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 (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) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}^{3}-\mathrm{CHO}\)

Step-by-Step Solution

Answer

(b) \( \mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}_{2} \mathrm{OH}\)

Step 1: Understanding the Reaction

The compound \( \text{CH}_{2}=\text{CH}-\text{COOH} \) is acrylic acid, which contains an alkene group and a carboxylic acid group. \( \text{LiAlH}_{4} \) is a strong reducing agent that is typically used to convert carboxylic acids to alcohols.

Step 2: Identify the Group Affected

Since \( \text{LiAlH}_{4} \) is a strong reducing agent, it will reduce the \( \text{-COOH} \) group in acrylic acid to \( \text{-CH}_2\text{OH}. \) It's important to note that \( \text{LiAlH}_4 \) does not typically reduce the carbon-carbon double bond (\( \text{C=C} \)) under these conditions.

Step 3: Write the Resulting Compound

Reducing \( \text{CH}_{2}=\text{CH}-\text{COOH} \) with \( \text{LiAlH}_{4} \) converts the carboxylic acid to a primary alcohol. This results in the compound \( \text{CH}_{2}=\text{CH}-\text{CH}_{2}\text{OH}. \)

Step 4: Select the Correct Option

From the multiple-choice options given, \( \text{CH}_{2}=\text{CH}-\text{CH}_{2}\text{OH} \) matches with option (b). Therefore, the correct answer is option (b).

Key Concepts

Acrylic Acid

Acrylic acid is a simple unsaturated carboxylic acid, characterized by the presence of a single carbon-carbon double bond in the structure, denoted as \( \text{CH}_2 = \text{CH} - \text{COOH} \). This double bond is a key feature that classifies it as an alkene. The carboxylic acid group \( \text{-COOH} \) is a functional group known for its acidic properties and ability to form hydrogen bonds.
For students, it's vital to understand why acrylic acid is significant in organic chemistry. Its reactivity, particularly the ability to undergo polymerization, makes it a critical precursor in the production of polymers like polyacrylic acid. These materials are widely used in the production of superabsorbent polymers found in everything from diapers to water retention agents in agriculture.
Remember that, despite its alkene bond, the reaction conditions may selectively target the carboxylic acid group for transformation without affecting the double bond.

LiAlH4 Reduction

Lithium aluminium hydride (\(\text{LiAlH}_4\)) is a powerful reducing agent used in organic chemistry to convert various functional groups to their reduced forms. One of its hallmark features is its ability to reduce carboxylic acids (\(\text{-COOH}\)) to primary alcohols (\(\text{-CH}_2\text{OH}\)). This makes \(\text{LiAlH}_4\) particularly useful when a chemist needs to remove the carboxylic acid group while preserving other features of the molecule.
When applying \(\text{LiAlH}_4\) to molecules such as acrylic acid, it selectively targets the most oxidized functional group, which, in this case, is the carboxylic acid. The alkene group remains unaffected during this reduction because \(\text{LiAlH}_4\) does not typically interact with carbon-carbon double bonds under standard conditions.

• High reduction potential: Makes it useful for reducing strong bonds.
• Specificity: Targets specific groups without altering others, like \(\text{C=C}\) bonds.
• Reactivity: Not suitable for work in air or water due to high reactivity.

Learning how \(\text{LiAlH}_4\) functions is crucial for any student aspiring to master organic transformation reactions.

Carboxylic Acid to Alcohol

Carboxylic acids (\(\text{R-COOH}\)) are organic compounds with a wide array of chemical properties, one of which is their ability to be reduced to alcohols. This transformation is pivotal in synthetic chemistry and is often carried out using strong reducing agents like \(\text{LiAlH}_4\).

**Why Convert Carboxylic Acids to Alcohols?**

• Industrial and Synthetic Uses: Alcohols are crucial building blocks in the chemical industry, used in everything from fuels to pharmaceuticals.
• Simplifying Complex Structures: Reduction to an alcohol may be a step towards further chemical manipulation or simplification of a molecule.

Understanding the mechanism of this conversion is key. \(\text{LiAlH}_4\) transfers hydride ions \(\text{H}^-\) to the carbonyl carbon of the carboxylic acid, ultimately replacing the \(\text{-COOH}\) group with a hydroxyl group \(\text{-OH}\), thereby forming an alcohol.

This conversion is not only a fundamental reaction in organic chemistry courses but also an excellent way to comprehend the delicate manipulations possible with organometallic reagents.