Key ideas
Carboxylic acids contain –COOH and are weak acids. Their derivatives, acyl chlorides, anhydrides, esters and amides, differ greatly in reactivity and are made from one another by nucleophilic acyl substitution.
- Acidity: carboxylic acids are stronger than phenols and alcohols, and they release CO₂ from sodium hydrogencarbonate, which alcohols and phenols do not.
- Esterification (Fischer): acid + alcohol ⇌ ester + water with an acid catalyst. It is reversible, and removing water pushes it forward.
- Saponification: an ester heated with NaOH gives the carboxylate salt and the alcohol. It is irreversible.
- Reactivity of derivatives: acyl chloride > anhydride > ester > amide. A more reactive derivative can be turned into a less reactive one.
Key equations
RCOOH + R′OH ⇌ RCOOR′ + H₂ORCOOR′ + NaOH → RCOONa + R′OHRCOOH + NaHCO₃ → RCOONa + CO₂ + H₂Oreactivity: RCOCl > (RCO)₂O > RCOOR′ > RCONH₂
Worked example
Ethanoic acid + ethanol ⇌ ethyl ethanoate + water. The ester name starts with the alkyl group from the alcohol and ends with the acid’s -oate. Heating ethyl ethanoate with NaOH gives sodium ethanoate and ethanol. Ethanoyl chloride reacts with ammonia at once to give ethanamide.
Common mistakes
- Naming the ester from the acid first. The alcohol part comes first, as in ethyl ethanoate.
- Treating esterification as complete. It reaches equilibrium unless water is removed or one reagent is in large excess.
- Writing the acid, not its salt, as the product of saponification in basic solution.
Practice
Test yourself with practice questions on this topic: Carboxylic acids and derivatives