Question

Write the full mechanism of the reaction between \(\mathrm{LiAlH}_{4}\) and benzoyl chloride to yield benzyl alcohol.

Short answer

The reaction involves nucleophilic attacks by hydride ions, forming benzyl alcohol from benzoyl chloride via aldehyde intermediates.

Step-by-step solution

Identify the Reactants

The reactants in this reaction are lithium aluminum hydride (\(\mathrm{LiAlH}_{4}\)) and benzoyl chloride, which is an acyl chloride derived from benzoic acid. Benzoyl chloride has the functional group \(-COCl\).

Nucleophilic Attack

In the first step of the reaction, a hydride ion \((\mathrm{H}^{-})\) from \(\mathrm{LiAlH}_{4}\) attacks the carbonyl carbon of benzoyl chloride. This forms a tetrahedral intermediate and breaks the \(\pi\)-bond of the carbonyl group, shifting electrons to the oxygen atom.

Chloride Ion Departure

The tetrahedral intermediate collapses, and the chloride ion \((\mathrm{Cl}^{-})\) is expelled as a leaving group. This results in the formation of an aldehyde intermediate (benzaldehyde).

Second Nucleophilic Attack

Another hydride ion from \(\mathrm{LiAlH}_{4}\) performs a nucleophilic attack on the carbonyl carbon of the benzaldehyde intermediate. This again results in the formation of a tetrahedral intermediate.

Protonation and Quenching of Reaction

In the final steps, the reaction is typically quenched with water or an acid to protonate the oxygen bearing a negative charge, forming benzyl alcohol as the final product.

Key concepts

Lithium Aluminum Hydride

Lithium aluminum hydride, expressed as \( \mathrm{LiAlH}_{4} \), is a potent reducing agent often used in organic chemistry. It is known for its ability to donate hydride ions (\( \mathrm{H}^{-} \)) which play a crucial role in reducing carbonyl groups into alcohols.

Some key features include:

• Excellent reducer: Effective at reducing esters, aldehydes, and ketones.
• Strong agent: Very reactive, usually requiring careful handling.
• Hydride source: Provides hydride ions used in nucleophilic attacks.

In our reaction, \( \mathrm{LiAlH}_{4} \) donates hydride ions in two separate nucleophilic attacks, highlighting its multiple roles in complex reactions.

Nucleophilic Attack

A nucleophilic attack is a fundamental step in many organic reactions. It involves a nucleophile, which is a species with a lone pair of electrons, attacking a positively charged or electronegative atom. In our reaction with \( \mathrm{LiAlH}_{4} \) and benzoyl chloride, the nucleophile involved is the hydride ion \( \mathrm{H}^{-} \).

The hydride ion attacks the carbonyl carbon of benzoyl chloride, due to its partial positive charge. This causes:

• Breaking of the \( \pi \) bond in the carbonyl group.
• Formation of a new \( \sigma \) bond with the carbonyl carbon.
• Creation of a tetrahedral intermediate.

Understanding the role of nucleophiles and how they interact with electrophiles like the carbonyl carbon helps you predict the outcome of many reactions.

Benzoyl Chloride Reaction

Benzoyl chloride is a reactive acyl chloride derived from benzoic acid, with the functional group \(-\mathrm{COCl}\). Due to the presence of a highly electronegative chlorine atom and a carbonyl group, benzoyl chloride is quite reactive, making it a perfect candidate to undergo nucleophilic substitution reactions.

In this reaction:

• The \( \mathrm{Cl} \) group acts as a leaving group.
• Forming a tetrahedral intermediate releases \( \mathrm{Cl}^{-} \).
• Leads to the formation of benzaldehyde before another attack to form benzyl alcohol.

The double role of benzoyl chloride in accepting nucleophiles and releasing groups defines its utility in forming various chemical structures.

Benzyl Alcohol Formation

The formation of benzyl alcohol is the final step in this reaction after two impactful phases of nucleophilic attack. As the reaction progresses, the intermediate formed after the first hydride attack becomes more reactive benzaldehyde.

Once the aldehyde is formed, another hydride attack takes place:

• This second attack further reduces the carbonyl group.
• Forms another tetrahedral intermediate, which is eventually protonated.
• Quenching the reaction, commonly with water, finalizes the alcohol formation.

Transforming benzoyl chloride into benzyl alcohol showcases the power of hydride-based reductions in organic synthesis, demonstrating how intricate steps lead to the formation of specific functional groups.