Problem 177 - A Complete Resource Book in Chemistry for JEE Main

Question

Identify the products formed \((\mathrm{X})\) and (Y) in the following reaction \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}+\mathrm{HCHO}_{\text {conc. } \mathrm{OH}}{\longrightarrow}(\mathrm{X})+(\mathrm{Y})\) (a) \((\mathrm{X})\) is \(\mathrm{HCOO}^{\mathrm{e}}\) (b) \((\mathrm{Y})\) is \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{OH}\) (c) (Y) is \(\mathrm{CH}_{3} \mathrm{OH}\) (d) \((\mathrm{X})\) is \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COO}^{\Theta}\)

Step-by-Step Solution

Answer

(X) is \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{COO}^{\Theta}\) and (Y) is \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{CH}_{2}\mathrm{OH}\).

Step 1: Identify the Reaction Type

The given reaction involves benzaldehyde \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{CHO}\) with formaldehyde \(\mathrm{HCHO}\) in the presence of concentrated \(\mathrm{OH}^-\). This setup suggests a Cannizzaro reaction, which is an organic redox reaction.

Step 2: Understand the Cannizzaro Reaction

In a Cannizzaro reaction, a non-enolizable aldehyde reacts in the presence of a strong base to produce an alcohol and a carboxylate ion. Here, benzaldehyde and formaldehyde will react.

Step 3: Predict the Products

In the reaction, benzaldehyde will undergo disproportionation, where one molecule is reduced to benzyl alcohol \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{CH}_{2}\mathrm{OH}\) and the other is oxidized to benzoate \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{COO}^-\). Formaldehyde will be reduced to methanol \(\mathrm{CH}_{3}\mathrm{OH}\).

Step 4: Assign \((X)\) and \((Y)\)

Based on the reaction predictions, (X) is benzoate ion \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{COO}^-\), and (Y) is benzyl alcohol \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{CH}_{2}\mathrm{OH}\).

Key Concepts

Organic Redox Reaction

A Cannizzaro reaction is a classic example of an organic redox reaction. Redox is short for reduction-oxidation, where one substance is reduced and another is oxidized. In the realm of organic chemistry, such a reaction typically involves the transformation of aldehydes without a hydrogen atom on the carbon adjacent to the carbonyl group.
The Cannizzaro reaction specifically occurs in the presence of a strong base. It is unique in that it does not require the presence of an enolizable hydrogen. One aldehyde molecule acts as a reducing agent, leading to the production of an alcohol, while another acts as an oxidizing agent, resulting in the formation of a carboxylate ion. As a result, you end up with two distinct products from identical reactants. This fascinating interplay is a perfect example of how redox reactions can operate in organic systems.

Non-enolizable Aldehydes

Within organic chemistry, aldehydes are classified based on the presence or absence of enolizable hydrogens. Enolizable aldehydes have a hydrogen atom on the alpha-carbon next to the carbonyl group which can be deprotonated to form an enolate ion. Conversely, non-enolizable aldehydes lack this feature.
Benzaldehyde and formaldehyde are prime examples of non-enolizable aldehydes. Benzaldehyde · lacks an alpha-hydrogen since the carbonyl group is directly attached to an aromatic ring, · Meanwhile, formaldehyde ( · whose structure is simply · lacks any carbon-carbon bonds to house an alpha-hydrogen, making it quintessentially non-enolizable.
The absence of enolizable hydrogens makes these molecules suitable candidates for the Cannizzaro reaction. Their inability to form enolates guides them toward other reaction pathways, emphasizing the importance of structure in dictating chemical reactivity.

Benzaldehyde and Formaldehyde Reaction

The reaction between benzaldehyde and formaldehyde in a Cannizzaro reaction is a textbook case of organic chemists leveraging structural features to drive product formation. When these aldehydes meet in a concentrated hydroxide environment, a unique chain of events begins to unfold.
Benzaldehyde, a · non-enolizable aldehyde, becomes oxidized to form benzoate ions, while · Formaldehyde undergoes reduction processes, yielding methanol.
Of course, the presence of a strong base like · hydroxide ion ( · is crucial. It abstracts a hydrogen from one aldehyde, allowing the redox process to proceed seamlessly. This reaction vividly demonstrates how functional groups and catalytic conditions work harmoniously to direct the transformation of simple organic molecules into more complex ones.

Benzyl Alcohol Production

Benzyl alcohol production is a key outcome of the Cannizzaro reaction involving benzaldehyde. During this process, one of the benzaldehyde molecules reduces to form benzyl alcohol · characterized by its hydroxyl (-OH) group connected to a benzyl moiety.
In the presence of a strong base like concentrated hydroxide ( · the aldehyde form of benzaldehyde transitions by gaining hydrogen atoms, resulting in the creation of benzyl alcohol.
This reaction showcases the ability of redox chemistry to facilitate the conversion of aldehyde functionalities into alcohols. The subtle shifts in atomic arrangements yield a completely different compound, emphasizing how such transformations are central to synthetic organic chemistry.

• Benzyl alcohol presents itself as a versatile compound used in numerous applications, ranging from pharmaceuticals to the fragrance industry.
• Understanding its formation through Cannizzaro reactions provides insights into larger scale industrial processes.