Ethanoic Acid and Esters

Ethanoic acid (also called acetic acid) has formula (CH3COOH). It is the acid in vinegar (about 5%). Its functional group is the carboxyl group, –COOH. Ethanoic acid is a weak acid: it only partly releases hydrogen ions in water.

Weak acid equation:

[ \mathrm{CH_3COOH(aq) \rightleftharpoons H^+(aq) + CH_3COO^-(aq)} ]

Reactions of Ethanoic Acid

• With metals (e.g., magnesium): makes a salt (an ethanoate) and hydrogen gas. [ \mathrm{2CH_3COOH(aq) + Mg(s) \rightarrow Mg(CH_3COO)_2(aq) + H_2(g)} ] The gas pops with a lighted splint.
• With bases (alkalis/oxides): neutralisation to make a salt and water. Example with sodium hydroxide: [ \mathrm{CH_3COOH + NaOH \rightarrow CH_3COONa + H_2O} ]
• With carbonates: makes a salt, water, and carbon dioxide. Example with sodium carbonate: [ \mathrm{2CH_3COOH + Na_2CO_3 \rightarrow 2CH_3COONa + H_2O + CO_2} ] Carbon dioxide turns limewater milky.

Making Ethanoic Acid from Ethanol

• Chemical oxidation using acidified potassium manganate(VII), KMnO4: purple to colourless. [ \mathrm{C_2H_5OH + 2[O] \rightarrow CH_3COOH + H_2O} ]
• Bacterial oxidation (vinegar making): air and bacteria change ethanol to ethanoic acid. [ \mathrm{C_2H_5OH + O_2 \rightarrow CH_3COOH + H_2O} ]

Esters from Ethanoic Acid

An ester forms when a carboxylic acid reacts with an alcohol, using an acid catalyst (often concentrated sulfuric acid) and warming. Esters often smell fruity and are used in flavourings and solvents.

Key example (ethyl ethanoate): [ \mathrm{CH_3COOH + C_2H_5OH \rightleftharpoons CH_3COOC_2H_5 + H_2O} \quad \text{(H_2SO_4, heat)} ]

• Naming rule: alcohol gives the first part (alkyl), acid gives the second part (…oate). Ethanol + ethanoic acid → ethyl ethanoate. Methanol + ethanoic acid → methyl ethanoate.
• Displayed link in esters is the –COO– group joining the acid part to the alcohol part.

Common misconceptions

• Not all acids are strong. Ethanoic acid is weak but still reacts in the same patterns as other acids.
• In ester names, the alcohol part comes first (alkyl), then the acid part (…oate).