tags
acetic anhydrideDCMsodium propionatebrominedistillatedichloromethanepropionylacyl chloridesdisulfurpropionicsulfur
video tutorial Propionic anhydride - testing two preparation methods
- Two preparations of propionic anhydride were tested: method A using bromine in dichloromethane with dry sodium propionate; method B (classical) using propionyl chloride with sodium propionate.
- Method A setup involved a 500 mL flask with an ice bath, thermometer, and stirrer. Bromine solution in dichloromethane (57 mL, ~97 g, ~35 g Br2) and 15 g dry sulfur were added to 50 g sodium propionate. After partial reaction and reflux, distillation showed DCM first; the distillate became pale orange, sulfur dioxide was emitted, solids did not dissolve readily, and crystals of sulfur formed. The final product was not propionic anhydride; the mixture ultimately appeared black and was deemed a failure.
- Method B used a classical approach: 25 g sodium propionate in a 250 mL flask with thermometer, adding 17 mL propionyl chloride in portions while cooling. Temperature rose sharply; after completion, distillation yielded 20 g propionic anhydride (79% yield) with a slightly sweet, sharp, lacrimatory aroma and a fruity note, indicating good sodium propionate quality.
- Physical check: a drop test in cold water showed the product is insoluble and sinks as a blob, similar to acetic anhydride.
- Conclusion: method A likely does not work for propionic anhydride (possible need for longer reflux or DCM effects); method B successfully produces propionic anhydride in about 79% yield.
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