Question

Ketones react slowly with benzeneselenenyl chloride in the presence of HCl to yield \(\alpha\) -phenylseleno ketones. Propose a mechanism for this acid- catalyzed \(\alpha\) -substitution reaction.

Short answer

The mechanism involves enol formation, electrophilic attack by benzeneselenenyl chloride, and a proton transfer to yield α-phenylseleno ketones.

Step-by-step solution

Formation of the Enol

In the presence of HCl, the ketone undergoes tautomerization to form the enol. This step is crucial as the enol form is the reactive species in the subsequent steps of the mechanism. The hydrogen ion from HCl protonates the oxygen of the keto form, creating a positively charged intermediate. This intermediate can convert into the more stable enol form by shifting a proton and forming a carbon-carbon double bond.

Electrophilic Attack by Benzeneselenenyl Chloride

The enol attacks the benzeneselenenyl chloride. The double bond of the enol acts as a nucleophile, attacking the electrophilic selenium center in benzeneselenenyl chloride. This results in the formation of a C-Se bond and the release of a chloride ion.

Proton Transfer

A proton transfer occurs to stabilize the newly formed intermediate. This step ensures that the seleno group is in the correct oxidation state and that the molecule returns to a neutral charge state. This typically involves a proton from the hydroxyl group of the enol migrating to the oxygen to produce a carbonyl group, thereby completing the α-phenylseleno ketone structure.

Key concepts

Enol Formation

Enol formation is a key process in the mechanism of many chemical reactions. It occurs when a ketone tautomerizes to an enol, with the presence of an acid catalyst, such as HCl, facilitating this transformation. When an acid is added, it provides a hydrogen ion (H⁺) which protonates the oxygen atom in the ketone. This creates a positively charged intermediate. The molecule then rearranges, moving a proton from the carbon next to the carbonyl group, and forms a carbon-carbon double bond. This step results in the enol form which is characterized by an alcohol group (hydroxyl group) adjacent to a carbon-carbon double bond. The enol form is more reactive than the ketone form, making it essential for the next steps of the mechanism. In essence, enol formation transforms a relatively stable structure into a reactive one, poised for subsequent reactions.

Electrophilic Attack

An electrophilic attack occurs in this mechanism when the enol, formed in the previous step, reacts with benzeneselenenyl chloride. In this scenario, the enol acts as a nucleophile, meaning it donates electrons. The carbon-carbon double bond in the enol is electron-rich and is attracted to the electron-deficient selenium atom in the benzeneselenenyl chloride. As the enol approaches the selenium, the electrons from the double bond are used to form a new bond with selenium. This results in the creation of a C-Se bond. The chloride ion is released as a leaving group, resulting from this nucleophilic attack. The formation of this new bond is crucial, as it properly incorporates the selenium atom into the organic compound, moving the reaction towards the formation of the final product.

Proton Transfer

Proton transfer is a crucial step for stabilizing the intermediate that forms after the electrophilic attack. Often, this step involves balancing charges to return to a stable, neutral state. In our example, the intermediate product of the reaction still contains a hydroxyl group. One of the protons from this group transfers back to the oxygen atom. This process transforms the hydroxyl group back into a carbonyl group, effectively re-establishing a double bond between the carbon and oxygen atoms. This proton migration completes the formation of the α-phenylseleno ketone structure by ensuring the seleno group is stabilized in its desired oxidation state. Proton transfer might seem simple, but it plays a pivotal role in ensuring the stability and continuation of the reaction mechanism. It importantly adjusts the molecular geometry and ensures the reaction proceeds to yield the desired product efficiently.