Problem 173 - A Complete Resource Book in Chemistry for JEE Main

Question

Carbonyl compounds on reduction with selective reducing agents give alcohols. The structure of alcohol formed depends upon the nature of reducing agents. \(\operatorname{LiAlH}_{4}\), \(\mathrm{NaBH}_{,}\), sodium alcohol, \(\mathrm{Mg}(\mathrm{Hg}) \mathrm{H}, \mathrm{O}\) etc can be used. 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) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}^{2}-\mathrm{CH}_{2}^{2}-\mathrm{CH}_{3}\) (d) \(\mathrm{CH}_{3}^{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2} \mathrm{OH}\)

Step-by-Step Solution

Answer

Option (a): \(\text{CH}_3-\text{CH}=\text{CH}-\text{CH}_2\text{OH}\) is the product.

Step 1: Understand Reduction Process

Reduction involves the addition of hydrogen to the carbonyl group (C=O) to convert it into an alcohol (C-OH). Different reducing agents can selectively reduce carbonyl compounds without affecting other functional groups, such as double bonds.

Step 2: Identify the Reducible Group

In the given compound \(\text{CH}_3-\text{CH}=\text{CH}-\text{CHO}\), the focus is on the aldehyde group \(-\text{CHO}\) at the end of the molecule. This group is the target for reduction by \(\text{NaBH}_4\).

Step 3: Selectivity of NaBH4

Sodium borohydride (\(\text{NaBH}_4\)) is a mild reducing agent that specifically reduces aldehydes and ketones to alcohols. It does not typically affect C=C double bonds present in alkenes.

Step 4: Perform the Reduction

NaBH4 will reduce the aldehyde group (CHO) in \(\text{CH}_3-\text{CH}=\text{CH}-\text{CHO}\) to a primary alcohol (CH2OH), resulting in \(\text{CH}_3-\text{CH}=\text{CH}-\text{CH}_2\text{OH}\).

Step 5: Analyze the Options

Review the multiple choice options: (a) \(\text{CH}_3-\text{CH}=\text{CH}-\text{CH}_2\text{OH}\), (b) \(\text{CH}_3-\text{CH}_2-\text{CH}_2-\text{CHO}\), (c) \(\text{CH}_3-\text{CH}_2^2-\text{CH}_2^2-\text{CH}_3\), and (d) \(\text{CH}_3^3-\text{CH}_2-\text{CH}_2-\text{CH}_2 \text{OH}\). The structure from the reduction of the aldehyde group by NaBH4 is given by option (a).

Key Concepts

Selective Reducing Agents

In chemical reactions, particularly in organic chemistry, reducing agents play a crucial role. These agents facilitate the addition of hydrogen or the removal of oxygen from a compound.
Selective reducing agents are specifically chosen based on their ability to target certain functional groups without interfering with others. This selectivity allows chemists to perform precise modifications on complex molecules.

• Targeted Reduction: Selective reducing agents are used to reduce specific groups like carbonyls while leaving other sensitive bonds, like C=C double bonds, intact.
• Diverse Applications: They are employed in synthesizing alcohols from carbonyl compounds such as aldehydes and ketones.

By understanding the nature and selectivity of these reducing agents, chemists can predict and control the outcomes of reduction processes effectively.

Sodium Borohydride (NaBH4)

When it comes to mild and selective reduction of carbonyl groups, sodium borohydride (NaBH4) is a preferred choice.
NaBH4 is celebrated for its ability to reduce aldehydes and ketones into alcohols without affecting alkenes or alkynes.

• Structure: NaBH4 consists of sodium, boron, and hydrogen in a stable ion arrangement that provides its reducing characteristics.
• Reactivity: It works well in protic solvents like water, methanol, or ethanol, where it readily donates hydrogen.
• Mild Conditions: NaBH4 operates under mild conditions, meaning it neither requires high temperatures nor harsh conditions.

Because of its gentleness and precision, NaBH4 is a staple in labs for the reduction of carbonyl compounds.

Alcohol Formation

One of the primary outcomes of reducing carbonyl compounds is the formation of alcohols. This transformation is essential in chemical synthesis and industrial applications.
When an aldehyde or ketone is reduced, the carbonyl group \(-\text{C=O}\) is transformed into a hydroxyl group \(-\text{OH}\). This change takes the compound from a carbonyl derivative to an alcohol.

• Primary Alcohols: Reduction of aldehydes results in primary alcohols. For instance, reducing ethanal yields ethanol.
• Secondary Alcohols: When ketones are reduced, the product is a secondary alcohol. Take acetone, for example, which when reduced, forms isopropanol.
• Versatility: Alcohols, being very versatile, serve as fundamental building blocks in pharmaceuticals and polymers.

The ease with which carbonyls can be reduced to alcohols illustrates the efficiency of groups like NaBH4 in yielding desired chemical transformations.

Aldehyde Reduction

Aldehyde reduction is a specific process that focuses on simplifying an aldehyde into a primary alcohol through the addition of hydrogen molecules.
This process is made possible by the use of reducing agents such as NaBH4, which selectively target the aldehyde's carbonyl group without disrupting other elements of the molecular structure.

• Selective Targeting: The aldehyde group \(-\text{CHO}\) is efficiently reduced to \(-\text{CH}_2\text{OH}\), transforming it into a simple primary alcohol.
• Predictable Outcomes: Equipped with mild reducing agents, chemists can accurately predict the transformation products when working with aldehydes.
• Example: Consider the reduction of formaldehyde to methanol - a straightforward conversion illustrating this concept clearly.

Such predictable transformations highlight the power of selective reducing agents like NaBH4, which enable targeted chemical reactions with limited side effects or by-products.