Problem 2 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Preparation of Bakelite proceeds via reactions: [Main Jan. 08, 2020 (II)] (a) Electrophilic addition and dehydration (b) Condensation and elimination (c) Electrophilic substitution and dehydration (d) Nucleophilic addition and dehydration

Step-by-Step Solution

Answer

Bakelite formation involves electrophilic substitution and dehydration, option (c).

Step 1: Identifying the Reactants and the Initial Reaction

Bakelite is formed from the polymerization of phenol and formaldehyde. The initial reaction in the formation is the reaction between phenol (an aromatic compound) and formaldehyde (an aldehyde). This reaction involves the creation of a methylolphenol or hydroxymethylphenol intermediate.

Step 2: Understanding the Electrophilic Substitution Component

Phenol reacts with formaldehyde through an electrophilic substitution reaction. The hydroxymethyl group (-CH2OH) substitutes a hydrogen on the aromatic ring of phenol. This step involves the aromatic ring reacting with an electrophile (formaldehyde) to form hydroxymethylphenol.

Step 3: Dehydration Reaction in Bakelite Formation

Following the formation of hydroxymethylphenol, a dehydration reaction occurs where water is eliminated, leading to the formation of methylene bridges (-CH2-) that link the phenol units together. This dehydration occurs as the polymer chain builds.

Step 4: Concluding the Type of Reaction Involved in Bakelite Formation

The formation of Bakelite involves both electrophilic substitution when the formaldehyde substitutes on the phenol, and dehydration as the hydroxymethyl groups form methylene links by water loss.

Key Concepts

Electrophilic Substitution

In the context of Bakelite preparation, electrophilic substitution is a pivotal step. It occurs when an electrophile, such as formaldehyde, reacts with phenol. Phenol, being an aromatic compound, possesses a benzene ring enriched with electrons. This makes it ready to react with electrophiles. An electrophile is a species that accepts electrons, forming a new bond.

In electrophilic substitution, the formaldehyde (the electrophile) penetrates the electron cloud of phenol and substitutes one of its hydrogen atoms. This substitution occurs at the carbon atom of the aromatic ring, mainly at the ortho or para position. This results in the formation of hydroxymethylphenol, an important intermediate in Bakelite production.

The key points to remember about electrophilic substitution in this scenario are:

• The electrophile (formaldehyde) takes the place of a hydrogen atom.
• The aromatic nature of phenol allows it to stabilize the intermediate.
• This reaction leads to the creation of hydroxymethylphenol.

Dehydration Reaction

Dehydration reactions play a crucial role in creating Bakelite. After the electrophilic substitution forms hydroxymethylphenol, dehydration comes into play. A dehydration reaction involves the removal of water (H₂O) from a molecule. In Bakelite synthesis, this is a polymerization step that links molecules together, forming a complex network.

During this process, hydroxymethylphenol loses a molecule of water. As water is removed, methylene bridges (-CH₂-) form between phenol units. These bridges are crucial as they connect multiple phenol molecules, creating a sturdy and thermoset polymer known as Bakelite.

Key features of dehydration in Bakelite formation include:

• Elimination of water molecules.
• Formation of methylene bridges between phenol units.
• This process results in a strong, cross-linked polymer network.

Phenol and Formaldehyde Reaction

The reaction between phenol and formaldehyde is the starting point for Bakelite synthesis. Phenol, a ring-like structure (aromatic), is reactive due to its hydroxyl group (-OH). Formaldehyde, on the other hand, is simpler, with a carbon that's easily attacked due to its double-bonded oxygen. This makes it an essential electrophile in the reaction.

Initially, phenol interacts with formaldehyde to form hydroxymethylphenol through an electrophilic substitution. This then undergoes further reactions, specifically dehydration, to establish links between phenol units by forming methylene bridges. This ultimately results in Bakelite, a durable, heat-resistant polymer.

Important considerations when discussing the phenol and formaldehyde reaction include:

• Use of phenol's aromatic nature to facilitate reactions.
• Formaldehyde's electrophilic properties allow it to bond with phenol.
• Formation of a highly cross-linked and durable polymer as an end product.