Problem 214 - A Complete Resource Book in Chemistry for JEE Main

Question

When \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{COOH}\) is reduced with \(\mathrm{LiAlH}_{4}\) the compound obtained will be \(\quad (a) \)\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{COOH}\( (b) \)\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}_{2} \mathrm{OH}\( (c) \)\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2} \mathrm{OH}\( (d) \)\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CHO}$

Step-by-Step Solution

Answer

The reduced compound is (b) \(\text{CH}_2=\text{CH}-\text{CH}_2\text{OH}\).

Step 1: Understanding the Reagent

Lithium aluminium hydride (\(\text{LiAlH}_4\)) is a strong reducing agent known for reducing carboxylic acids (\(\text{COOH}\)) to alcohols (\(\text{CH}_2\text{OH}\)). It does not affect carbon-carbon double bonds in alkenes during the reduction process.

Step 2: Identifying the Structure of the Reactant

The given reactant is acrylic acid \(\text{CH}_2=\text{CH}-\text{COOH}\). It contains a carboxylic acid group and a carbon-carbon double bond (alkene).

Step 3: Performing the Reduction

When \(\text{LiAlH}_4\) is added to acrylic acid, it specifically reduces the carboxylic acid \(\text{COOH}\) group to a primary alcohol \(\text{CH}_2\text{OH}\). The alkene double bond \(\text{CH}_2=\text{CH}\) remains unaffected. Therefore, the product formed is \(\text{CH}_2=\text{CH}-\text{CH}_2\text{OH}\).

Step 4: Matching the Product with the Given Options

The reduced product \(\text{CH}_2=\text{CH}-\text{CH}_2\text{OH}\) matches with option (b). Thus, the compound obtained after reduction with \(\text{LiAlH}_4\) is \(\text{CH}_2=\text{CH}-\text{CH}_2\text{OH}\).

Key Concepts

Lithium Aluminium Hydride

Lithium Aluminium Hydride, commonly abbreviated as \( \text{LiAlH}_4 \), is an important reagent in organic chemistry for reduction reactions. It is particularly known for its ability to reduce carboxylic acids to alcohols. This powerful reducing agent provides a source of hydride ions \((\text{H}^-)\), which are essential for the reduction process.

In reduction, the \(\text{LiAlH}_4\) doesn’t randomly choose any hydrogen to attach. Instead, it specifically targets the electronegative regions, like the carboxylic acid group \((\text{COOH})\), reducing it to a primary alcohol \((\text{CH}_2\text{OH})\). An important trait of lithium aluminium hydride is its selectivity; it does not affect alkene double bonds, so structures like \(\text{CH}_2=\text{CH}-\) remain unchanged during the process. This selectivity makes it extremely useful in synthetic chemistry, where altering specific functional groups without affecting others is crucial. For students, understanding the specificity of \(\text{LiAlH}_4\) is key to mastering reduction reactions in organic chemistry.

Acrylic Acid

Acrylic acid is an unsaturated carboxylic acid with the molecular formula \(\text{CH}_2=\text{CH}-\text{COOH}\). It contains two notable features: a carbon-carbon double bond and a carboxylic acid group. Acrylic acid ranks high in industrial importance because it serves as a precursor for many polymers.

When considering reactions of acrylic acid, it's crucial to recognize the two reactive sites present in the molecule:

• The alkene group (\(\text{CH}_2=\text{CH}-\))
• The carboxylic acid group (\(-\text{COOH}\))

Despite its simplicity, acrylic acid's dual reactivity offers diverse possibilities in chemical synthesis, from polymer production to reaction derivatives like primary alcohols. During a reduction reaction, such as with \( \text{LiAlH}_4 \), the focus is typically on the \(-\text{COOH}\) group, as the double bond remains intact. Understanding this structure helps in predicting the outcome of reduction reactions.

Primary Alcohol

Primary alcohols are organic compounds characterized by the \(\text{CH}_2\text{OH}\) group, where a hydroxyl group (\(\text{-OH}\)) is connected to a carbon atom bonded to only one other carbon atom. This distinct structure differentiates primary alcohols from secondary and tertiary alcohols, which have more carbon attachments.

In the context of reduction reactions, compounds like \(\text{LiAlH}_4\) can transform carboxylic acids to primary alcohols. This conversion involves breaking the \(-\text{C}=\text{O}\) (carboxylic) bond and replacing it with \(-\text{C}-\text{H}\) to form \(\text{CH}_2\text{OH}\). For instance, acrylic acid's carboxylic acid group becomes a primary alcohol when reduced. This creates the foundation for many more complex reactions and compounds, highlighting the significance of primary alcohols in organic synthesis.

Carboxylic Acid Reduction

Carboxylic acid reduction is a process by which the acidic functional group \((-\text{COOH})\) is converted into a less oxidized form, typically an alcohol. This transformation is fundamental in organic chemistry, allowing chemists to manipulate molecular structures.

The reduction of carboxylic acids generally requires a strong reducing agent like lithium aluminium hydride \((\text{LiAlH}_4)\). During this process, the \(-\text{C}=\text{O}\) group of the carboxylic acid is reduced to \(-\text{C}-\text{H}\), resulting in the formation of an alcohol.

• Example: Upon reduction by \(\text{LiAlH}_4\), acrylic acid \((\text{CH}_2=\text{CH}-\text{COOH})\) is transformed into \(\text{CH}_2=\text{CH}-\text{CH}_2\text{OH}\), maintaining the double bond while converting \(\text{COOH}\) to \(\text{CH}_2\text{OH}\).

This ability to selectively reduce carboxylic acids without affecting other groups, like alkenes, makes carboxylic acid reduction a valuable technique in synthesizing alcohol derivatives and modifying molecular frameworks in chemistry.