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 

The first step, which is rate determining, is an ionization to a carbocation (carbonium ion in earlier terminology) intermediate, which reacts with the nucleophile in the second step. Because the transition state for the rate-determining step includes R-X but not Y , the reaction is unimolecular and is labeled S l. First-order kinetics are involved, with the rate being independent of the nucleophile identity and concentration. 

This bimolecular process is called the S/ 2 mechanism. It yields overall second-order kinetics (unless the nucleophile is the solvent, in which case apparent first-order kinetics are seen). 

The evidence supporting the duality of mechanisms is of several kinds. The kinetic behavior is an obvious feature. This is somewhat more complex than is implied by the preceding treatment. A quantitative description of the SnI mechanism requires recognition of the reversibility of the ionization step, thus 

Because the cationic intermediate is unstable, it will be permissible to apply the steady-state approximation, leading to Eq. (8-65) for the reaction rate. 

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With SECM, almost any kind of electrochemical measurement may be carried out, whether voltaimnetric or potentiometric, and the addition of spatial resolution greatly increases the possibilities for the characterization of interfaces and kinetic measurements [, and 59]. It may be employed as an electrochemical tool... 

Relaxation kinetics may be monitored in transient studies tlirough a variety of metliods, usually involving some fonn of spectroscopy. Transient teclmiques and spectrophotometry are combined in time resolved spectroscopy to provide botli tire stmctural infonnation from spectral measurements and tire dynamical infonnation from kinetic measurements that are generally needed to characterize tire mechanisms of relaxation processes. The presence and nature of kinetic intennediates, metastable chemical or physical states not present at equilibrium, may be directly examined in tliis way. 

One otlier common source of nonlinear response, singlet-triplet annihilation, is often tire reason for a discrepancy between fluorometric and absorjDtion kinetic measurements [27, 28 and 29]. 

Since for the kinetic measurements a large excess of catalyst is used, the concentration of free catalyst [M" ]/ essentially equals the total concentration of catalyst. Moreover, since ko k jt and under the conditions of the measurements the concentration of free 2.4 is small, the contribution of the uncatalysed reaction can be neglected and equation 7 simplifies to ... 

Kinetic measurements were performed employii UV-vis spectroscopy (Perkin Elmer "K2, X5 or 12 spectrophotometer) using quartz cuvettes of 1 cm pathlength at 25 0.1 C. Second-order rate constants of the reaction of methyl vinyl ketone (4.8) with cyclopentadiene (4.6) were determined from the pseudo-first-order rate constants obtained by followirg the absorption of 4.6 at 253-260 nm in the presence of an excess of 4.8. Typical concentrations were [4.8] = 18 mM and [4.6] = 0.1 mM. In order to ensure rapid dissolution of 4.6, this compound was added from a stock solution of 5.0 )j1 in 2.00 g of 1-propanol. In order to prevent evaporation of the extremely volatile 4.6, the cuvettes were filled almost completely and sealed carefully. The water used for the experiments with MeReOj was degassed by purging with argon for 0.5 hours prior to the measurements. All rate constants were reproducible to within 3%. 

All kinetic measurements were performed using UV-vis spectroscopy (Perkin Elmer 2, 5 or 12 photo spectrometers) as described in Chapter 2. 

This method is exemplified by its application to quinoline, isoquinoline, cinnoline, and isoquinoline 2-oxide, which are nitrated as their conjugate acids. The rate profiles for these compounds and their N- or O-methyl perchlorates show closely parallel dependences upon acidity (fig. 2.4). Quaternisation had in each case only a small effect upon the rate, making the criterion a very reliable one. It has the additional advantage of being applicable at any temperature for which kinetic measurements can be made (table 8.1, sections B and D). 

ANTIPARASITICAGENTS - ANTIMYCOTICS] (Vol3) -kinetic measurements [KINETIC MEASUREMENTS] (Vol 14)... 

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