Reaction rates, comparison substituent effects

The applicability of the two-parameter equation and the constants devised by Brown to electrophilic aromatic substitutions was tested by plotting values of the partial rate factors for a reaction against the appropriate substituent constants. It was maintained that such comparisons yielded satisfactory linear correlations for the results of many electrophilic substitutions, the slopes of the correlations giving the values of the reaction constants. If the existence of linear free energy relationships in electrophilic aromatic substitutions were not in dispute, the above procedure would suffice, and the precision of the correlation would measure the usefulness of the p+cr+ equation. However, a point at issue was whether the effect of a substituent could be represented by a constant, or whether its nature depended on the specific reaction. To investigate the effect of a particular substituent in different reactions, the values for the various reactions of the logarithms of the partial rate factors for the substituent were plotted against the p+ values of the reactions. This procedure should show more readily whether the effect of a substituent depends on the reaction, in which case deviations from a hnear relationship would occur. It was concluded that any variation in substituent effects was random, and not a function of electron demand by the electrophile. ... 

A difficulty in the analysis of reactions suspected to take place with concerted movement of atoms lies in the irreversibility of most of the processes and the consequent impossibility of characterizing rates at the reversible potential. An approach which has been suggested is the comparison of the substituent effects of appropriate electrode reductions and homogeneous reactions as in the example (Marcus, 1968)... 

The relative rates of cycloaddition of 88b-88e were measured in comparison with that of the parent 88a as a reference. The methoxy substituent has practically no effect on the reaction rate. However, it is apparent that electron-withdrawing substituents (88b, 88c and 88e) significantly accelerate the anti-addition, whereas in xyn-addition the acceleration is not as large the rate is comparable to that of the reference compound (88a). In the reactions of the tetrafluoro-substituted dienophile 88d, we found significant rate acceleration on both sides, though anti-side addition was still substantially favored. 

Finally, in many of the perturbation calculations of the effect of substituents and other structural changes, an important tacit assumption is made and it is far from obvious that it is always fulfilled. As already discussed, the physical argument on which the calculation is based is that the value of the initial slope, or the height of a small barrier along the way, determine the rate at which the photochemical reaction occurs. However, the experimental value with which comparison is made usually is not the reaction rate but the quantum yield, which of course also depends on rates of other competing processes and these may be affected by substitution as well. For instance, the rate at which fluorescence occurs is related to the absorption intensity of the first transition, the rate of intersystem crossing may be affected by introduction of heavy atoms... 

Taking the comparison of 67 and its a-Cp3 analogue 85 as the example (Eq. 25), a 10 -10 difference is observed in the rate constants k(oi for formation of the cations, but there is virtually no difference in the rate constants rev for their reactions with nucleophiles. In other words, the a-CF3 cation is considerably less stable in the thermodynamic sense but of comparable stability in the kinetic sense. This finding has been explained by a model separating the substituent effects into polar and resonance contributions. The polar effect, which substantially destabilizes... 

The rates of sodium borohydride reduction of the ketones (46) in propan-2-ol have been measured (78JCS(P2)1232). The strategy was to vary X keeping Y = H, and then to vary Y with X = H. This permitted direct comparisons to be made of transmission of substituent effects through the aromatic rings to the same reaction centre. It was concluded that the... 

Detailed mechanisms of intramolecular rearrangement reactions have been difficult to determine. Classical rate measurements seldom lead to unambiguous mechanistic predictions. Generally only after extensive examination of concentration, solvent, and substituent effects on the reaction rate can a general mechanistic class be proposed for example, intra vs intermolecular or bond rupture of a bidentate chelate vs non-bond rupture twist pathways. Indeed, only two examples of slow complexes are known where detailed rate comparisons for geometrical and optical iso-merizations were made and used to eliminate several mechanisms however, a single most probable pathway was not demonstrated in either case.12,13 Only with DNMR can detailed environmental site interchanges be directly observed and with this in-... 

Freeman chose substrates D-I to study the influence of steric bulk on the free energy of activation for the permanganate dihydroxylation. It is difficult to separate these substituent effects on the reaction rate, because it is not clear how to determine the relative contributions of steric and electronic effects to the overall observed effect. However, some comparisons are useful. For example, in the case of trans-crotonic acid D and 4,4-dimethyl-fra/rv-2-pentenoic acid I the large bulk of the t-butyl group compared to that of a methyl group should result in a dominance of the steric over the electronic substituent effect. 

Meisenheimer complex formation as separate reactions. Meisenheimer complexes can be considered as the anionic pseudobases derived from neutral aromatic molecules, and in this light it is clear that heterocyclic Meisenheimer complexes are appropriately considered in the current Review. By so doing, it is hoped that attention can be drawn to potentially mutual benefits that may be derived from comparative studies of neutral and anionic pseudobases. Certainly, the spectroscopic techniques applicable to the study of pseudobase and Meisenheimer complex formation are identical. Quantitative studies of substituent effects and structural effects on rates and equilibria for nucleophilic addition should be relevant both to neutral and to anionic e-complex formation. The general rules enunciated by Strauss23,318 and Fendler319 for the prediction of the relative stabilities of Meisenheimer complexes should be directly applicable to analogous pseudobases. Terrier et al.2n have made an important contribution in this area with a detailed comparison of kinetic and thermodynamic parameters for formation of a benzofuroxan Meisenheimer complex and an isoquinoline pseudobase. 

The reverse reaction is determined by the nature of R. The generating [Pt(PR3)2] moiety is a rather stable intermediate. A comparison of the results obtained with the NMR data (22) of substituted phenylacetylenes led to the conclusion that there is a relation between equilibrium constant, first-order rate constants, and chemical shifts of the acetylenic proton they all depend on substituent effects on the electron density in the triple bond. 

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