Reaction Time Control

Fluoro-l-phenylethanone is also reduced by triphenyltin hydride in a 2,2 -azobisisobutyroni-trile initiated reaction to yield almost exclusively acetophenone. The selectivity decreases as the solvent becomes more polar.96 Electrolytic reduction of 2,2,2-trifiuoro-l-phenylethanone (8) does not proceed to the expected trifluoromethylated pinacol, but results in acetophenone and/or acetophenone pinacol depending on the potential employed the reaction time controls the product ratio.97... 

The temperature-controlling features of this reaction scheme dominate selection and use of the reactor. However, the semibatch reactor does have some of the advantages of batch reactors temperature programming with time and variable reaction time control. 

Yoshida J (2010) Flash chemistry flow microreactor synthesis based on high-resolution reaction time control. Chem Rec 10 332-341... 

Concentration control Reaction time control Temperature control... 

The ratio of the three products depends on the molar ratio of chloroform used as well as on reaction time. Control experiments showed that generation of dichlorocarbene in the presence of I led to II and III, while generation of dichlorocarbene in the presence of II led to high yields of III. The lack of reactivity of the carbon-carbon double bond in the second cyclobutene ring may result from factors similar to those operating in the norbbrnadiene case. 

Automatic reaction control, variable ionization and reaction time control in internal ionization ion trap mass spectrometers for chemical ionization. A built-up of Cl ions in the ion trap is facihtated by the AGC control until the maximum capacity or preset maximum reaction time is reached. This results in the inherent high sensitivity of the ion trap analyser for full scan data acquisition in full scan mode. 

High-resolution reaction time control based on controlling the residence time discussed in this chapter is one of the most important features of flow microreactor synthesis. This feature enables chemical synthesis that cannot be done in batch. Some examples will be discussed in the following chapters. 

Fast Micromixing for High-Resolution Reaction Time Control... 

As described above, high-resolution reaction time control enables switching product selectivity at will. A product derived from a reactive species that has yet to be isomerized can be obtained by setting a shorter reaction time, or a product derived from an isomerized reactive species can be obtained by setting a longer reaction time. 

For some oxiranyllithium species, which we have discussed in Chap. 2, two stereoisomers can exist. In such cases, an oxiranyllithium species has another side reaction that needs attention Isomerization caused by inversion of configuration at its carbon atom bound to lithium in addition to its decomposition involving ring opening. To synthesize a substituted epoxide stereoselectively, we need to suppress such isomerization and allow only one isomer to selectively react with an electrophile. For this purpose, high-resolution reaction time control using a flow microreactor system is effective. 

The concept of high-resolution reaction time control using a flow microreactor system can also be effectively applied to asymmetric synthesis. In some cases, enantiomerically enriched intermediates can easily be isomerized (racemized) reducing the enantioselectivity of the reaction. Batch reaction would often give products with low enantiomeric purity because of such isomerization. Flow microreactor system, however, could allow the formation of products with high enantiomeric purity by first generating the intermediates and then promptly using it in subsequent reactions before isomerization. An example of such synthesis will now be described. 

Flash chemistry makes highly short-lived reactive species usable as starting materials or reagents for chemical synthesis. In short, unstable reactive species can be generated and transferred to another location to be used in the next reaction before they decompose or isomerize based on high-resolution reaction time control. Therefore, chemical reactions that are practically impossible in batch macroreactors should be made possible by flash chemistiy. 

The chain-growth polymerization that finds the widest industrial uses is radical polymerization. However, a carbon radical is typically highly reactive and has a shorter lifetime than a carbocation or a carbanion. Radial polymerization is thus difficult to control based on high-resolution reaction time control, which is effective for cationic and anionic polymerizations described in this chapter. In other words, by using a simple flow microreactor technique radical polymerization cannot be living... 

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