Sampling and analysis for kinetics

When data are collected automatically and calculations are also automated, hundreds of data points are often used to obtain rate constants (see Chapter 3). Readers contemplating their first foray, using a manual method of sampling, will be relieved to learn that reliable rate constants can be obtained from only about ten data points, collected at approximately equal extents of reaction, for a well-thermostatted, well-behaved reaction. Even fewer data points are adequate, especially if an infinity reading is available if the disappearance of starting material is being monitored, an infinity reading of zero may be assumed. 

For faster reactions, it maybe necessary to monitor the progress of the reaction in an NMR tube in a thermostatted NMR probe. If an aliquot method is used, it should be possible to remove an aliquot rapidly and quench it by adding cold solvent - reaction rates are lower under colder, more dilute conditions. In reactions involving bases or acids, reactions may be quenched by addition of acidic or basic buffer solutions. Of course, whether or not the reaction can be quenched efficiently, the aliquots should be analysed as expeditiously as possible to reduce the possibility of further reaction or degradation. 

Many different types of reactions can be monitored by reverse phase HPLC, in some cases by automated injections from a thermostatted HPLC sample tray [33], or alternatively after quenching aliquots in methanol. No additional work-up is required, and a wide range of product polarities can be accommodated. If all of the products dissolve in the methanol solution, all of the products should be detected, unless their retention times are exceedingly long. If reaction volumes are controlled accurately either by automated injections [33] or by controlling solvent volumes (of aliquots withdrawn, volumes of solvent used to quench 

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