The mechanism for hydrolysis of an ester in aqueous acid involves formation of a tetrahedral carbonyl addition intermediate. Evidence in support of this mechanism comes from an experiment designed by Myron Bender. He first prepared ethyl benzoate enriched with oxygen- 18 in the carbonyl oxygen and then carried out acidcatalyzed hydrolysis of the ester in water containing no enrichment in oxygen- 18 . If he stopped the experiment after only partial hydrolysis and isolated the remaining ester, the recovered ethyl benzoate had lost a portion of its enrichment in oxygen- 18 . In other words, some exchange had occurred between oxygen- 18 of the ester and oxygen- 16 of water. Show how this observation bears on the formation of a tetrahedral carbonyl addition intermediate during acid-catalyzed ester hydrolysis.\mathrm{O}$

Step by step solution

01

Acid-catalyzed hydrolysis of ester is represented by the following reaction: Ethyl benzoate + Water -> Acid + Alcohol Ethyl benzoate is specifically made with oxygen-18, to trace the mechanism of the reaction. #Step 2: Draw structure of both reactants and label oxygen atoms#

The ester (ethyl benzoate) has the following structure: C6H5COOCH2CH3 (Enriched with oxygen-18 in carbonyl group) The water molecule involved in the reaction: H2O (Containing oxygen-16, as specified) #Step 3: Analyze the mechanism for hydrolysis of ester in aqueous acid involving the formation of a tetrahedral carbonyl addition intermediate#

02

During the acid-catalyzed ester hydrolysis, it is proposed that a tetrahedral carbonyl addition intermediate is formed. This occurs in the following steps: 1. A proton from the acid catalyst attacks the carbonyl oxygen of the ester, which increases the electrophilicity of the carbonyl carbon. 2. Water (nucleophile) attacks the electrophilic carbonyl carbon, forming a tetrahedral intermediate compound. 3. The protonated carbonyl oxygen deprotonates to reform the carbonyl group, and the leaving group (ethoxide ion) is displaced, yielding the final acid and alcohol products. #Step 4: Show how the observation bears on the formation of a tetrahedral carbonyl addition intermediate during acid-catalyzed ester hydrolysis#

When Bender stopped the experiment after only partial hydrolysis, it means that the tetrahedral intermediate was formed. In the tetrahedral intermediate, the carbonyl oxygen-18 is still present in the ester, while the water molecule containing oxygen-16 now forms a bond with carbonyl carbon. This tetrahedral intermediate can revert to its reactants, exchanging the oxygen-18 in the carbonyl carbon with the oxygen-16 of water. This observation supports the existence of the tetrahedral carbonyl addition intermediate during acid-catalyzed ester hydrolysis because it explains the loss of enrichment of oxygen-18 in the recovered ethyl benzoate. The formation of a tetrahedral intermediate results from the interaction of electrophilic carbonyl carbon with a nucleophile (water), where oxygen-18 and oxygen-16 oxygen atoms can exchange their positions.

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