Kinetic evidence from monitoring reactions

The use of standard spectroscopic and chromatographic techniques of organic chemistry for mechanistic studies has been emphasised in this chapter. Monitoring of organic syntheses is usually desirable, not only to ensure a suitable extent of reaction before work-up, but also to avoid the apocryphal situation where a reaction claimed to be occurring overnight was actually complete in a few minutes With relatively small changes in experimental procedures, 

Instead of obtaining excellent fits to theoretical rate laws by studying reactions in very dilute solutions (10 2 to 10 r M), reactions could be monitored at the same concentrations as those used for syntheses (e.g. 1 M ). Although this is far from ideal because the reaction medium changes as the reaction occurs, the important point is that relatively few changes in the procedure (requiring little extra effort) are required. Methods of handling the raw kinetic data are considered in Chapter 3. 

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Applications of the bipolar pulse technique have demonstrated its utility in a variety of experiments, but it is particularly useful in monitoring reaction kinetics [18]. The technique has been shown to be useful on the stopped-flow time scale by the investigation of the dehydration of carbonic acid [20]. The study of this widely used text reaction demonstrates the accuracy and precision of the method. A sample data set from a single experiment is shown in Figure 8.16, and the excellent precision obtainable in such experiments is evident. The... 

PSLi to 48 was retarded relative to DPE. The addition reaction in benzene required 3-5 days for completion as monitored by UV-visible spectroscopy. The addition was accelerated in the presence of small amounts of THF ([THF]/[Li]=30) such that addition was complete in 12 h. Using the rate constant for addition of PSLi to 48 [1.7xlO M s ], the sigma value (a) for the tert-butyldimethylsiloxy substituent was calculated to be -0.3 compared to the a value of -0.46 calculated for this substituent determined from the kinetics of addition of PSLi to 24 under the same conditions [170]. The reduced effectiveness with two substituents is consistent with the X-ray structural evidence for diethyl ether solvated diphenylmethyllithiums which indicates that one of the phenyl rings is distorted by an angle of 37° from coplanarity with the other ring and the diphenylmethyl carbon [52] as discussed in Sect. 2.2. 

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