Intrinsic barrier solvation

Other measures of nucleophilicity have been proposed. Brauman et al. studied Sn2 reactions in the gas phase and applied Marcus theory to obtain the intrinsic barriers of identity reactions. These quantities were interpreted as intrinsic nucleo-philicities. Streitwieser has shown that the reactivity of anionic nucleophiles toward methyl iodide in dimethylformamide (DMF) is correlated with the overall heat of reaction in the gas phase he concludes that bond strength and electron affinity are the important factors controlling nucleophilicity. The dominant role of the solvent in controlling nucleophilicity was shown by Parker, who found solvent effects on nucleophilic reactivity of many orders of magnitude. For example, most anions are more nucleophilic in DMF than in methanol by factors as large as 10, because they are less effectively shielded by solvation in the aprotic solvent. Liotta et al. have measured rates of substitution by anionic nucleophiles in acetonitrile solution containing a crown ether, which forms an inclusion complex with the cation (K ) of the nucleophile. These rates correlate with gas phase rates of the same nucleophiles, which, in this crown ether-acetonitrile system, are considered to be naked anions. The solvation of anionic nucleophiles is treated in Section 8.3. 

Typical chemical reactions are characterized by sharp reaction barriers, often arising in part from the existence of a reaction barrier in the gas phase. Thus, even though the magnitude ofthe reactive solute-solvent coupling is strong [large (t=0)], the intrinsic barrier is of such high frequency that the nonadiabatic solvation limit... 

The interpretation of reactivities here provides a particular challenge, because differences in solvation and bond energies contribute differently to reaction rates and equilibria. Analysis in terms of the Marcus equation, in which effects on reactivity arising from changes in intrinsic barrier and equilibrium constant can be separated, is an undoubted advantage. Only rather recently, however, have equilibrium constants, essential to a Marcus analysis, become available for reactions of halide ions with relatively stable carbocations, such as the trityl cation, the bis-trifluoromethyl quinone methide (49), and the rather less stable benzhydryl cations.19,219... 

In cases where there is strong solvation of the carbanion, as for example hydrogen bonding solvation of enolate or nitronate ions in hydroxylic solvents, the intrinsic barrier is increased further because the transition state cannot benefit significantly from this solvation. This is the reason why AG for the deprotonation of nitroalkanes in water is particularly high, i.e., much higher than in dipolar aprotic solvents, see, e.g., entry 11 versus 15 and entry 13 versus 16 in Table 1. These solvation effects will be discussed in more detail below. 

Solvation can have a large effect on intrinsic barriers or intrinsic rate constants, especially hydrogen bonding solvation of nitronate or enolate ions in hydroxylic solvents. Table 4 reports intrinsic rate constants in water and aqueous DMSO for a number of representative examples.19,20,23 25,40,54 56 Entries 1-4 which refer to nitroalkanes show large increases in ogka when... 

A complementary aspect of solvation is that it affects the magnitude of the transition state imbalance. This can be seen for the reactions of ArCH2N02 in DMSO and MeCN where the imbalances are much smaller than in water (Table 2, entries 4 and 6). Again we see the connection between imbalance and intrinsic barriers the greater imbalance induced by solvation leads to an enhanced intrinsic barrier. 

A case where the late solvation of halogen leaving groups in a carbocation forming solvolysis reaction increases the intrinsic barrier is the one shown in Equation (54). 

Thermodynamically cleavage of a C-H bond in an alkylaromatic radical cation is strongly favored over C-C fragmentation due to the much higher solvation free energy of the proton as compared to a carbocation. However, the former process is characterized by significantly higher intrinsic barriers (0.5-0.6 eV for C-H... 

Predicting reactivity patterns of reductants in inner-sphere processes is more difficult because such processes, by definition, involve strong electronic coupling, and hence detailed quantum-mechanical calculations are needed to understand these processes [compare the classical calculations that lead to Eq. (a)]. A semiquantitative attempt is available . Nevertheless, similar factors operate in these inner-sphere processes e.g., an intrinsic barrier owing to changes in bond lengths and solvation of the reactants, is shown by oxidation of [Co(TIM)(HjO)j] and [Co(trans[14]diene)(H20)j] by... 

It has been proposed that part or all of the intrinsic barrier for deprotonation of a-carbonyl carbon is associated with the requirement for solvation of the negatively charged oxygen of the enolate anion [80]. However, the observation of small intrinsic barriers for deprotonation of oxygen acids by electronegative bases to form solvated anions [31] suggests that the requirement for a similar solvation of enolate anions should not make a large contribution to the intrinsic barrier for deprotonation of a-carbonyl carbon. 

Extremely strong solvational imbalances in the transition state for deprotonation of a trinitrobenzylic carbon acid (96) by oximate bases in 1 1 (v/v) H20-Me2S0 accounts for the occurrence of rapid Marcus curvature in the corresponding Brpnsted plots and the consequent levelling of oximate reactivity in the proton transfer process. " The desolvation of the oximate ion pair prior to actual proton transfer ensures that the intrinsic barrier becomes dominated by the work term for formation of the encounter complex. 

G. The unified model for barrier crossing in solution, (a) Construct the two intersecting two-dimensional parabolas, each being a function of the solvation coordinate r and the solute coordinate q. (b) Determine the reaction coordinate and the lowest barrier, (c) Determine the intrinsic barrier, when the reaction is symmetrical, (d) Derive Eq. (11.29) for the free energy at the barrier. 

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