Question

Write reasonable mechanisms for the different oxidation reactions of cycloöctatetraene with mercuric acetate in acetic acid, methanol, and water solutions. Note that compounds of the type \(\mathrm{Hg}(\mathrm{OR})_{2}\) appear to act in some cases as \- OR-donating agents and also that the oxide produced from the cycloöctatetraene and peracids rearranges readily to phenylacetaldehyde in the presence of acids.

Short answer

In the oxidation reactions of cyclooctatetraene with mercuric acetate in different solvents, the following mechanisms take place: In acetic acid, mercuric ion forms a complex with cyclooctatetraene, the acetate ion acts as an OR-donating agent to form an intermediate with Hg(OAc)2 and the ketone, which rearranges to phenylacetaldehyde under acidic conditions. In methanol, the Hg2+ complex is again formed, and methanol acts as an OR-donating agent to form an intermediate with Hg(OCH3)2 and the ketone, which is further attacked by methanol to form the final product. In water, the Hg2+ complex is formed, water attacks the mercuric ion to form an intermediate with Hg(OH)2 and the ketone, which then forms the corresponding product. If an acid is present, the oxide product may rearrange to phenylacetaldehyde.

Step-by-step solution

Determine the starting material and oxidizing agent

The starting material for all three mechanisms is cyclooctatetraene. The oxidizing agent is mercuric acetate, and the solvent is either acetic acid, methanol, or water.

Mechanism in acetic acid solution

\(1.\) The mercuric acetate dissociates to form mercuric ion (Hg2+) and acetate ion (OAc-) in the acetic acid solution. \(2.\) The mercuric ion forms a complex with cyclooctatetraene, where the π electrons of the double bonds coordinate to the positively charged Hg2+. \(3.\) The acetate ion can act as an OR-donating agent, meaning it can transfer its acetate group to a carbon atom of the cyclooctatetraene molecule, resulting in an intermediate with Hg(OAc)2 and the corresponding ketone. \(4.\) Finally, the intermediate rearranges to form phenylacetaldehyde under acidic conditions through the assistance of the acidic protons in the solution.

Mechanism in methanol solution

\(1.\) The mercuric acetate dissociates to form mercuric ion (Hg2+) and acetate ion (OAc-) in the methanol solution. \(2.\) The mercuric ion forms a complex with cyclooctatetraene, where the π electrons of the double bonds coordinate to the positively charged Hg2+. \(3.\) Methanol can also act as an OR-donating agent, meaning it can transfer its OCH3 group to the mercuric ion through a transition state. \(4.\) The intermediate with Hg(OCH3)2 and the corresponding ketone are formed. \(5.\) Then, methanol molecule attacks the Hg(OCH3)2 intermediate, replacing it with another OCH3 group. \(6.\) Finally, the Hg(OCH3)2 group is removed, and the corresponding product is formed.

Mechanism in water solution

\(1.\) The mercuric acetate dissociates to form mercuric ion (Hg2+) and acetate ion (OAc-) in the water solution. \(2.\) The mercuric ion forms a complex with cyclooctatetraene, where the π electrons of the double bonds coordinate to the positively charged Hg2+. \(3.\) Water can attack the mercuric ion, replacing the OR group from Hg(OR)2. \(4.\) The intermediate with Hg(OH)2 and the corresponding ketone are formed. \(5.\) Finally, the Hg(OH)2 group is removed, and the corresponding product is formed. (If an acid is present, the oxide product may rearrange to phenylacetaldehyde).

Key concepts

Mechanism of Organic Reactions

Organic reactions are the cornerstone of organic chemistry, involving the breaking and forming of chemical bonds within organic molecules. In the context of oxidation reactions of cyclooctatetraene, understanding the mechanism is crucial for predicting the products and intermediates formed during the reaction.

Typically, a mechanism will entail the stepwise movement of electrons, and it is visualized using arrow-pushing notation to indicate which bonds are broken or formed. For instance, in the reaction with mercuric acetate, the coordination of π electrons and subsequent OR group transfer are key steps illustrated in these mechanisms. Throughout the process, the reaction passes through various transition states and intermediates, often influenced by the solvent used.

Mercuric Acetate as Oxidizing Agent

Mercuric acetate acts as a versatile oxidizing agent in organic chemistry, capable of facilitating various oxidative transformations. In the mechanism with cyclooctatetraene, mercuric acetate dissociates into mercuric ions and acetate ions.

Oxidizing Ability

The mercuric ion (Hg^2+) plays a pivotal role by coordinating with the π electrons of cyclooctatetraene's double bonds. This interaction prompts the oxidation process, and it allows the subsequent steps, involving OR-donating agents, to occur and eventually lead to the transformation of the starting material into more oxidized products.

π Electrons Coordination

Coordination of π electrons is an essential concept in certain organic reactions, including the oxidation of cyclooctatetraene. Π electrons are the electrons found in the double bonds of organic molecules, which are delocalized and readily available for interactions.

In these reactions, π electrons form a complex with electrophiles, such as the mercuric ion (Hg^2+), attracted to regions of high electron density. This concept is fundamental in understanding how cyclooctatetraene, with its conjugated system of double bonds, can interact with mercuric acetate, marking the initiation of the oxidative process.

OR-donating Agents

OR-donating agents refer to molecules capable of donating an OR group, where 'R' is a hydrocarbon or hydrogen in the case of hydroxide. This concept is pivotal when considering how the oxidizing agent manipulates the course of the reaction.

Whether the OR group comes from acetate, methanol, or water, it is fundamental in the formation of various intermediates. For example, in the reaction with acetic acid, the acetate ion (OAc-) donates an acetate group to the cyclooctatetraene, creating the ketone intermediate. Similarly, in methanol and water, the OR groups (OCH3 and OH respectively) play a central role in progressing the reaction towards the formation of the final oxidized products.

Organic Reaction Solvents

Solvents used in organic reactions significantly influence the course and outcome of the reaction. Different solvents can affect the solubility of reactants, the ability of ions to dissociate, and the reaction rate.

Each solvent mentioned – acetic acid, methanol, and water – serves a unique purpose in the mechanisms depicted. Acetic acid acts not only as a solvent but also as an acidic catalyst in certain steps of the reaction. Methanol and water, being polar solvents, facilitate both the dissociation of mercuric acetate and the subsequent coordination of their respective OR groups to the mercury. This highlights how the choice of solvents is intrinsic to directing the path and intricacies of oxidation reactions in organic chemistry.