[26.3] carboxylic acids

• contain the carboxyl functional group COOH • bonds are in a planar arrangement • are functional group isomeric with esters (RCOOR)

• carbonyl (C=O) group • hydroxyl (O-H) group

• find longest chain of C atoms containing COOH group • remove the e and add oic acid after the basic name • number the chains starting from the end nearer the COOH group • prefix with alkyl substituents • side chain positions are based on the C in COOH being 1

• increases as size increases (due to increased london forces) • carboxylic acids have high boiling points for their relative mass • this arises from intermolecular hydrogen bonding due to polar O-H bonds • extra intermolecular attraction means more energy is needed to separate molecules

• soluble in water due to hydrogen bonding • small ones dissolve readily in water (C1-4) • as mass increases, solubility decreases as the non-polar carbon chain has a greater effect on the overall polarity of the molecule • benzoic acid is fairly insoluble in cold but soluble in hot water • dicarboxylic acids readily dissolve in water

• weak acids • when dissolved in water, carboxylic acids partially dissociate • RCOOH + H₂O (l) ⇌ RCOO⁻ (aq) + H₃O⁺ (aq)

take place in: • redox reactions with metals • neutralisation reactions with bases (alkalis, metal oxides, and carbonates)

• form hydrogen gas an a carboxylate salt • observe metal disappearing and effervescence as hydrogen gas is evolved • 2RCOOH + Mg (s) ➞ (RCOO⁻)₂Mg²⁺ (aq) + H₂ (g)

2RCOOH + CaO (s) ➞ (RCOO⁻)₂Ca²⁺ (aq) + H₂O (l)

• overall: RCOOH + NaOH (aq) ➞ RCOO⁻Na⁺ (aq) + H₂O (l) • ionic: H⁺ (aq) + OH⁻ ➞ H₂O (l)

2RCOOH + Na₂CO₃ (aq) ➞ 2RCOO⁻Na⁺ (aq) + H₂O (l) + CO₂ (g)

• by treatment with a stronger acid • conversion of an acid to its water soluble salt followed by acidification of the salt to restore the acid is often used to separate acids from a mixture • RCOO⁻Na⁺ (aq) + HCl (aq) ➞ RCOOH + NaCl (aq)

• add sodium carbonate • observation: bubbling • 2RCOOH + Na₂CO₃ (aq) ➞ 2RCOO⁻Na⁺ (aq) + H₂O (l) + CO₂ (g)

• they are both weak acids • reaction with sodium carbonate • phenol is too weak of an acid to react with carbonate