Transition state, charge separation

Transition state is more polar than starting state polar solvent can cluster about transition state so as to reduce electrostatic energy associated with separation of opposite charges... 

Retardation by a factor of 1000 by use of benzene in place of ethanol in the rearrangement of (58) points to appreciable charge separation in the transition state. 

STO-3G and 3-2IG MO calculations indicate a rotational barrier that is substantially reduced relative to the corresponding barrier in ethylene. The transition state for the rotation is calculated to have a charge separation of the type suggested by the dipolar... 

The ortho effect may consist of several components. The normal electronic effect may receive contributions from inductive and resonance factors, just as with tneta and para substituents. There may also be a proximity or field electronic effect that operates directly between the substituent and the reaction site. In addition there may exist a true steric effect, as a result of the space-filling nature of the substituent (itself ultimately an electronic effect). Finally it is possible that non-covalent interactions, such as hydrogen bonding or charge transfer, may take place. The role of the solvent in both the initial state and the transition state may be different in the presence of ortho substitution. Many attempts have been made to separate these several effects. For example. Farthing and Nam defined an ortho substituent constant in the usual way by = log (K/K ) for the ionization of benzoic acids, postulating that includes both electronic and steric components. They assumed that the electronic portion of the ortho effect is identical to the para effect, writing CTe = o-p, and that the steric component is equal to the difference between the total effect and the electronic effect, or cts = cr — cte- They then used a multiple LFER to correlate data for orrAo-substituted reactants. 

Pertiaps the most obvious experiment is to compare the rate of a reaction in the presence of a solvent and in the absence of the solvent (i.e., in the gas phase). This has long been possible for reactions proceeding homolytically, in which little charge separation occurs in the transition state for such reactions the rates in the gas phase and in the solution phase are similar. Very recently it has become possible to examine polar reactions in the gas phase, and the outcome is greatly different, with the gas-phase reactivity being as much as 10 greater than the reactivity in polar solvents. This reduced reactivity in solvents is ascribed to inhibition by solvation in such reactions the role of the solvent clearly overwhelms the intrinsic reactivity of the reactants. Gas-phase kinetic studies are a powerful means for interpreting the reaction coordinate at a molecular level. 

The neutral reactants possess permanent dipoles, the product is ionic, and the transition state must be intermediate in its charge separation, so an increase in solvent polarity should increase the rate. Except for selective solvation effects of the type cited in the preceding section, this qualitative prediction is correct. 

Calculate activation energies for Sn2 reactions of ammonia and trimethylamine with methyl iodide via transition states ammonia+methyl iodide and trimethyl-amine+methyl iodide, respectively. Is attack by ammonia or trimethylamine more facile Rationalize your observation by comparing electrostatic potential maps for the two transition states. Which transition state requires more charge separation Is this also the higher-energy transition state ... 

For unactivated aromatics, the activation energy (ca. 30 kcaF ) is less than the calculated localization energies (ca. 40 kcal for complete separation of charges in pyridine ), and, therefore, complete localization prior to reaching the transition state appears to be unnecessary... 

Polar factors can play an extremely important role in determining the overall reactivity and specificity of hotnolytic substitution.97 Theoretical studies on atom abstraction reactions support this view by showing that the transition state has a degree of charge separation.101 10 ... 

One possible explanation for the above results is that the transition state for the uncatalyzed reaction is either more ionic or has its charges more highly separated than does the transition state for the catalyzed reaction. A consideration of possible transition state structures makes this explanation improbable, since the transition state for the catalyzed reaction would, in fact, be expected to show the greater charge separation, and this would be equally the case for both the transition state for intermediate formation and the transition state for conversion of intermediate to product. 

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