The hydrolysis of the lactone 19-A shows catalysis by...

The hydrolysis of the lactone 19-A shows catalysis by acetate ion, with the rate expression being $$ k_{\text {obs }}=1.6 \times 10^{-6}+6.4 \times 10^{-4}\left[\mathrm{H}^{+}\right]+2.08 \times 10^{-5}\left[{ }^{-} \mathrm{OAc}\right]+49\left[{ }^{-} \mathrm{OH}\right] \mathrm{s}^{-1} $$ This expression results in a pH-rate profile shown in Figure 7.P19, with acetate catalysis being significant in the $\mathrm{pH}$ range 3-6. The reaction shows a solvent isotope effect of $2.65$. Discuss how the catalysis by acetate might occur. What are the likely mechanisms for hydrolysis at $\mathrm{pH}<1$ and $\mathrm{pH}>7$, where the rates are linearly dependent on $\left[\mathrm{H}^{+}\right]$and $\left[{ }^{-} \mathrm{OH}\right]$, respectively?

The hydrolysis of the lactone 19-A shows catalysis by acetate ion, with the rate expression being
$$
k_{\text {obs }}=1.6 \times 10^{-6}+6.4 \times 10^{-4}\left[\mathrm{H}^{+}\right]+2.08 \times 10^{-5}\left[{ }^{-} \mathrm{OAc}\right]+49\left[{ }^{-} \mathrm{OH}\right] \mathrm{s}^{-1}
$$
This expression results in a pH-rate profile shown in Figure 7.P19, with acetate catalysis being significant in the $\mathrm{pH}$ range 3-6. The reaction shows a solvent isotope effect of $2.65$. Discuss how the catalysis by acetate might occur. What are the likely mechanisms for hydrolysis at $\mathrm{pH}<1$ and $\mathrm{pH}>7$, where the rates are linearly dependent on $\left[\mathrm{H}^{+}\right]$and $\left[{ }^{-} \mathrm{OH}\right]$, respectively?

Key Concepts

Acid-Catalyzed Hydrolysis

Acid-catalyzed hydrolysis involves the protonation of the electrophilic center (often the carbonyl group) in a substrate, increasing its susceptibility to nucleophilic attack by water. In the context of lactone hydrolysis at very low pH, high proton concentrations facilitate the formation of a protonated intermediate, which can more readily be attacked by water to open the ring and complete the reaction. This mechanism is typically characterized by a linear dependence of the rate on [H+].

Nucleophilic (Acetate) Catalysis

Nucleophilic catalysis by acetate involves the acetate ion acting not only as a base but as a nucleophile that attacks the electrophilic center of the substrate. In the moderate pH range (pH 3–6), acetate can transiently form an intermediate by attacking the carbonyl carbon of the lactone. The resulting intermediate subsequently reacts with water, leading to hydrolysis. This pathway highlights the dual role of some catalysts which can enhance the reaction rate through direct nucleophilic assistance rather than solely by general acid-base catalysis.

Base-Catalyzed Hydrolysis

Base-catalyzed hydrolysis occurs when hydroxide ions act as nucleophiles to attack the electrophilic center of a substrate, such as the carbonyl carbon in a lactone, leading directly to ring-opening. At higher pH values (above 7), the reaction rate often shows a linear dependence on [OH–], as the reaction mechanism is driven by the concentration of hydroxide ions. This process is typically faster due to the high nucleophilicity of the hydroxide ion compared to water.

Solvent Isotope Effect

The solvent isotope effect, observed as a change in reaction rate upon substituting the hydrogen in the solvent with its isotope (deuterium), suggests that proton transfer is involved in the rate-determining step of the reaction. A significant solvent isotope effect, like the reported value of 2.65, indicates that the breaking or forming of O–H bonds plays a crucial role in the mechanism, supporting the notion that specific proton transfer events, possibly during nucleophilic attack or formation of intermediates, are critical to the overall reaction rate.

pH-Rate Profile Interpretation

The pH-rate profile of a reaction provides insight into the dominant catalytic pathways at different pH levels. In this case, the profile reveals distinct regions: at low pH (<1) acid catalysis prevails, at intermediate pH (3–6) acetate catalysis (nucleophilic catalysis) is significant, and at high pH (>7) base catalysis by hydroxide increases the rate. This profile assists in elucidating the different mechanisms operative under various conditions and reflects how catalyst concentration and type dictate the reaction pathway.

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