Even back in 2013 when this blog was first written, continuous flow reactors were increasingly popular. They have been available commercially for many years. They have become mechanically sophisticated in their pumping and controls. But even in Organic Synthesis Coll. Vol. III pg. 172 the synthesis of Carboxymethoxyamine Hydrochloride is described and it uses a continuous flow reactor in the first step.
The reactor works by gravity flow and is made from simple glassware and operates at 100 C using steam heating.
The procedure can be expected to work for reactions that are slow at room temperature or below but procedure rapidly at 100 C. The Organic Synthesis procedure combines acetone oxime with bromoacetic acid using an aqueous base:
“A mixture of 612 g. (4.4 moles) of bromoacetic acid and 500 g. of crushed ice is chilled in an ice-salt bath and made distinctly alkaline to litmus with sodium hydroxide ( about 440 g. of a 40% solution). During the neutralization, an additional 500 g. of ice is added. To the solution are then added 292 g. (4.0 moles) of acetoxime and 440 g. of 40% sodium hydroxide (4.4 moles), the temperature being held below 20 C during the addition of the alkali. The mixture is then allowed to flow dropwise, during 3-4 hours, through the inner tube of a steam-heated Liebig condenser (jacket 75 cm. long; inner tube 10-mm diameter; angle of inclination about 20 degrees) into a 5-l. round-bottomed flask cooled with running water (Note 2).”
“A mixture of 612 g. (4.4 moles) of bromoacetic acid and 500 g. of crushed ice is chilled in an ice-salt bath and made distinctly alkaline to litmus with sodium hydroxide ( about 440 g. of a 40% solution). During the neutralization, an additional 500 g. of ice is added. To the solution are then added 292 g. (4.0 moles) of acetoxime and 440 g. of 40% sodium hydroxide (4.4 moles), the temperature being held below 20 C during the addition of the alkali. The mixture is then allowed to flow dropwise, during 3-4 hours, through the inner tube of a steam-heated Liebig condenser (jacket 75 cm. long; inner tube 10-mm diameter; angle of inclination about 20 degrees) into a 5-l. round-bottomed flask cooled with running water (Note 2).”
Note 2 says that “[b]y this procedure, the reaction takes place in a few seconds, and the formation of by-products is minimized. If the solution of the reactants is heated in bulk, the reaction temperature cannot be controlled and a lower yield is obtained of a dark product which, however, can be purified by distillation under reduced pressure.”
The total throughput can be calculated to be 2784 g of solution which passes, in we can approximate, about 3.5 hours. That is 13.3 g. per minute. The actual duration that material is heated within the steam-heated 100 C zone is determined by the angle of declination of the condenser tube. One can imagine that using instead of a Liebig condenser an Allihn condenser, that has a series of bulbs through which the liquid must pass, would imitate the effect of a series of continuously stirred tank reactors and the condenser would not need to be so long to have the heat contact time.
The total throughput can be calculated to be 2784 g of solution which passes, in we can approximate, about 3.5 hours. That is 13.3 g. per minute. The actual duration that material is heated within the steam-heated 100 C zone is determined by the angle of declination of the condenser tube. One can imagine that using instead of a Liebig condenser an Allihn condenser, that has a series of bulbs through which the liquid must pass, would imitate the effect of a series of continuously stirred tank reactors and the condenser would not need to be so long to have the heat contact time.
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