Question

Reaction of cyclohexene with mercury(II) acetate in \(\mathrm{CH}_{3} \mathrm{OH}\) rather than \(\mathrm{H}_{2} \mathrm{O}\), followed by treatment with \(\mathrm{NaBH}_{4}\), yields cyclohexyl methyl ether rather than cyclohexanol. Suggest a mechanism.

Short answer

Cyclohexyl methyl ether is formed through a nucleophilic attack of methanol via a mercurinium ion intermediate, followed by deprotonation and demurcuration.

Step-by-step solution

Initiate the Reaction with Mercury(II) Acetate

The reaction begins when cyclohexene, an alkene, is exposed to mercury(II) acetate in methanol ( CH_3OH ). Mercury(II) acetate acts as a Lewis acid, initiating the formation of a mercurinium ion intermediate. This occurs as the Hg^2+ from mercury(II) acetate coordinates with the double bond in cyclohexene, creating a three-membered ring intermediate.

Add Methanol Nucleophile

Next, the methanol (CH_3OH) acts as a nucleophile. It attacks the more substituted carbon of the mercurinium ion intermediate, leading to the opening of the three-membered ring. This results in a carbocation that is stabilized by the presence of the mercury acetate ion, forming an organomercury intermediate bonded to the methanol.

Deprotonation

Following nucleophilic attack, a deprotonation occurs. The oxygen in the methanol group, now bonded to the cyclohexyl ring, will lose a proton. This step stabilizes the organomercury compound by neutralizing charges and forms a cyclohexyl methyl ether with a mercury group still attached.

Demercuration with Sodium Borohydride

In the final step, sodium borohydride ( NaBH_4 ) is used for demurcuration. NaBH_4 acts as a reducing agent, replacing the mercury group with a hydrogen atom. This transformation completes the synthesis of cyclohexyl methyl ether, as the mercury group is fully removed and only the ether linkage remains.

Key concepts

Mercurinium Ion Formation

When an alkene like cyclohexene reacts with mercury(II) acetate, a special type of intermediate is formed known as the **mercirinium ion**. This happens because mercury(II) acetate acts as a Lewis acid, trying to accept electrons. It coordinates with the double bond in the alkene.
This coordination results in the creation of a three-membered ring intermediate, consisting of the alkene and the mercury ion (Hg²⁺).
Such a ring is highly strained and temporarily stabilizes the reactive mercury ion. Here, the mercury ion forms bonds with the carbon atoms that previously composed the double bond. The formation of this ion is essential as it sets the stage for subsequent reactions, providing a structured and directional framework for further chemical changes.

Nucleophilic Attack with Methanol

Once the mercurinium ion has been formed, the next key step involves a **nucleophilic attack** by methanol (CH₃OH). A nucleophile is a species that loves to attack positive charges or electron-deficient sites.
In our reaction, the more substituted carbon in the mercurinium ion is typically more susceptible to such attacks.
Methanol, having an oxygen atom with lone pairs of electrons, acts as a nucleophile and attacks this carbon atom.

• This leads to the breaking open of the three-membered mercurinium ring.
• In this process, a carbocation is formed, which is a positively charged ion species.

The presence of the mercury acetate helps stabilize this carbocation, facilitating the next stage in our reaction process.

Understanding the Reduction Reaction

The final steps of the reaction involve a **reduction reaction** courtesy of sodium borohydride (NaBH₄). Reduction reactions involve the gain of electrons or the loss of an oxygen atom.
In this context, sodium borohydride serves as a reducing agent, aiming to remove the mercury group that was added earlier in the mercurinium ion formation stage.

• NaBH₄ transfers hydride ions (H^-) to the organomercury intermediate, effectively replacing the mercury with hydrogen.
• This process is called demercuration.

Demerucration through reduction ensures that the initial structure is altered just enough to maintain the functional group as an ether, achieving the formation of cyclohexyl methyl ether from the organomercury species.

Organomercury Intermediate

After the nucleophilic attack and subsequent deprotonation, the reaction forms an **organomercury intermediate**. This intermediate is central to the mechanism as it contains both the functional groups and mercury.
Organomercury compounds are unique as they often have mercury bonded directly to carbon. This bond is important for stabilizing what would otherwise be a reactive or unstable compound.

• In our reaction, the organomercury intermediate features a cyclohexyl group bonded to the methoxy group.
• It's further stabilized by the mercury presence.

Even though mercury is a heavier atom, its involvement is crucial for the selectivity and direction of the reaction pathway, ensuring that we eventually get our desired product: cyclohexyl methyl ether.