Esters

An ester contains the –COO– link and is named [alcohol]yl [acid]oate (the alcohol-derived alkyl group is written first). For example, CH₃CH₂COOCH₃ is methyl propanoate (from propanoic acid + methanol).

There are two ways to make an ester:

• Acid + alcohol (conc H₂SO₄ catalyst) — a slow, reversible equilibrium: $\text{CH}_3\text{COOH} + \text{C}_2\text{H}_5\text{OH} \rightleftharpoons \text{CH}_3\text{COOC}_2\text{H}_5 + \text{H}_2\text{O}$
• Acyl chloride + alcohol — fast and irreversible, releasing HCl: $\text{CH}_3\text{COCl} + \text{C}_2\text{H}_5\text{OH} \rightarrow \text{CH}_3\text{COOC}_2\text{H}_5 + \text{HCl}$

The acyl-chloride route is preferred when a high yield is needed (it goes to completion).

Esters are hydrolysed (split by water) in two ways:

• Acid hydrolysis (reflux, dilute acid) — reversible, giving the carboxylic acid + alcohol: $\text{CH}_3\text{COOC}_2\text{H}_5 + \text{H}_2\text{O} \rightleftharpoons \text{CH}_3\text{COOH} + \text{C}_2\text{H}_5\text{OH}$
• Alkaline hydrolysis (reflux, dilute NaOH) — irreversible, giving the carboxylate salt + alcohol: $\text{CH}_3\text{COOC}_2\text{H}_5 + \text{NaOH} \rightarrow \text{CH}_3\text{COONa} + \text{C}_2\text{H}_5\text{OH}$

Alkaline hydrolysis goes to completion because the carboxylate salt cannot recombine — useful when you want full conversion. To deduce an unknown ester, identify the acid and alcohol parts from the hydrolysis products.

Fats and oils are triglycerides — esters of glycerol (propane-1,2,3-triol) with three long-chain fatty acids, so they contain three –COO– ester links and undergo ester reactions.

Saponification is the alkaline hydrolysis of a triglyceride: boiling a fat with NaOH gives glycerol + the sodium salts of the fatty acids (soap).

Maximising ester yield (for the reversible acid + alcohol route) uses Le Chatelier: use an excess of one reagent, remove water (conc H₂SO₄ absorbs it), or distil off the ester — or simply use the acyl-chloride route, which is irreversible.