In order to control, optimize, and analyze organic chemical reactions, it is vital to gain a mechanistic understanding of how they occur. It can be hard to study reaction kinetics, however, when the rate of reaction is extremely fast. This presents an obstacle to this understanding.
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Guy Lloyd-Jones, the Professor of Organic Chemistry at Forbes, gave a webinar in which he presented some new, exciting techniques he had develop with his colleagues in order to overcome this issue. He described how these techniques can be applied to simple, regular laboratory instruments.
Professor Lloyd-Jones began his webinar by outlining the meaning of “fast” in this context, explaining why fast reactions can be hard to monitor. Fast reactions present problems in the following areas: fully initiating the reaction; temperature control during the initiation of the reaction and the reaction itself; and precisely assembling the reaction with the correct stoichiometries and component volumes. Furthermore, the reaction needs to be homogenous, which is difficult in the case of NMR-based experiments in which a long, thin reaction vessel is used.
As an example, Professor Lloyd-Jones outlined the lack of homogeneity and poor mixing which can occur using the protodeboronation of difluorinated boronic acid. He showed how complete homogenous mixing has not taken place, as indicated by the NMR spectra generated.
Focusing on the classic stop-flow (SF) technique, he outlined the methods which can be used for in situ analysis. Along with TgK and Bruker four years previously, Professor Lloyd-Jones adapted this technique in order to provide an efficient, variable ratio multi-mixing system which saves both money and time. As well as introducing Bruker’s InsightXpress stop-flow solution, Professor Lloyd-Jones explained how it is possible to interface the system with NMR and infra-red (IR) spectrometers.
With the classic SF technique, component A meets component B before they react while passing down a tube. Once the flow has stopped, the reaction is analyzed at a particular point along the tube. Outlining this method’s advantages and limitations, Professor Lloyd-Jones gave a schematic overview of the SF set up and process.
Benefits of SF Technique
Benefits of this method include the ability to obtain kinetics data which is highly reproducible; the ability to purge all contaminants out; and the useful communication between the spectrometer, SF device, and the computer used to monitor the process.
Professor Lloyd-Jones then adumbrated how the reaction was performed four times in order to obtain four kinetic datasets, and how different sets of kinetics can be generated by altering the concentration of individual components in the reaction. Using the different rates, a graph can then be generated which shows the way the system responds to the concentration of this component and the kinetic dependents on it.
Even though this is very useful information, Professor Lloyd-Jones explained that three stock solutions of the individual component need to be made up in order to obtain these three data sets. Even though the SF technique is quick and very reproducible, valuable time and materials must be used as it is labor intensive to vary the component concentration and each solution must be washed out completely.
Consequently, Professor Lloyd-Jones described the new system he, Bruker, and TgK had created four years earlier. They developed a variable ratio multi-mixing system in which at least three inputs, which are independently driven, are used. This allows for the generation of different outputs, different reaction mixtures, and the possibility of keeping one component constant, if necessary.
Following this, the professor discussed this new system in the context of NMR and IR studies.
In relation to IR, he outlined the CF3 transfer’s kinetics, as well as the advantages of pairing multi-mixing with an attenuated total reflectance (ATR) set-up. Furthermore, the professor offered an example of using MX-SF-ATR Fourier transform infrared spectroscopy to deliver an analysis of a fast and air-sensitive reaction which is both time- and reagent-efficient.
Professor Lloyd-Jones described some of the issues inherent in NMR-based experiments, such as the lack of reproducibility, inefficient mixing, and poor temperature control. He then continued to explain how the multi-mixing SF solution can address these issues.
The professor illustrated how it is possible to use the technique for a protodeboronation reaction using Bruker’s InsightXpress SF solution. He then has a schematic overview of this reaction. He also explained the technique’s benefits, such as high reproducibility, sample efficiency, simple cleaning, high quality data density, and precise and variable temperature control.
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