Problem 172 - A Complete Resource Book in Chemistry for JEE Main

Question

2\. \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{C}-\mathrm{CH}_{3} \quad \stackrel{\mathrm{LiAlH}_{4}}{\longrightarrow} \mathrm{P}\) \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CH}=\mathrm{CH}-\mathrm{CHO} \stackrel{\mathrm{LiAlH}_{4}}{\longrightarrow} \mathrm{Q}\)(P) and (Q) are respectively (a) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CHOH}-\mathrm{CH}_{3}\) and \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{2} \mathrm{OH}\) (b) \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CHOH}-\mathrm{CH}_{3}\) and \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{2} \mathrm{OH}\) (c) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CHOH}-\mathrm{CH}_{3}\) and \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2} \mathrm{OH}\) (d) \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CHOH}-\mathrm{CH}_{3}\) and \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2} \mathrm{OH}\)

Step-by-Step Solution

Answer

Option (b) is correct: \( \mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CHOH}-\mathrm{CH}_{3} \) and \( \mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{2}\mathrm{OH} \).

Step 1: Understanding the Reagent

Lithium aluminium hydride (LiAlH₄) is a strong reducing agent that is commonly used to reduce aldehydes, ketones, and carboxylic acids to alcohols. It does not affect carbon-carbon double bonds (alkenes). It will convert the -CHO group into -CH₂OH, and similarly, reduce other forms of carbon bound to oxygen like carboxylic acids to alcohols.

Step 2: Analyzing the First Reaction

The first reagent is an alkene with a carbonyl group. Upon treatment with LiAlH₄, the carbonyl group (the C=O double bond from the ketone or aldehyde group) is reduced to an alcohol group (-OH). Therefore, \ \( \mathrm{CH}_{2}=\mathrm{CH}-\mathrm{C}-\mathrm{CH}_{3} \) becomes \ \( \mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CHOH}-\mathrm{CH}_{3} \). The double bond in the alkene part remains unchanged.

Step 3: Analyzing the Second Reaction

The second reagent has a phenyl group attached to an unsaturated aldehyde group. LiAlH₄ will reduce the -CHO group to -CH₂OH, resulting in \ \( \mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CH} = \mathrm{CH}-\mathrm{CHO} \) transforming to \ \( \mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{2}\mathrm{OH} \). Again, the double bond in the alkene part is not affected.

Step 4: Matching Products to Options

Option (b) matches the products exactly as derived: \ \( \mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CHOH}-\mathrm{CH}_{3} \) and \ \( \mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{2}\mathrm{OH} \). Options (a), (c), and (d) either change the position of reduction or the length of the carbon chain, which does not occur with LiAlH₄ in this reaction.

Key Concepts

Reduction reactions

Reduction reactions in organic chemistry typically involve the gain of electrons, which results in a decrease in the oxidation state of a molecule. In the context of organic compounds, reduction often implies the addition of hydrogen atoms to the compound or the removal of oxygen atoms. These reactions are vital in synthetic organic chemistry as they can transform functional groups, thereby altering the properties and reactivity of the molecules.

Reduction is particularly important because it allows chemists to convert complex molecules into simpler forms or to modify specific parts of a molecule while leaving others unchanged. For example, in the conversion of aldehydes and ketones to alcohols, the carbonyl group's double bond is converted into a single bond with the addition of hydrogen. This transformation significantly changes the chemical behavior of the compound.

Reduction reactions are categorized not just by their chemical activity but also by the specific reagents used, which brings us to the crucial role of reducing agents like LiAlH₄.

Reagents

Reagents are substances or compounds that are added to a system to cause a chemical reaction, or are present for reaction facilitation. In the realm of reduction reactions, particular reagents are known for their specificity. For example, reducing agents are chemicals that donate electrons to another substance in a chemical reaction, thereby reducing it.

Strong reducing agents like Lithium Aluminium Hydride (LiAlH₄) play a significant role. They are incredibly potent, particularly in reducing carbonyl compounds such as ketones and aldehydes to alcohols. Their strength lies in their ability to transfer hydride ions (H⁻), which are a source of hydrogen, to the substrate molecule, allowing for the reduction process.

Thus, understanding the properties and specificities of reagents like LiAlH₄ makes predicting and executing reduction reactions more manageable and efficient.

Alkenes

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond (C=C). This unsaturation provides a site of reactivity in organic molecules. However, in the context of reduction reactions using reagents like LiAlH₄, it's crucial to know that these double bonds remain unaffected.

The stability and reactivity of alkenes are essential in organic synthesis. Alkenes can undergo a variety of reactions, but specific reducing agents like LiAlH₄ do not impact them. This characteristic allows chemists to selectively reduce other functional groups, such as carbonyls, without altering the structure of the alkene section.

This selective reactivity is invaluable, as it ensures that the compound's structural integrity is preserved while still picking up desired reductions elsewhere in the molecule.

LiAlH₄

Lithium Aluminium Hydride, known as LiAlH₄, is one of the most powerful and commonly used reducing agents in organic chemistry. It is especially noted for its ability to reduce a wide range of carbonyl-containing functional groups, such as esters, aldehydes, and ketones, to their corresponding alcohols.

LiAlH₄ acts by transferring hydride ions (H⁻) to the electrophilic carbon in the carbonyl group. This conversion changes a double bond (C=O) to a single bond (C-OH). However, it is important to handle LiAlH₄ carefully as it reacts vigorously, especially in the presence of protic solvents (water or alcohol) which can cause violent reactions.

This selective reduction is what makes LiAlH₄ so valuable in organic synthesis, allowing scientists to achieve conversions without affecting other reactive parts of the molecule such as alkenes.

Alcohols

Alcohols are organic compounds in which the hydroxyl functional group (-OH) is bonded to a carbon atom. They are a significant product of reduction reactions involving aldehydes or ketones. The reduction of these compounds results in the addition of hydrogen atoms, transforming the carbonyl group into an alcohol.

The formation of alcohols via reduction is crucial in organic synthesis because alcohols serve as important intermediates and functional precursors in the production of a wide range of chemical products. They are versatile in chemical reactions, participating in dehydration, oxidation, and esterification, to name a few processes.

Knowing how to transform carbonyl compounds into alcohols effectively allows for the design and implementation of complex synthetic strategies in both laboratory and industrial settings. Thus, alcohols are not just end-products but crucial building blocks in further chemical transformations.