Ester reactions – what causes low yields?
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Hello, everyone! I have carried out several pilot-scale reactions involving esterification, but the yields have always been low (lower than those in lab-scale experiments). I now suspect that an excessively long period of heating under reflux is causing the esters to hydrolyze; I seek guidance from experienced experts. PS: The previous esterification reaction was simple: 5-chlorosalicylic acid + methanol + concentrated sulfuric acid = methyl 5-chlorosalicylate. Due to insufficient flow of cooling freshwater during the cooling and temperature maintenance phase, the overall production time increased by 4–5 hours.
The lipid reaction is itself a reversible reaction; to increase the yield, it is necessary to continuously remove the products and enable a continuous cycle of the reactants
This is quite difficult, as water and methanol are azeotropic; for now, concentrated sulfuric acid is used to absorb the moisture.
That’s exactly what we do: the reaction enters a cyclic pattern, and the reaction mixture is continuously cooled to allow the product to precipitate out. The reason for the low yield at present is unknown; it’s not clear whether too long reaction time and holding cycle time might affect the yield
The reaction vessel should be designed for continuous reaction; an additional separation vessel should be used to cool down and precipitate the product, while the remaining reaction mixture is returned to the reaction vessel to continue the reaction, in order to assess the results
In general, esterification reactions proceed more rapidly when the water present in the products is continuously and promptly removed. Considering the mutual solubility and azeotropy between water and methanol, could 5-chlorosalicylic acid methyl ester be continuously separated out?
It is suspected that methanol volatilized and entered the reflux stream, causing losses; therefore, the temperature during the first holding period can be reduced appropriately so that there is virtually no reflux during this phase. It should be around 120 degrees. After 2 hours, the temperature can be increased directly to 160 degrees, or it can be raised gradually over one hour to 160 degrees – let’s see if this results in a higher yield
Thank you for your feedback! I’ll try it out first to see how it works :)
Yes, it’s just that it’s difficult to obtain the product during the reaction process; generally, the reactants in the reactor are allowed to react completely, after which the mixture enters a cooling and holding phase to allow the product to crystallize, followed by centrifugal separation.
Thank you for your reply. I forgot to mention that the backflow temperature is around 70°C, with the maximum temperature also being 70 degrees. And I think most of the reflux heating is methanol reflux.