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

Question

vWhen \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}-\mathrm{CHO}\) 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 correct product is (a) \( \mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{2} \mathrm{OH} \).

Step 1: Understanding the Reaction

The given compound \( \mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}-\mathrm{CHO} \) is an \( \alpha,\beta\)-unsaturated carbonyl compound. When treated with \( \mathrm{NaBH}_{4} \), a reducing agent, it will reduce the carbonyl group \( \mathrm{CHO} \) to \( \mathrm{CH}_{2} \mathrm{OH} \). NaBH4 generally does not reduce double bonds present in conjugation with the carbonyl group, thus the alkene part \( \mathrm{CH}=\mathrm{CH} \) remains unaffected.

Step 2: Analyzing the Reduction Result

Upon the reduction of the aldehyde group by \( \mathrm{NaBH}_{4} \), the structure \( \mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}-\mathrm{CHO} \) converts specifically to \( \mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{2}\mathrm{OH} \). This involves the conversion of the carbonyl \( \mathrm{CHO} \) group to a primary alcohol \( \mathrm{CH}_{2}\mathrm{OH} \) while the rest of the molecule remains the same.

Step 3: Identifying the Correct Option

From the above analysis, the product of the reaction is \( \mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{2} \mathrm{OH} \), which directly matches option (a). This confirms that option (a) is the correct answer as the NaBH4 reduction does not affect the double bonds because they are conjugated.

Key Concepts

Reduction Reactions

In organic chemistry, reduction reactions are pivotal in transforming carbonyl compounds into alcohols. Such reactions typically involve the gain of electrons or hydrogen. It's essential to understand that reduction reactions decrease the oxidation state of a molecule.

• To determine if a compound undergoes reduction, check for the presence of oxygenated groups like carbonyls (C=O).
• In these reactions, the carbonyl group usually gains hydrogen atoms, replacing double-bonded oxygen with a hydroxyl group (-OH).
• Reduction reactions can vary in selectivity and scope depending on the agent used.

Thus, these reactions are invaluable, especially when selectively producing alcohols from aldehydes and ketones.

NaBH4 Reduction

Sodium borohydride (NaBH4) is a mild reducing agent renowned for its ability to selectively reduce carbonyl compounds like aldehydes and ketones.

• NaBH4 specifically targets and reduces the carbonyl group without affecting other functional groups.
• Its mild nature compared to other reducing agents (like LiAlH4) makes it ideal for reactions where selectivity is crucial.
• The reduction process involves the transfer of hydride ions (H-) to the carbonyl carbon, converting it into an alcohol.

For the compound \( \mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}-\mathrm{CHO} \), NaBH4 will specifically reduce the \( \mathrm{CHO} \) group to \( \mathrm{CH}_{2} \mathrm{OH} \) while leaving other parts of the molecule unchanged.

Alpha,Beta-Unsaturated Carbonyl Compounds

Alpha,beta-unsaturated carbonyl compounds are characterized by a carbonyl group adjacent to a carbon-carbon double bond. This structural configuration results in some interesting chemical behavior.

• These compounds feature conjugation between the carbonyl and double bond, stabilizing the molecule.
• The unsaturated character implies that not just the carbonyl carbon but also the adjacent carbons can engage in reactions.
• Due to this conjugation, not all reducing agents will fully reduce both the carbonyl and the double bonds.

In the exercise, this is why NaBH4 selective reduces only the carbonyl group and does not affect the \( \mathrm{CH} = \mathrm{CH} \) double bond, thanks to its mild nature.

Hydrogenation Selectivity

Hydrogenation selectivity is crucial in organic synthesis, as it refers to the ability of a reducing agent to target specific bonds or functional groups while sparing others.

• Selective hydrogenation allows chemists to precisely modify specific sections of a molecule without altering others.
• For example, NaBH4 selectively reduces carbonyls without affecting unsaturated sections like double bonds conjugated with carbonyls.
• This selectivity is vital when working with multifunctional molecules, ensuring only desired transformations occur.

In summary, knowing the selectivity of your reducing agent, like NaBH4, allows for more controlled and efficacious reactions when reducing complex organic molecules.