isomer possesses a greater degree of charge dispersal, which would be consistent with a bridged structure. This fact, along with the rate enhancement of the exo isomer, indicates that the c participation commences prior to the transition state being attained, so that it can be concluded that bridging is a characteristic of the solvolysis intermediate, as well as of the stable-ion structure. ... 

Z7. The cotr arison of activation parameters for reactions in two different solvents requires consideration of differences in solvation of both the reactants and the transition states. This can be done using a potential energy diagram such as that illustrated below, where A and B refer to two different solvents. By thermodynamic methods, it is possible to establish values which correspond to the enthalpy... 

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