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Kinetics-Rates revision notes Part 7

7.5 Ester hydrolysis

RCOOR'_(aq) + H₂O/OH^–_(aq) ===> RCOOH/RCOO^–_(aq) + R'OH_(aq)

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Case study 4.5 Ester hydrolysis

Explanation and derivation of orders of reactants and how to write the rate expression

• Esters can be hydrolysed by (i) water alone (but very slow), but is (II) catalysed by acids (H⁺_(aq) specifically) and (iii) alkalis (OH^–_(aq) specifically). The latter hydrolysis (iii), is sometimes called a saponification reaction.

  • equation for (i)/(ii): RCOOR'_(aq) + H₂O_(l) RCOOH_(aq) + R'OH_(aq)

  • equation for (iii): RCOOR'_(aq) + OH^–_(aq) ====> RCOO^–_(aq) + R'OH_(aq)

• Hydrolysis with water alone is usually too slow to obtain meaningful rate data.

• However, with a fixed amount of acid catalyst e.g. HCl_(aq), it is possible to follow the reaction by alkali titration and show that ...

  • rate = k₁[RCOOR'_(aq)], that is 1st order with respect to ester, however,

  • if the concentration of acid catalyst is varied, things are more complicated,

  • and the 2nd order expression, rate = k₂[RCOOR'_(aq)][H⁺_(aq)] then applies.

• With alkali, 2nd order kinetics are observed overall, and the reaction can be followed by titrating the remaining alkali with acid. Phenolphthalein (P_k(ind) = 9.3), would be a suitable indicator because one of the reaction products is the salt of a weak carboxylic acid (approx. pH 9). The product of the titration is neutral sodium chloride.

  • i.e. rate = k₂[RCOOR'_(aq)][OH^–_(aq)], 1st order with respect to each reactant, 2nd order overall.

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