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Advanced level chemistry kinetics notes: Ester hydrolysis rate expressions

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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) doc b 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 (Pk(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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