Problem 174 - A Complete Resource Book in Chemistry for JEE Main

Question

When \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}-\mathrm{CHO}\) is reduced with \(\mathrm{NaBH}_{4}\), the product formed is (a) \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{2} \mathrm{OH}\) (b) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CHO}\) (c) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{3}\) (d) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2} \mathrm{OH}\)

Step-by-Step Solution

Answer

The product is \( \mathrm{CH}_3-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_2 \mathrm{OH} \) (choice a).

Step 1: Identify Reactants and Reaction Type

The compound \( \mathrm{CH}_3-\mathrm{CH}=\mathrm{CH}-\mathrm{CHO} \) is reduced using sodium borohydride (\( \mathrm{NaBH}_4 \)). \( \mathrm{NaBH}_4 \) is a selective reducing agent that typically reduces aldehydes and ketones to their corresponding alcohols.

Step 2: Determine the Reaction on the Functional Group

The compound has an aldehyde group \( -\mathrm{CHO} \). When reduced by \( \mathrm{NaBH}_4 \), the aldehyde group \( \mathrm{CHO} \) is specifically transformed into a primary alcohol group \( -\mathrm{CH}_2\mathrm{OH} \). The double bond \( \mathrm{CH}=\mathrm{CH} \) does not react with \( \mathrm{NaBH}_4 \), as \( \mathrm{NaBH}_4 \) does not affect carbon-carbon double bonds.

Step 3: Write the Structure of the Product

After the selective reduction of the aldehyde group, the structure of the resulting product is \( \mathrm{CH}_3-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_2\mathrm{OH} \). This corresponds to choice (a).

Key Concepts

Sodium borohydride (NaBH4)

Sodium borohydride, commonly denoted as \(\mathrm{NaBH}_4\), plays a crucial role in organic chemistry as a reducing agent. It's specifically known for its ability to reduce aldehydes and ketones into alcohols. This compound is a white crystalline powder which is easy to handle. Its importance stems from the fact that it is selective in its action, meaning it effectively reduces only specific functional groups without affecting others.

• \(\mathrm{NaBH}_4\) selectively targets carbonyl groups (like aldehydes and ketones) and does not affect other sensitive groups in a molecule.
• It is commonly used in the laboratory for synthesizing alcohols from simpler starting materials.
• The selectivity of \(\mathrm{NaBH}_4\) allows chemists to reduce specific parts of a molecule without altering the molecule's overall structure.

In practice, the action of \(\mathrm{NaBH}_4\) provides a straightforward method to approach organic synthesis, making it an invaluable tool in the chemist's toolkit.

Aldehyde reduction

Aldehyde reduction is a fundamental reaction in organic chemistry wherein an aldehyde group \((-CHO)\) gets converted into a primary alcohol \((-CH_2OH)\). This transformation is made possible through specific reducing agents like \(\mathrm{NaBH}_4\). The process is essentially a gain of electrons (reduction) resulting in the addition of hydrogen atoms to the carbonyl group of the aldehyde.This selective reduction relies on the reactivity of the carbonyl bond. The oxygen in the carbonyl group is highly electronegative, making the carbon atom partially positive and highly susceptible to nucleophilic attack, which is ideal for agents like \(\mathrm{NaBH}_4\).

• Aldehydes are more reactive than ketones, allowing \(\mathrm{NaBH}_4\) to selectively reduce these groups even in the presence of less reactive ketones.
• The resulting primary alcohol retains the original carbon skeleton of the molecule except with an additional hydrogen, thus maintaining the integrity of the compound's framework.
• This reaction does not affect carbon-carbon double bonds, which remains intact during the process.

This characteristic allows for the careful adjustment of molecular structures, creating possibilities for more complex synthesis in organic chemistry.

Alcohol synthesis

Alcohol synthesis in organic chemistry is an indispensable reaction since alcohols serve as vital building blocks for further chemical transformations. Converting aldehydes into alcohols using \(\mathrm{NaBH}_4\) underscores one of the most straightforward methods of alcohol synthesis.The synthesis of alcohol through this process involves hydride transfer, where a hydride ion \((H^-)\) from \(\mathrm{NaBH}_4\) interacts with the carbonyl carbon in an aldehyde, leading to the formation of a primary alcohol.

• This reaction proceeds under mild conditions, often in solvents like ethanol or water, enhancing its practicality.
• The alcohols formed from aldehydes like \(\mathrm{CH}_3-\mathrm{CH} = \mathrm{CH}-\mathrm{CHO}\) often find usefulness in both academic and industrial settings due to their functional versatility.
• Such reductions are integral to synthesizing components in pharmaceuticals, fragrances, and plastics.

Given its simplicity and efficiency, alcohol synthesis via aldehyde reduction with \(\mathrm{NaBH}_4\) stands out as a robust and practical method for generating primary alcohols from aldehydes in an array of chemical contexts.