Problem 167 - A Complete Resource Book in Chemistry for JEE Main

Question

A new carbon-carbon bond formation is possible in 1\. Cannizzro reaction 2\. Feirdel-Craft's reaction 3\. Clemmensen reduction 4\. Reimer - Teimann reaction (a) 2,4 (b) 1,2 (c) 2,3 (d) \(1,2,4\)

Step-by-Step Solution

Answer

(a) 2,4

Step 1: Understanding the Options

First, we need to understand each of the reactions mentioned to determine if they involve carbon-carbon bond formation: 1. **Cannizzro reaction** - This is a redox reaction involving aldehydes without an alpha hydrogen, which does not lead to carbon-carbon bond formation. 2. **Friedel-Craft's reaction** - This is a type of electrophilic aromatic substitution that results in the formation of carbon-carbon bonds by attaching alkyl or acyl groups to an aromatic ring. 3. **Clemmensen reduction** - This involves the reduction of ketones to alkanes and does not create new carbon-carbon bonds. 4. **Reimer-Tiemann reaction** - This is a formylation reaction where a new carbon-carbon bond is formed when introducing a formyl group to a phenol.

Step 2: Identifying Correct Reactions

Based on the analysis: - Friedel-Craft's reaction does involve the formation of new carbon-carbon bonds. - Reimer-Tiemann reaction also involves the formation of new carbon-carbon bonds. Thus, options 2 and 4 lead to carbon-carbon bond formation.

Step 3: Choosing the Correct Answer

Given the analysis, we have determined that options 2 and 4 lead to the formation of carbon-carbon bonds. Therefore, the correct option that reflects these reactions is (a) 2,4.

Key Concepts

Friedel-Craft's Reaction

Friedel-Craft's reaction is a powerful technique in organic chemistry used for introducing substituents onto aromatic rings. It falls under the broader category of electrophilic aromatic substitution, which involves the replacement of a hydrogen atom on an aromatic ring with an electrophile. This process specifically leads to carbon-carbon bond formation, a fundamental step in synthesizing various complex organic molecules.

There are two main types of Friedel-Craft reactions:

• Alkylation: In this form, an alkyl group is introduced onto an aromatic ring. An alkyl chloride reacts with an aromatic compound in the presence of a strong Lewis acid, such as aluminum chloride (AlCl₃), forming a carbon-carbon bond.
• Acylation: This involves adding an acyl group to the aromatic ring using an acyl chloride and a Lewis acid. It results in the formation of a ketone, which is useful for further synthetic reactions due to the directing effects of the newly formed group.

Despite its utility, Friedel-Craft's reaction has limitations, such as the possible rearrangement of carbocations and the requirement of non-deactivating substituents on the ring for the reaction to take place effectively.

Reimer-Tiemann Reaction

The Reimer-Tiemann reaction specializes in the formylation of phenolic compounds, an essential process in aromatic chemistry. This reaction introduces a formyl group, producing ortho-hydroxyaldehydes from phenols, which is a valuable method for creating carbon-carbon bonds.

The mechanism involves:

• Starting with chloroform (CHCl₃), which reacts with strong base, like sodium hydroxide (NaOH), to form dichlorocarbene (CCl₂).
• Dichlorocarbene acts as the electrophile, inserting into the ortho-position of the phenol due to the electron-rich nature of the aromatic ring.
• Subsequent hydrolysis then converts the substituted adduct into the final aldehyde product.

This reaction is particularly useful because it selectively adds formyl groups to aromatic rings, lending itself to further functionalizations and synthetic applications. The Reimer-Tiemann reaction is limited mainly to phenolic substrates due to the need for electron-rich aromatic systems to stabilize the intermediates.

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution (EAS) is a key class of reactions in organic chemistry characterized by the replacement of a hydrogen atom on an aromatic ring with an electrophile. This process maintains the integrity of the aromatic system while allowing for intricate functionalizations of aromatic molecules.

During EAS, the aromatic compound first interacts with an active electrophile, creating a non-aromatic carbocation intermediate. This intermediate is stabilized by resonance, allowing the reaction to proceed. Finally, a proton is lost from the intermediate, regenerating the aromatic system.

Common variants of EAS include:

• Nitration: Formation of nitrobenzene from benzene using nitric acid and sulfuric acid.
• Halogenation: Introduction of halogens, like chlorine or bromine, often using a Lewis acid catalyst.
• Sulfonation: Sulfonic groups are added using sulfur trioxide or oleum.

Each type of EAS reaction selects for different types of electrophiles and offers diverse methods for modifying aromatic compounds, paving the way for further development in pharmaceutical, material, and chemical industries.