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According to experience, in small-scale reactions below 30g, the above factors can only be considered qualitatively.
Another important point is that when the reaction heat exceeds the safe heat capacity of the organic solvent or the entire reaction system, the risk can be effectively reduced by removing the reaction heat in time
Case 1
A R&D personnel reacted as follows:
To a solution of the substrate 3L 4mol diethyl ether solution (three bottles 5L) was added dropwise phosphorus oxychloride , copper-zinc mixture trichloroacetyl chloride and diethyl ether overshoot occurs after the mixture was added dropwise the whole amount of 1/4 Therefore, the dripping was stopped
Reason analysis: the solvent ether has a small specific heat capacity, a low boiling point, and a small dosage ratio, resulting in a small overall heat capacity; while the reaction exotherms more, which greatly exceeds the safe heat capacity of the acetaldehyde used, and when the temperature rises slightly, flushing occurs
Countermeasures: Use isopropyl ether, methyl tetrahydrofuran or methyl tertiary butyl ether and other solvents with high specific heat capacity and higher boiling point, and increase the proportion of the amount of anhydrous solvents used to increase the safe heat capacity of the system and avoid the problem of flushing.
Case 2
The reaction using ether as the solvent is as follows:
In a 5L three-necked flask, 200g of sodium sand was prepared with toluene under the protection of nitrogen, 3L of ether and 450g of TMSC1 were sequentially added , and then 500g of ethyl chloropropionate was added dropwise
Reason analysis: It is caused by many reasons.
Figure 24-17 Punching and burning and explosion scenes
Other R&D personnel have also done this reaction, but use isopropyl ether or methyl tert-butyl ether with high specific heat capacity and high boiling point instead of diethyl ether.
In the design of organic synthesis, the use of ether as a solvent is most inconsistent with the safety requirements of heat capacity.
After analyzing some closed explosive reaction cases, it is found that if the reaction liquid can be properly diluted with a solvent in advance, the reaction heat can be absorbed and stored, the temperature of the reaction liquid will not rise sharply, and there will be no explosion and the substrate or product Carbonization at high temperature
Related Links: Safe Heat Capacity and Reaction Heat Conduction (1)