Problem 193 - A Complete Resource Book in Chemistry for JEE Main

Question

Match the following: List I (Reaction) 1\. \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}+\mathrm{NaOH} \longrightarrow \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOH}\) 4\. \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}+\mathrm{NaOH}+\mathrm{CO}_{2} \stackrel{\text { 1. heat, 2. } \mathrm{H}^{+}}{\longrightarrow}\) List II (Name of reactions) (i) Rosenmund reduction (ii) Kolbe's reaction (iii) Claisen reaction (iv) Cannizzaro reaction (v) Friedal-Craft's reaction The correct matching is:

Step-by-Step Solution

Answer

1. Cannizzaro reaction (iv); 4. Kolbe's reaction (ii).

Step 1: Identify Reaction 1

The reaction given is \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}+\mathrm{NaOH} \longrightarrow \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOH} \). This reaction involves the conversion of benzaldehyde (\(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{CHO}\)) to benzoic acid (\(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{COOH}\)) in the presence of a strong base such as sodium hydroxide (\(\mathrm{NaOH}\)). This is indicative of the Cannizzaro reaction.

Step 2: Identify Reaction 4

The reaction \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}+\mathrm{NaOH}+\mathrm{CO}_{2} \stackrel{\text { 1. heat, 2. } \mathrm{H}^{+}}{\longrightarrow} \) occurs through the Kolbe's reaction mechanism. In Kolbe's reaction, phenol (\(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\)) reacts with carbon dioxide (\(\mathrm{CO}_2\)) and sodium hydroxide (\(\mathrm{NaOH}\)) to form sodium phenoxide, which upon heating and subsequent acidification yields a salicylic acid derivative.

Step 3: Match Reaction 1 with List II

The Cannizzaro reaction (iv) involves the self-oxidation and reduction of two molecules of aldehydes (without α-H) in the presence of a base to form an alcohol and a carboxylic acid. Therefore, reaction 1 matches with option (iv), Cannizzaro reaction.

Step 4: Match Reaction 4 with List II

Kolbe's reaction (ii) is the process of preparing salicylic acid by reaction of sodium phenoxide with carbon dioxide and subsequent acidification. Therefore, reaction 4 corresponds to option (ii), Kolbe's reaction.

Key Concepts

Cannizzaro Reaction

The Cannizzaro reaction is a chemical process where aldehydes, which lack a hydrogen atom on the carbon adjacent to the carbonyl group (known as α-hydrogen), undergo self-oxidation and reduction. Essentially, in the presence of a strong base like sodium hydroxide (\(\mathrm{NaOH}\)), two molecules of an aldehyde react.

• One molecule of the aldehyde is oxidized to form a carboxylic acid.
• The other molecule is reduced to form an alcohol.

An example of this reaction is the transformation of benzaldehyde (\(\mathrm{C}_6\mathrm{H}_5\mathrm{CHO}\)) into benzoic acid (\(\mathrm{C}_6\mathrm{H}_5\mathrm{COOH}\)) and benzyl alcohol under the influence of a base.
It’s interesting because it doesn't require external oxidizing or reducing agents. The reaction is rather unique among aldehyde chemistry as it effectively redistributes electrons between two aldehyde molecules.
Taking advantage of this versatility, chemists use the reaction to synthesize various organic compounds, making it an important reaction in synthetic organic chemistry.

Kolbe's Reaction

Kolbe's reaction primarily involves the conversion of phenol derivatives into aromatic acids using carbon dioxide. In a typical scenario, phenol (\(\mathrm{C}_6\mathrm{H}_5\mathrm{OH}\)) is reacted with sodium hydroxide (\(\mathrm{NaOH}\)) to form sodium phenoxide.

When this sodium phenoxide is exposed to carbon dioxide (\(\mathrm{CO}_2\)) and heat is applied, an interesting transformation occurs:

• The sodium phenoxide converts to salicylic acid, crucial for synthesizing aspirin.
• This happens through an intermediate where the carbon dioxide attacks the phenoxide ion, facilitating this thoughtful reaction.

Subsequent reaction conditions, such as acidification, help in further refining the product.
The reaction is pivotal in organic synthesis, helping chemists manufacture compounds that are vital for many industries.
Unique to this reaction is its ability to introduce carbon extensions into aromatic rings. This effectively enables chemists to build more complex structures from simpler starting materials.

Phenol Chemistry

Phenol chemistry is an intriguing area of organic chemistry that deals with compounds containing the phenol group, \(\mathrm{C}_6\mathrm{H}_5\mathrm{OH}\). Phenols have a hydroxyl group (\(\mathrm{OH}\)) attached directly to an aromatic benzene ring. This structure renders phenols both acidic and reactive:

• The hydroxyl group contributes to the acidic nature of phenols. They are more acidic than alcohols due to resonance stabilization of the phenoxide ion formed after losing a hydrogen ion.
• This acidity plays a significant role in reactions, enabling substitution in the aromatic ring.

A distinctive feature is their reactivity under different conditions, such as when reacting with \(\mathrm{NaOH}\), allowing for processes like the Kolbe's reaction.
In addition, phenols engage in a variety of reactions:

• They can participate in electrophilic substitution reactions, such as halogenation, nitration, and sulfonation.
• Phenols also undergo esterification reactions, forming esters when reacting with acid chlorides or anhydrides.

Understanding phenol chemistry opens doors to numerous industrial and pharmaceutical applications, making it an essential cornerstone in understanding organic reactions.