Transition state productlike

Figure 5-5. Reaction coordinate diagram corresponding to Fig. 5-4, showing that the initial state is more stable than the final state and the transition state is productlike.

This valence bond description leads to an interesting conclusion. Because the transition state occurs at the point where the initial and final state VB configurations cross, the transition state receives equal contributions from each. This is so whether the transition state is early or late. Thus, the nucleophile Y and the leaving group X possess about equal charge densities in the transition state. This conclusion means that an early transition state is not (in this sense) reactantlike , for a reactantlike transition state should have most of the charge on Y. Similarly, a late transition state is not necessarily productlike. This view is at variance with other interpretations. 

These results suggest that the transition state which leads for example to the compound (38) is not symmetrical and completely productlike. It may well be that bond formation at the position para to the f-butyl group is more advanced in the transition state, as represented schematically in (40). 

Conformational control is important (a) when transition state interactions between the reagents are weak (p. 166), (b) in the absence of highly polar substituents (e.g. propanal) and also (c) in reactions with very early (reactant-like) or very late (productlike) transition states.3... 

JK 1 mol"1 (Stewart and Teo, 1980). For this series also, a plot of log k for reduction against log K for hydration yields a line of slope 1.12, consistent with a transition state with substantial hydride bonding to carbonyl carbon. A correlation (r = 0.9785) between logk for reduction of a series of aliphatic ketones and log K for equilibration of the ketone and alcohol has a slope of 1.1, which has also been taken to support a productlike transition state (Muller and Blanc, 1980, 1981 Boyer et al., 1979). 

Factors controlling the stereochemistry of conjugate additions are not well understood. Mixtures of isomers are often produced, but generally one isomer predominates. Both steric and electronic factors play a role. ° Generally, Michael-type additions have late and hence productlike—and chairlike— transition states. In the example shown below, for stereoelectronic reasons antiparallel attack by the nucleophilic CH3 is favored over parallel attack. 

The mechanism proposed for the ( )-alkene selectivity involves a nonsynchro-nous cycloaddition with a relatively advanced, productlike transition state leading to the kinetic fran -oxaphosphetane intermediate. Extrusion of triphenylphosphine oxide produces the ( )-alkene. ... 

A related study on the addition of OH to formamide in water by Weiner et al. should be noted.They obtained potential functions from quantum mechanical calculations and estimated the solvent effect by molecular mechanics energy minimization for the reacting system in a box of TIP3P water. The latter procedure does not provide proper configurational averaging. Nevertheless the computed and AE in water of 22 and 9 kcal/mol are consistent with experimental data on the hydrolyses of amides. As in the present case, the transition state in water is very productlike and is computed to occur at a C-O distance of about 2.0 A. 

To study this discrimination, generally expressed as an M value, the ratio of the O2 and CO equilibrium constant values, researchers have used sterically hindered porphyrins (capped, pocket and hybrid porphyrins) with a view to providing an environment which would permit normal binding of dioxygen whilst hindering the binding of carbon monoxide. Kinetically, the major effect is a decrease in CO association rates, consistent with steric blocking in productlike transition states. 

The SN1 reaction of RY is believed to have a rate-determining ionization of RY, equation 14, and the transition state is believed to be very productlike. Thus, a close parallelism occurs between rate and stability of the product ions, although this statement is generally made with respect to variation in R rather than in Y. 

Figure 10 shows the calculated electron density map that slices through the four C centers of the transition state. Again it verifies that the transition state is quite productlike there is almost no electron overlap between the two P carbon centers. The left two C centers show the characteristics of an ethylene (double bond) while the right two C centers look like an ethyl radical. 

Figure 11 shows the fully optimized geometry of the transition state of reaction (17) as well as the calculated energies. The results indicate that the transition state is quite similar to that of reaction (16), with a long transitional C-C bond (2.256 A) and productlike structure. The calculated Ea is 34.96 kcal/mol at the PMP2/6-31G level, and 33.24 kcal/mol at the B3LYP/6-31G level. Both of them are very close to that of reaction (16). It can be concluded that without other reactions (H-transfer reaction, termination reaction, and addition reaction), the radical decomposition will proceed in a chain reaction fashion until the P position is eliminated. 

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