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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Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced level physical theoretical chemistry students US K12 grade 11 grade 12 physical theoretical chemistry courses topics including kinetics rates of reaction speeds AQA Edexcel OCR Salters

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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