Esters and esterification — Chemistry, 14–17
An ester forms when an alcohol and a carboxylic acid react, usually making a sweet-smelling molecule and water. The reaction links structure to smell, reversibility and useful materials.
Making an ester
In esterification, an alcohol and a carboxylic acid join and release water. The new molecule contains the ester link, –COO–, and often has a noticeable fruity or sweet smell, although smell alone is not a safe identification method.
Why make esters?
Chemists wanted to turn small, strongly smelling substances into compounds with useful scents and properties. The reaction also solves a practical problem: joining two molecules gives a new substance whose boiling point, smell and solubility can differ greatly.
Ethanol plus ethanoic acid
Ethanoic acid, CH₃COOH, reacts with ethanol, CH₃CH₂OH. Remove OH from the acid and H from the alcohol to make H₂O; join the remaining parts to get CH₃COOCH₂CH₃, ethyl ethanoate.
The trap: it goes to completion
Because water is produced, it is tempting to think all the acid and alcohol must become ester. In fact, esterification is reversible: the ester and water can react back, so an equilibrium mixture remains unless conditions remove a product.
Where esters appear
Esters are used in fragrances and flavourings, where their smells are valuable in tiny amounts. Larger esters are also used as solvents, and reactions involving esters help make biodiesel and some biodegradable materials.
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