Substitution reactions qualitative theories

With this exception we can see that the impact of the configuration mixing model on nucleophilic substitution reactions, which constitute the most widely studied organic reaction, is indeed extensive. The model readily rationalizes much available experimental data, relates the entire mechanistic spectrum within a single framework, challenges some fundamental precepts of physical organic chemistry and enables one to make reactivity predictions about reactions yet to be investigated. For such a simple, qualitative theory, this is no mean achievement. 

Qualitative Theory of Solvent Effects on Reaction Rates 165 Table 5-4. Predicted solvent effects on rates of nucleophilic substitution reactions [16, 44-46],... 

More important, the failure of many transition metal aqua ions to fit the correlations of Figure 8.4 highlights the influence of d electron configuration on the reactivity of metal aqua ions in substitution reactions. The importance of d electron configuration was first noted by Taube in 19521 and explained qualitatively in terms of valence bond theory. Taube, with his predilection for simple test tube demonstrations, distinguished labile metal complexes (ones which underwent substitution within the time of mixing) from inert ones, the latter being typically octahedral complexes... 

I would like to take the rest of the space available to discuss some general qualitative theory about a-d rate effects and some results obtained from the application of this technique to nucleophilic solvolytic substitution reactions. 

In the past, transition state theory(l) was typically used in a qualitative manner to explain the effect of medium changes on reaction rates. This approach reached its quintessence in the systematic prediction by Hughes and Ingold(2) of the effect of increased solvent polarity on the rates of various types of nucleophilic substitution reactions. Their classification of S 2 reactions... 

Nucleophilic vinylic substitutions of 4//-pyran-4-onc and 2,6-dimethyl-4//-pyran-4-one with a hydroxide ion in aqueous solution were calculated by the density functional theory (B3LYP) and ab initio (MP2) methods using the 6-31+G(d) and 6-31G (d) basis sets. The aqueous solution was modelled by a supermolecular approach, where 11 water molecules were involved in the reaction system. The calculations confirmed a different addition-elimination mechanism of the reaction compared with that in the gas phase or non-polar solution. Addition of OH- at the C(2) vinylic carbon of the pyranone ring with an activation barrier of 10-11 kcalmol-1 (B3LYP) has been identified as the rate-determining step, in good quantitative and qualitative agreement with experimental kinetics. Solvent effects increase the activation barrier of the addition step and, conversely, decrease the barrier of the elimination step.138... 

As stated by Chatt and co-workers, their early speculation on the role of trans-n bonding groups in ligand substitution of platinum(ii) complexes was based on the assumption that the reactions proceed by an 8 2 mechanism. However, at the time (1955) most of the observations reported on such reactions were qualitative and little had been done to use detailed kinetic studies in attempts to elucidate the mechanism of ligand substitution. Since the valence bond theory in use then assigned dsp hybridisation to the square-planar plati-num(ii) complexes, coordination chemists believed an entering nucleophile would readily attack the low energy vacant p orbital on the metal and substitution would take place by an 8 2 mechanism. Furthermore, a coordination... 

Qualitative studies of the effects of allylic substituents, acyl substituents, and reaction conditions on rates of isomerization of allylic esters played a prominent role in the development of mechanistic theories of anionotropic rearrangements . Burton and Ingold observed that a-phenylallyl p-nitrobenzoate isomerizes more rapidly than a-phenylallyl acetate, that electron-releasing a-substituents facilitate isomerization of a-substituted allyl acetates, and that the rate of isomerization of a-phenylallyl p-nitrobenzoate increases as the dielectric constant of the solvent increases. All of these observations support the mechanism, which involves dissociation of the allylic ester into an allylic carbonium carboxylate ion pair, which reassociates to the starting material and its allylic isomer, viz. 

Electron affinities for 35 substituted nitrobenzenes have been reported and provided a comprehensive data set for the examination of substituent effects. The data were used to derive Taft gas-phase substituent parameters and discussed qualitatively based on frontier orbital molecular theory. The rate constants for the exo-energetic electron-transfer reactions were found to be close to those predicted by the ADO (average dipole orientation) theory... 

MO theory has been used to draw qualitative conclusions about the course of chemical reactions. The most fi-uitful applications have come from the Woodward-Hqffmemn rules, which predict the preferred path and stereochemistry for many important classes of organic reactions. As an example of the application of these rules, we consider the cyclization of a substituted 5-cis-butadiene to a substituted cyclobutene. There are two possible steric courses the reaction can take, described as con-rotatory or disrotatory, depending on whether the terminal groups rotate in the same or opposite senses as the reaction proceeds. Note the difference in products in Fig. 16.13. 

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