Question

Predict the distribution of oxygen- 18 in the products obtained from hydrolysis of ethyl benzoate labeled in the ethoxy oxygen under the following conditions. (a) In aqueous \(\mathrm{NaOH}\) (b) In aqueous \(\mathrm{HCl}\) (c) What distribution would you predict if the reaction were done with the tert-butyl ester in HCl?

Short answer

Answer: Under all given conditions, the labeled oxygen-18 remains in the carboxylic acid or carboxylate ion, and does not get incorporated into the alcohol product.

Step-by-step solution

Write the general reaction of ester hydrolysis

Ester hydrolysis is the chemical process of breaking an ester into its constituent alcohol and carboxylic acid in the presence of a base or an acid. The general chemical equation for ester hydrolysis can be written as: Ester + Water → Alcohol + Carboxylic acid

Determine the hydrolysis mechanism for each condition

Ester hydrolysis can occur via two mechanisms depending on the conditions: base-catalyzed hydrolysis (also called saponification) and acid-catalyzed hydrolysis. For each condition given in the exercise, determine which mechanism will take place: (a) In aqueous NaOH, a strong base will promote base-catalyzed hydrolysis. (b) In aqueous HCl, a strong acid will promote acid-catalyzed hydrolysis. (c) With a tert-butyl ester and HCl, a strong acid will promote acid-catalyzed hydrolysis.

Write the specific equations for the reactions under each condition

For each condition, write the specific reaction where the ester is Ethyl benzoate labeled in the ethoxy oxygen (oxygen-18 labeled as \(\stackrel{18}{\mathrm{O}}\)): (a) Base-catalyzed hydrolysis (NaOH): Ethyl benzoate-\(\stackrel{18}{\mathrm{O}}\) + \(\mathrm{H_2O}\) → Ethanol + Benzoate ion (\(\stackrel{18}{\mathrm{O}}\)) + \(\mathrm{Na^+}\) (b) Acid-catalyzed hydrolysis (HCl): Ethyl benzoate-\(\stackrel{18}{\mathrm{O}}\) + \(\mathrm{H_2O}\) → Ethanol + Benzoic acid (\(\stackrel{18}{\mathrm{O}}\)) + \(\mathrm{Cl^-}\) (c) Acid-catalyzed hydrolysis with tert-butyl ester (HCl): Tert-butyl benzoate-\(\stackrel{18}{\mathrm{O}}\) + \(\mathrm{H_2O}\) → tert-Butanol + Benzoic acid (\(\stackrel{18}{\mathrm{O}}\)) + \(\mathrm{Cl^-}\)

Predict the distribution of oxygen-18 in the products

Now that we have the specific reactions for each condition, predict the distribution of the labeled oxygen-18 in the products: (a) In aqueous NaOH, the labeled oxygen-18 remains in the carboxylate ion (benzoate ion) because base-catalyzed hydrolysis attacks the carbonyl carbon of the ester leaving the labeled oxygen undisturbed in the products. (b) In aqueous HCl, the labeled oxygen-18 remains in the carboxylic acid (benzoic acid) because acid-catalyzed hydrolysis involves the attack of water to the carbonyl carbon, followed by protonation and cleavage of ester bond. The labeled oxygen stays in the carboxylic acid group, similar to the base-catalyzed hydrolysis. (c) With the tert-butyl ester in HCl, the labeled oxygen-18 remains in the carboxylic acid (benzoic acid) because the hydrolysis mechanism is analogous to condition (b), where the labeled oxygen stays in the carboxylic acid group and does not get exchanged or removed during the reaction. In conclusion, under all given conditions, the labeled oxygen-18 remains in the carboxylic acid or carboxylate ion and does not get incorporated into the alcohol product.