Problem 84 - A Complete Resource Book in Chemistry for JEE Main

Question

Bakelite is made from phenol and formaldehyde. The initial reaction between the two compounds is an example of (a) free radical reaction (b) aromatic nucleophilic substitution (c) aromatic electrophilic substitution (d) aldol reaction

Step-by-Step Solution

Answer

The initial reaction is an example of aromatic electrophilic substitution.

Step 1: Identify the Reaction Type

Bakelite is a polymer formed from the reaction of phenol and formaldehyde. To determine the type of reaction, we look at the functional groups involved in the reaction. Phenol has a hydroxyl group attached directly to an aromatic ring, while formaldehyde contains a carbonyl group.

Step 2: Consider the Reaction Mechanism

The reaction involves the electrophilic substitution on the aromatic ring of phenol. Formaldehyde acts as an electrophile, and it interacts with the aromatic ring to form a new carbon-carbon bond. This reaction often involves the formation of intermediate compounds before setting into a polymer structure.

Step 3: Select the Correct Answer

Given that formaldehyde behaves as an electrophile and attaches to the aromatic ring of phenol, this reaction exemplifies an electrophilic substitution. Among the given choices, this matches with aromatic electrophilic substitution.

Key Concepts

Phenol-Formaldehyde Reaction

The reaction between phenol and formaldehyde is a foundational process in chemistry that results in the creation of Bakelite, one of the earliest synthetic polymers. This reaction is essential in polymer chemistry as it forms the basis for the production of many other phenolic resins.
At its core, the process begins with the interaction between phenol, which consists of an aromatic benzene ring bonded to a hydroxyl group, and formaldehyde, a simple aldehyde with a carbonyl group.

• Initially, formaldehyde behaves like an electrophile, meaning it seeks out electron-rich areas to attach to.
• Phenol's benzene ring, being rich in electron density due to its aromatic structure, acts as a perfect target for formaldehyde.

This interaction is a critical step that sets the stage for further polymerization, leading to the formation of Bakelite. As a result, understanding this reaction not only elucidates the synthesis of Bakelite but also demonstrates key principles of how aldehydes react with aromatic compounds.

Aromatic Electrophilic Substitution

Aromatic electrophilic substitution is a prevalent type of chemical reaction, especially in the formation of polymers like Bakelite. In this process, an electrophile selectively reacts with an aromatic compound, replacing a substituent on the ring with itself.
In the case of phenol and formaldehyde:

• The aromatic ring in phenol contains electrons that are delocalized, making them accessible and reactive towards electrophiles like formaldehyde.
• The reaction involves the formation of a carbon-carbon bond, as formaldehyde substitutes a position on the aromatic ring of phenol.

This reaction mechanism is facilitated by the resonance stability of the aromatic ring, allowing the reaction to proceed without disrupting the integrity of the ring structure. When phenol and formaldehyde interact through this electrophilic substitution, intermediate compounds form, which eventually polymerize to create the solid structure of Bakelite.

Polymer Chemistry

Polymer chemistry explores the development and characteristics of polymers, which are large molecules made by repeating smaller units, known as monomers. The synthesis of Bakelite is a classic example within this field, showcasing the creation of a synthetic polymer through a stepwise polymerization process.
In the context of Bakelite formation:

• The phenol-formaldehyde reaction acts as a condensation reaction, where water molecules are typically eliminated as new carbon-carbon bonds form.
• After the initial reaction, phenol and formaldehyde monomers link to form long, interconnected chains or structures, leading to the hardened polymer known as Bakelite.

The resulting polymer is characterized by high thermal stability and mechanical strength, which is why Bakelite historically found extensive use in electrical insulators and other heat-resistant applications. Understanding polymer chemistry helps explain not only the formation of materials like Bakelite but also the broader principles of materials science and engineering.