Question

Which of the following products is formed when benzaldehyde is treated with \(\mathrm{CH}_{3} \mathrm{MgBr}\) and the addition product so obtained is subjected to acid hydrolysis? (a) a primary alcohol (b) phenol (c) secondary alcohol (d) tert-Butyl alcohol

Short answer

The product is a secondary alcohol.

Step-by-step solution

Identify Reactants and Products

The reaction involves benzaldehyde (C₆H₅CHO) and (CH₃MgBr), a Grignard reagent. The process proceeds with acid hydrolysis after the Grignard reaction.

Reaction with Grignard Reagent

Benzaldehyde reacts with (CH₃MgBr) to form an intermediate adduct, where the (CH₃) group adds to the carbonyl carbon, disrupting the C=O bond and forming a magnesium alkoxide.

Acid Hydrolysis of the Intermediate

The intermediate alkoxide is then subjected to acid hydrolysis, where it reacts with water under acidic conditions. This reaction converts the alkoxide to an alcohol.

Determine the Structure of the Final Product

After hydrolysis, the newly formed carbon attaches an OH group, resulting in the formation of a secondary alcohol, C₆H₅(CHOH)(CH₃) or benzyl alcohol with a methyl group attached.

Key concepts

Acid Hydrolysis

Acid hydrolysis is an essential step in transforming intermediates into stable compounds. In the context of the Grignard reaction, it involves breaking down the magnesium alkoxide intermediate to form the corresponding alcohols. The process works by introducing water into an acidic environment where the magnesium component splits apart, allowing the alkoxide ion to capture a hydrogen atom and form a hydroxyl group. This conversion is crucial because it stabilizes the otherwise reactive intermediate and effectively transforms it into a more easily handled alcohol compound. Acid hydrolysis not only aids in Grignard reactions but is also a fundamental process in various organic synthesis reactions, ensuring that intermediates reach their desired product forms efficiently.

Benzaldehyde

Benzaldehyde is an aromatic aldehyde known for its distinctive pleasant almond-like smell. Its chemical formula is C₆H₅CHO. This compound serves as a key starting material in many organic syntheses, such as reactions involving Grignard reagents. In a Grignard reaction, benzaldehyde's carbonyl group (C=O) is reactive towards nucleophiles like the Grignard reagent (CH₃MgBr), allowing the addition of an alkyl group to the carbon that's doubly bonded to oxygen. This interaction is pivotal for synthesizing secondary alcohols as the presence of the phenyl group (C₆H₅-) in benzaldehyde also influences the properties of the resulting alcohol product, providing distinct aromatic features.

Secondary Alcohol

Secondary alcohols are a type of alcohol characterized by the hydroxyl (-OH) group being attached to a carbon atom that is itself connected to two other carbon atoms. In the given reaction, the secondary alcohol formed has the formula C₆H₅(CHOH)(CH₃), where a hydroxyl group is positioned on a secondary carbon. These alcohols are significant due to their versatile presence in various biological and chemical processes. The formation of secondary alcohols through reactions like those involving Grignard reagents shows how organic chemistry can leverage simple nucleophilic additions to achieve complex structures. They typically display distinct boiling points and solubilities, making them easily distinguishable in mixtures.

Magnesium Alkoxide

During the Grignard reaction, the first product to form is a magnesium alkoxide intermediate. This happens when the Grignard reagent, CH₃MgBr, reacts with benzaldehyde. In this reaction, the CH₃ group attaches to the carbon of the carbonyl group in benzaldehyde, and the magnesium atom links with the resultant alkoxide oxygen. The formation of this magnesium alkoxide is crucial as it initiates the transition from a carbonyl to an alcohol through subsequent steps, such as acid hydrolysis. Without this intermediate, the formation of the desired secondary alcohol product would not be possible. Magnesium alkoxides are often used in organic chemistry settings as intermediates in synthesizing various alcohols, making them a cornerstone in reaction mechanisms.