Jun 2017
For my Organic Chemistry lab, we are synthesizing esters via microwave-assisted Fischer esterification. My task is to synthesize n-butyl benzoate. The procedure provides us with two options, either use excess alcohol or excess carboxylic acid to obtain the highest yield and purest product. We get to pick which procedure we think is best. After looking into the equilibria behind this reaction, I am leaning towards using excess carboxylic acid over excess alcohol. According to Le Chateliers principle, doing either will drive the reaction towards the synthesis of the ester. Here's my reasoning:

(1) Because I am synthesizing n-butyl benzoate, my alcohol/solvent is going to be 1-butanol. This has a relatively high boiling point (about 118 degrees Celcius), which is going to be hard to evaporate off by using steam distillation/rotary evaporation. Even though the n-butyl benzoate has a boiling point of about 250 degrees celcius and wouldn't be effected by steam distillation/rotary evaporation, I feel like it would be hard to evaporate off all of the excess alcohol and obtain the purest product (even boiling off excess methanol in a previous lab took a really long time using rotary evaporation)

(2) Adding excess carboxylic acid seems to be a lot easier to extract off by doing liquid-liquid extraction. Just rinse the solution with sodium bicarbonate, you create a carboxylate salt and that will cause it to associate with the aqueous layer. Even though I still have to do steam distillation/rotary evaporation, it would be much easier to boil off the remaining alcohol than if I were to use it in excess to begin with.

Additionally, one concern about the procedure I do have is during the liquid-liquid extraction of the ester. Because the density of the ester is at 1.00g/mL +/- 0.01 and water is also at 1.00g/mL, how does the ester separate from the aqueous layer if the densities are the same? Or is it just because the n-butyl benzoate is not polar and water is? In that case, despite their similar densities, ultimately the separation comes down to the differences in polarities, not the densities?

Finally, my only hesitation with using excess carboxylic acid is that standard procedures involving esterifications use excess alcohol. Can you please explain the costs and benefits to using both excess carboxylic acid excess alcohol? Any help would be appreciated, thank you.