KINETICS study 7.5 Ester hydrolysis

RCOOR'_(aq) + H₂O_(l) ===> RCOOH_(aq) + R'OH_(aq)

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

[Author ©  Dr Phil Brown PhD: Doc Brown's exam revision notes suitable for students of advanced pre-university/college level theoretical-physical chemistry courses: rates of reaction factors: KINETICS of alkaline hydrolysis of an ester [kinetics page updated Mar 13th 2026 *]

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)]

Note

(i) In general for ester hydrolysis, you find experimentally the kinetics gives 1st order with respect to each reactant, 2nd order overall.

(ii) Except if the hydrolysis is in just water as the solvent.

In this case, because the water is effectively constant (the molarity of pure water is 55), overall the kinetics is 1st order i.e. theoretically the rate expression is 2nd order,

rate = k[RCOOR'_(aq)][H₂O_(l)]

so the experiment kinetics rate expression is actually 1st order.

rate = k₁[RCOOR'_(aq)]