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: The labeled oxygen-18 is found in the alcohol products: ethanol in the case of hydrolysis of ethyl benzoate (either with NaOH or HCl) and tert-butyl alcohol in the case of the hydrolysis of tert-butyl ester with HCl.

Step-by-step solution

1. Hydrolysis of ethyl benzoate with aqueous NaOH

To understand the product distribution, we need to look at the mechanism of hydrolysis in the presence of NaOH. NaOH is a strong base, so it will act as a nucleophile, attacking the carbonyl carbon of the ester (ethyl benzoate) and forming a tetrahedral intermediate. The intermediate then collapses back to reform the carbonyl by ejecting the ethoxide ion which takes the labeled oxygen-18. The final products of this reaction are sodium benzoate and ethanol, with ethanol containing the labeled oxygen-18.

2. Hydrolysis of ethyl benzoate with aqueous HCl

In the presence of HCl, ethyl benzoate undergoes an acid-catalyzed hydrolysis. The mechanism involves the protonation of the carbonyl oxygen, followed by the nucleophilic attack of water on the carbonyl carbon, forming a tetrahedral intermediate. The intermediate collapses back by ejecting the ethoxy group with the labeled oxygen-18. The products are benzoic acid and ethyl alcohol. In this reaction, labeled oxygen-18 is again found in the ethanol.

3. Hydrolysis of tert-butyl ester with HCl

The hydrolysis of tert-butyl ester in HCl follows the same acid-catalyzed hydrolysis mechanism as ethyl benzoate in HCl. Thus, the products of the reaction will again have the labeled oxygen-18 in the alcohol. In this case, the products would be benzoic acid and tert-butyl alcohol, where tert-butyl alcohol contains this labeled oxygen-18. In conclusion, under all conditions, the labeled oxygen-18 is found in the alcohol products: ethanol in the case of hydrolysis of ethyl benzoate (either with NaOH or HCl) and tert-butyl alcohol in the case of the hydrolysis of tert-butyl ester with HCl.