3.17 Esters
The union of a carboxylic acid and an alcohol forms an ester. The two molecules readily react, one of them giving up a hydrogen and an oxygen, the other giving up just a hydrogen, releasing water and becoming conjoined on the remaining oxygen atom. This reaction is called esterification. The reaction is reversible, so to make esters a dessicant is used, which is a substance like sulfuric acid or calcium chloride that's thirsty for water and dries out other substances. Reversing an esterification is an example of hydrolysis, meaning loosening by means of water. Esters tend to be solvent-like liquids, insoluble in water.
One simple ester is ethyl acetate CH3CH2OCOCH3, present in wine and contributing to the flavor. The alcohol in the wine is constantly being slowly converted to acetic acid; that's why wine that is left to ferment too long turns to vinegar. Acetic acid can then react with more of the ethanol to make ethyl acetate.
Other esters generally have various sweet, fruity, and flowery aromas. Don't worry about memorizing them all, this is just to give an idea of the kinds of esters that exist:
isoamyl acetate banana |
isoamyl isovalerate apple |
n-butyl butyrate overripe fruit |
ethyl levulinate pleasant, fruit, peony |
methyl salicylate wintergreen |
methyl anthranilate grape |
methyl cinnamate strawberry |
allyl phenoxyacetate floral, honey-like |
fructone fruity, apple, minty |
maltyl isobutyrate sugary, berry, cotton candy |
p-cresyl pivalate leathery, horse-stable, narcissus |
1-octen-3-yl acetate potato skins, mushroom |