Solvent effects kinetic measures

Most of the kinetic measures of solvent effects have been developed for the study of nucleophilic substitution (Sn) at saturated carbon, solvolytic reactions in particular. It may, therefore, be helpful to give a brief review of aliphatic nucleophilic substitution. Two mechanistic routes have been clearly identified. One of these is shown by... 

Because solvent viscosity experiments indicated that the rate-determining step in the PLCBc reaction was likely to be a chemical one, deuterium isotope effects were measured to probe whether proton transfer might be occurring in this step. Toward this end, the kinetic parameters for the PLCBc catalyzed hydrolysis of the soluble substrate C6PC were determined in D20, and a normal primary deuterium isotope effect of 1.9 on kcat/Km was observed for the reaction [34]. A primary isotope effect of magnitude of 1.9 is commonly seen in enzymatic reactions in which proton transfer is rate-limiting, although effects of up to 4.0 have been recorded [107-110]. 

The plateau region corresponds to effectively complete N-protonation. The pK value measured spectrophotometrically (1.90) agreed with that derived from the kinetic measurements. Similar good agreement was obtained for the N-Et and 4-Me reactants and also for the unsubstituted phenylhydroxylamine in D2O. The measured solvent KIE was also in agreement with the mechanism in Scheme 7. Acid catalysis at high acidity is believed... 

Hydrogen abstraction from propan-2-ol and propan-2-ol- /7 by hydrogen and deuterium atoms has been studied by pulsed radiolysis FT-ESR. A secondary kinetic isotope effect was observed for H (D ) abstraction from the C—H (C—D) bonds. The results were compared with ab initio data. In similar work, the kinetic isotope effects in H and D abstraction from a variety of other alcohols in aqueous solvents have been measured. It was found that, compared with the gas phase, the reactions exhibit higher activation energies in agreement with the ability of solvation to decrease the dipole moment from the reactant alcohol to the transition state. 

This chapter deals with the fundamental aspects of redox reactions in non-aque-ous solutions. In Section 4.1, we discuss solvent effects on the potentials of various types of redox couples and on reaction mechanisms. Solvent effects on redox potentials are important in connection with the electrochemical studies of such basic problems as ion solvation and electronic properties of chemical species. We then consider solvent effects on reaction kinetics, paying attention to the role of dynamical solvent properties in electron transfer processes. In Section 4.2, we deal with the potential windows in various solvents, in order to show the advantages of non-aqueous solvents as media for redox reactions. In Section 4.3, we describe some examples of practical redox titrations in non-aqueous solvents. Because many of the redox reactions are realized as electrode reactions, the subjects covered in this chapter will also appear in Part II in connection with electrochemical measurements. 

The apparent ability of simple absorption band measurements to provide direct information about kinetic barriers to electron transfer constitutes a significant demonstration of the predicted relationship between optical and thermal electron transfer. Turning the argument around, it is possible to use variations in EoP with solvent, which are easily measured, to gain insight into solvent effects in thermal electron transfer. For example, equation (70) in the form of equation (74) has... 

The first unequivocal evidence for the effective electrophile in an acylation reaction has been presented.59 Reaction of aromatics with aroyl triflates in organic solvents needs no catalyst and allows kinetic investigation in homogeneous solution. The rate-limiting step can be either dissociation of the triflate to the acylium ion or reaction of the latter with the substrate. Kinetic measurements in the presence of base establish firmly the mtennediacy of acylium ions. 

Foti M, Ruberto G, 2001. Kinetic solvent effects on phenolic antioxidants determined by spectrophotometric measurements. J Agric Food Chem 49 342-348. 

Kinetic measurements also show that the solvolysis is of the SnI-type small solvent polarity effects were found in the correlation with the ionizing power parameter qts, with a small m value of 0.12, characteristic of a reaction of a cationic substrate to give a cationic product. Furthermore, the rate data show that the leaving group ability of the phenyliodonio group is about 10 times as great as triflate or 10 -fold higher than iodide. ... 

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