Substituent effects on reactions

Substituent effects on reactions between monosubstituted pyrazines and Mel in DMSO are very nearly the same as those for the corresponding pyridines. Since two different annular nitrogen atoms are present in the pyrazines, isomeric products 14 and 15 may be produced. When the total rate of alkylation is corrected for the isomer content (total rate constant x % isomer = rate constant for isomer) it is found... 

Thus, although quantitative interpretation of substituent effects on reaction rates is possible in some cases, it is much less generally applicable in crystals than in fluids. 

Substituent Effects on Reaction Rates of Diels-Alder Reactions... 

This kind of procedure, i. e. empirical estimation of solvent polarity with the aid of actual chemical or physical reference processes, is very common in chemistry. The well-known Hammett equation for the calculation of substituent effects on reaction rates and chemical equilibria, was introduced in 1937 by Hammett using the ionization of meta-ox /iflra-substituted benzoic acids in water at 25 °C as a reference process in much the same way [10]. Usually, the functional relationships between substituent or solvent parameters and various substituent- or solvent-dependent processes take the form of a linear Gibbs energy relationship, frequently still referred to as a linear free-energy (LFE) relationship [11-15, 125-127]. 

Cocyclotrimerization of Aryl Acetylenes Substituent Effects on Reaction Rate... 

Historically, QSRR has its foundation in the efforts of physical organic chemists who rationalize solute substituent effects on reaction rates and equilibria. The most notorious of all, the Hammett equation (9), inaugurates the linear free energy relationships (LFER), where the logarithm of the reaction equilibrium constant, K, is a linear function of the substituent constant (o), an arbitrarily derived parameter based on the ionization of benzoic acid derivatives in water, as follows ... 

Hammett LFER. An LFER with broad implications was developed in the 1930s by Hammett (1937) to explain substituent effects on reactions of meta- or para-substituted benzene compounds. Hammett found that hydrolysis of substituted ethyl benzoates, ionization of substituted benzoic acids, and many related reactions are affected by nature and position of aromatic substituents, and that the effect could be predicted quantitatively. Using rate and equilibrium constants for ionization of benzoic acid as references, Hammett defined the LFER ... 

The two seminal contributions of Mimoun and Sharpless laboratories led to a controversy on the reaction mechanism that was lasting longer than for two decades [82] and expanded to the olefin epoxidation with other metal peroxo complexes, in particular those of rhenium. Kinetic studies of Al-Ajlouni and Espenson [83,84] on the MTO-catalyzed olefin epoxidation with H2O2 revealed the importance of both mono- and diperoxo species in the catalytic process as well as substituent effects on reaction rates, but the molecular mechanism remained uncertain. 

CASE STUDIES SUBSTITUENT EFFECTS ON REACTIONS OF PHENETHYL PHENYL ETHERS... 

Case Studies Substituent Effects on Reactions of Phenethyl Phenyl Ethers... 

Substituent effects on Reaction (178) are quite informative. It was found, for example, that the reaction cannot be detected for X = CH3CO, NO2, or (CH3)2N. It appears that for these substituents the S state of the m-stilbene isomer does not have an electronic distribution which is suitable for cyclization. The change in electronic distribution is most easily visualized in the case of the acetyl substituent. The acetylstUbenes would be expected to have S states which are of the n- -nr rather than of the tt —> tt type. Since bromostilbenes undergo photocyclization despite the enhanced rates of S — T intersystem crossing in these molecules, it may be concluded that the rate of cyclization is extremely fast provided the process is favored by the electron density distribution of the lowest excited singlet state of the cis isomer. 

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