The hydrolysis of the ester group in 2-acetoxybenzaldehyde...

The hydrolysis of the ester group in 2-acetoxybenzaldehyde is accelerated by about $10^{4}$, relative to the 4 -isomer. The rate of hydrolysis in the $\mathrm{pH}$ range 6.0-8.5 follows the rate expression $$ \text { Rate }=k_{0}+k\left[{ }^{-} \mathrm{OH}\right] $$ Both the $k_{0}$ and $k\left[^{-} \mathrm{OH}\right]$ terms are larger than for the 4 -isomer. When the hydrolysis is carried out in ${ }^{18} \mathrm{O}$-labeled water, the acetic acid contains $50 \%$ ${ }^{18} \mathrm{O}$. Suggest a mechanism that is consistent with these observations.

The hydrolysis of the ester group in 2-acetoxybenzaldehyde is accelerated by about $10^{4}$, relative to the 4 -isomer. The rate of hydrolysis in the $\mathrm{pH}$ range 6.0-8.5 follows the rate expression
$$
\text { Rate }=k_{0}+k\left[{ }^{-} \mathrm{OH}\right]
$$
Both the $k_{0}$ and $k\left[^{-} \mathrm{OH}\right]$ terms are larger than for the 4 -isomer. When the hydrolysis is carried out in ${ }^{18} \mathrm{O}$-labeled water, the acetic acid contains $50 \%$ ${ }^{18} \mathrm{O}$. Suggest a mechanism that is consistent with these observations.

Key Concepts

Ester Hydrolysis Mechanism

This concept involves the transformation of an ester into its corresponding acid and alcohol (or acid derivative) via nucleophilic attack by water or hydroxide ion. In both base?catalyzed and neutral pathways, the reaction proceeds through formation of a tetrahedral intermediate, which then collapses to expel the leaving group. Understanding the general pathway for ester hydrolysis is crucial for analyzing how substituents or neighboring groups may accelerate or alter the reaction course.

Neighboring Group Participation

Neighboring group participation occurs when a group within the same molecule assists in the reaction process by stabilizing intermediates or transition states. In the context of this ester hydrolysis, the proximity of an aldehyde group in the ortho position can facilitate intramolecular nucleophilic attack or proton transfer, leading to a cyclic intermediate or a transition state that is lower in energy. This internal assistance results in enhanced reactivity relative to nonadjacent isomers, exemplifying the importance of intramolecular interactions in organic reaction mechanisms.

Isotope Labeling in Mechanistic Studies

Isotope labeling, such as using H2^18O, is a powerful tool for elucidating reaction pathways. By tracking the incorporation of the labeled isotope into specific products, chemists can identify which oxygen atoms in the reactants and intermediates originate from water. In this case, the observation that 50% of the acetic acid contains ^18O provides evidence for the involvement of water-derived nucleophilic attack in the formation of the acetic acid product, thereby supporting a mechanism that involves exchange of oxygen atoms during the reaction.

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