Organic Synthesis

Synthesis of isoamyl acetate (pear essence)



3-methyl-1-butyl acetate or isoamyl acetate is a colorless liquid with b.p. 142 °C. It is best known for its characteristic odor. It is used worldwide as a flavoring agent – either as a banana flavor or as a pear oil (referring to an alcoholic solution of isoamyl acetate). It can be naturally found in a honey bee´s sting apparatus, serving as a pheromone. The compound can be simply prepared by means of Fischer esterification when isoamyl alcohol and glacial acetic acid are used as reactants. For its price, acetic acid is used in excess.  Typically, sulfuric acid is used as a catalyst. Reaction mechanism is depicted below.



3-methylbutan-1-ol (5.3 mL, 50 mmol), acetic acid (11.5 mL, 200 mmol) and a few boiling chips were placed in 50 mL round bottom flask. 1 mL of concentrated sulfuric acid was added as a catalyst. The flask with the mixture was fitted to a reflux condenser and refluxed for 1.5 h on a hot oil bath.

The flask was cooled down a little by immersing under the stream of cold water and then the mixture was poured into a beaker containing approximately 25 g of cracked ice. The mixture was stirred for another few minutes and transferred to a 250 mL separatory funnel. The reaction flask and the beaker were rinsed with 20 mL of diethyl ether. The organic solvent was added to the funnel. After shaking the funnel and allowing layers to separate, the aqueous phase has been discharged. The process was repeated one more time. The organic phase was washed with 30 mL of 5%  iron(II) sulfate solution and then with 2×15 mL portions of the sodium carbonate solution. The organic phase was dried over anhydrous sodium sulfate, filtered under gravity and evaporated using rotary vacuum evaporator.  (For reaching colorless liquid, the product should be distilled over).