Problem 112 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

Benzyl alcohol is obtained from benzaldehyde by: \(\quad\) [AMU 2001] (a) Fittig reaction (b) reduction with LiAlH \(_{4}\) (c) Cannizzarp's reaction (d) both (b) and (c)

Step-by-Step Solution

Answer

(d) both (b) and (c)

Step 1: Understanding the Options

The question asks which methods can convert benzaldehyde to benzyl alcohol. We will consider each option: (a) Fittig reaction, (b) reduction with LiAlH _{4}, (c) Cannizzaro's reaction, and (d) both (b) and (c).

Step 2: Analyzing Option (a): Fittig Reaction

The Fittig reaction is a coupling reaction used to synthesize biaryl compounds from aryl halides. It involves combining two aromatic compounds, which is not suitable for converting benzaldehyde to benzyl alcohol, as no alcohol formation is involved.

Step 3: Analyzing Option (b): Reduction with LiAlH _{4}

Reduction with LiAlH _{4} (lithium aluminium hydride) is a common method to reduce carbonyl groups to alcohols. In this reaction, the carbon-oxygen double bond in aldehydes is reduced to a single bond, forming the corresponding alcohol. Benzaldehyde can be converted to benzyl alcohol using this method.

Step 4: Analyzing Option (c): Cannizzaro's Reaction

Cannizzaro's reaction is a redox reaction involving aldehydes without α-hydrogen atoms. In this disproportionation reaction, one molecule of the aldehyde is reduced to form the corresponding alcohol, while another is oxidized. Hence, benzaldehyde can also be converted to benzyl alcohol through this reaction.

Step 5: Analyzing Option (d): Both (b) and (c)

This option combines the reductions from both option (b) Reduction with LiAlH _{4} and option (c) Cannizzaro's reaction. Since both individually are suitable for converting benzaldehyde to benzyl alcohol, option (d) is correct.

Step 6: Conclusion

Options (b) and (c) independently and collectively (option (d)) can convert benzaldehyde to benzyl alcohol. The correct answer is (d) both (b) and (c).

Key Concepts

Lithium Aluminium Hydride

Lithium Aluminium Hydride (LiAlH \(_{4}\)) is a powerful reducing agent commonly used in organic chemistry. It is particularly effective in the reduction of carbonyl compounds, such as aldehydes and ketones, to alcohols. This makes it ideal for reducing benzaldehyde to benzyl alcohol.

The chemical structure of lithium aluminium hydride allows it to effectively donate hydride ions (H\(^-\)). These hydride ions are crucial in attacking the carbonyl carbon of benzaldehyde, effectively breaking the double bond between carbon and oxygen.

Some important points about lithium aluminium hydride reduction include:

• It is a selective and strong reducing agent.
• Reactions involving LiAlH \(_{4}\) occur in anhydrous conditions as it reacts violently with water.
• The reaction produces a primary alcohol from aldehydes through hydrogen addition.

Understanding the role of lithium aluminium hydride is crucial in mastering organic synthesis pathways, especially when converting aldehydes like benzaldehyde into alcohols.

Cannizzaro Reaction

The Cannizzaro reaction is a unique organic chemical reaction involving non-enolizable aldehydes such as benzaldehyde. This reaction is a redox process where an aldehyde simultaneously undergoes oxidation and reduction, also known as disproportionation, without the need for an external reducing agent.

In the presence of a strong base, typically concentrated alkali, benzaldehyde undergoes the Cannizzaro reaction. Here's what happens:

• One molecule of benzaldehyde is reduced to benzyl alcohol.
• Another molecule of benzaldehyde is oxidized to benzoic acid.

This process results in two different products from the same aldehyde. The Cannizzaro reaction is significant for compounds lacking α-hydrogen, and it plays a vital role in synthetic organic chemistry when dealing with such substrates.

To summarize, the Cannizzaro reaction provides an alternative pathway to produce benzyl alcohol from benzaldehyde, showcasing the diversity of mechanisms in organic reactions.

Organic Chemistry Reactions

Organic chemistry reactions encompass a wide variety of transformations that allow the conversion of one organic molecule into another. Two such important reactions, as seen in the conversion of benzaldehyde to benzyl alcohol, involve reduction and disproportionation processes.

Reduction reactions in organic chemistry, such as those using lithium aluminium hydride, focus on decreasing the oxidation state of a molecule. They are crucial in the preparation of alcohols from aldehydes and ketones. These reactions involve the addition of hydrogen or removal of oxygen.

Disproportionation reactions, like the Cannizzaro reaction, uniquely involve a single reactive species undergoing both oxidation and reduction simultaneously. This type of reaction expands the versatility in synthetic approaches when external reductants or oxidants are undesirable.

When studying organic chemistry:

• Remember to identify functional groups and their transformations.
• Understand the conditions and reagents required for each type of reaction.
• Practice predicting products based on known mechanisms and reaction types.

Being adept at recognizing and applying different organic reactions helps in building complex organic synthesis pathways.