Problem 235 - A Complete Resource Book in Chemistry for JEE Main

Question

In the reaction, \(\mathrm{CH}_{3} \mathrm{COOH} \stackrel{\mathrm{LiAH}_{4}}{\longrightarrow} \mathrm{A} \stackrel{\mathrm{PCI}_{5}}{\longrightarrow} \mathrm{B} \stackrel{\text { AIC. } \mathrm{KOH}}{\longrightarrow} \mathrm{C}\) the product \(\mathrm{C}\) is: (a) Ethylene (b) Acetyl chloride (c) Acetaldehyde (d) Acetylene

Step-by-Step Solution

Answer

(a) Ethylene

Step 1: Identify the first reaction

In the first step, acetic acid ( CH_3COOH ) is reduced by lithium aluminum hydride ( LiAlH_4 ) to form ethanol. This is a common reduction reaction where carboxylic acids are converted to primary alcohols.

Step 2: Analyze the second reaction

In the second step, ethanol, the product from Step 1, reacts with phosphorus pentachloride ( PCl_5 ) to form ethyl chloride ( C_2H_5Cl ). This reaction replaces the OH group of the alcohol with a chlorine atom.

Step 3: Determine the final product

In the final step, ethyl chloride ( C_2H_5Cl ) undergoes elimination in the presence of alcoholic potassium hydroxide ( KOH ). This reaction removes the Cl and H leading to the formation of ethylene ( C_2H_4 ).

Key Concepts

Reduction Reactions

Reduction reactions are an essential part of organic chemistry. They involve the gain of electrons by a molecule, often accompanied by the addition of hydrogen. A classic example is when carboxylic acids are converted into alcohols. This process usually requires a reducing agent. One of the most potent and commonly used reducing agents is lithium aluminum hydride (\( \text{LiAlH}_4 \)).

In the context of our exercise, acetic acid (\( \text{CH}_3\text{COOH} \)) is reduced by lithium aluminum hydride. This results in the formation of ethanol (\( \text{CH}_3\text{CH}_2\text{OH} \)). This transformation is crucial because it changes a carboxylic acid into a primary alcohol.

Reduction reactions, especially with potent agents like \( \text{LiAlH}_4 \), are widely used in synthesis to increase the functional diversity of a molecule. This makes them invaluable in various fields including pharmaceuticals and materials science.

Carboxylic Acids

Carboxylic acids are a key functional group in organic chemistry characterized by the \( \text{COOH} \) group. This group comprises a carbonyl (\( \text{C=O} \)) and a hydroxyl (\( \text{OH} \)) group. This structure makes carboxylic acids both higher in acidity compared to alcohols and crucial in various chemical reactions.

In our exercise, acetic acid is the starting material which undergoes a reduction. This highlights one of the versatile reactions involving carboxylic acids: their ability to be reduced to alcohols.

Key points about carboxylic acids include:

• They can be reduced to primary alcohols.
• Their acidic properties allow them to donate protons (H\(^+\)).
• They participate in a multitude of reactions including esterification and formation of acyl chlorides.

These properties make them a critical component in organic synthesis, enabling the formation of various derivatives.

Elimination Reactions

Elimination reactions are another fundamental type of reaction in organic chemistry, where elements are removed from a molecule resulting in the formation of a double bond. In the final step of our example, ethyl chloride (\( \text{C}_2\text{H}_5\text{Cl} \)) is transformed into ethylene (\( \text{C}_2\text{H}_4 \)) through an elimination reaction.

When ethyl chloride is treated with alcoholic potassium hydroxide (\( \text{KOH} \)), the \( \text{Cl} \) and \( \text{H} \) atoms are removed as hydrogen chloride (\( \text{HCl} \)), resulting in the formation of a carbon-carbon double bond. This elimination process is called dehydrohalogenation.

Some important features of elimination reactions include:

• They often involve the removal of small molecules like \( \text{H}_2\text{O} \) or \( \text{HCl} \).
• They result in the formation of alkenes or alkynes, depending on the starting material.
• Elimination complement addition reactions, providing pathways to break and form double bonds.

Understanding these reactions is crucial, as it allows chemists to control the conversion of simple molecules into more complex structures.