Ester

An ester is a carbon compound that contains the ester linkage, –COO–, and is formed when a carboxylic acid reacts with an alcohol. Esters have sweet, fruity smells and are the compounds behind many natural and artificial flavours and scents. Malay: ester · Chinese: 酯.

An ester is made by esterification: a carboxylic acid and an alcohol are warmed together with a few drops of concentrated sulfuric acid, which acts as a catalyst, and water is removed at the same time. The –COO– group is built from the –COOH of the acid and the –OH of the alcohol, and the name of the ester carries both parents, so ethanoic acid and ethanol give ethyl ethanoate. The very same –COO– linkage is found in fats and oils, which are natural esters of glycerol and long-chain fatty acids, so the small esters of the laboratory and the large esters of living things share one core structure. Because they smell pleasant and dissolve many substances, small esters are widely used as artificial flavourings, as perfumes and as solvents.

Example. Warming ethanoic acid with ethanol and a little concentrated sulfuric acid produces ethyl ethanoate, an oily layer with a pleasant, glue-like smell that floats on the reaction mixture. Poured into water it can be smelled clearly, which is how the reaction is confirmed in the laboratory.

Confusion to avoid. Do not confuse the ester with the carboxylic acid it comes from. The acid has –COOH, is sour and turns litmus red; the ester has –COO– with no free –OH, is neutral and smells sweet. Take care with the naming order too: the alkyl part from the alcohol comes first (ethyl), then the part from the acid (ethanoate).

Esters draw together the alcohol and the carboxylic acid, the two oxygen-containing series of the chapter, through esterification, so learning this term ties a neat bow around the reactions of Carbon Compounds in SPM Chemistry.