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

Short answer

The initial reaction is an aromatic electrophilic substitution.

Step-by-step solution

Understanding the Starting Compounds

Before identifying the type of reaction, it's crucial to understand the nature of the starting compounds: phenol and formaldehyde. Phenol is an aromatic compound with an OH group attached to its benzene ring. Formaldehyde is the simplest aldehyde, containing a carbonyl group, which is known for its reactive nature.

Knowing the Possible Reactions

Next, we must consider the potential reactions listed in the choices: - **Free radical reaction**: Involves species with unpaired electrons, typically needing conditions initiating radical formation. - **Aromatic nucleophilic substitution**: Uncommon due to the stability of benzene's ring but typically involves substitution of a ring-bound group. - **Aromatic electrophilic substitution**: Common in aromatic compounds; electrophiles attack the electron-rich benzene ring. - **Aldol reaction**: Involves condensation between carbonyl compounds (ketones or aldehydes) typically resulting in 23-disubstituted butenal or butenol.

Match Reaction Type with Phenol and Formaldehyde

Phenol, as an aromatic compound, is prone to electrophilic attack on its electron-rich ring. Formaldehyde can act as an electrophile due to its electron-deficient carbonyl carbon. This setup aligns closely with aromatic electrophilic substitution where electrophiles substitute a hydrogen atom on an aromatic ring.

Identifying the Correct Reaction

Given that the reaction matches the characteristics of electrophilic substitution—specifically within an aromatic compound like phenol—the correct choice among the options is the aromatic electrophilic substitution reaction. This initial interaction is fundamental to the polymerization that forms Bakelite.

Key concepts

Bakelite Synthesis

Bakelite, one of the first synthetic plastics, is created through a polymerization process involving phenol and formaldehyde. This reaction is known for its significance in the development of synthetic resins. The synthesis begins with the initial combination of phenol and formaldehyde, undergoing a type of reaction that is pivotal for the creation of Bakelite. This involves aromatic electrophilic substitution, where the electron-rich benzene ring in phenol is attacked by the electrophile, which in this context is generated from formaldehyde.

During the reaction process, formaldehyde provides the reactive sites that lead to the formation of methylene bridges, connecting phenol units. This results in a network polymer structure, granting Bakelite its desirable properties such as rigidity, heat resistance, and electrical non-conductivity. The polymerization that solidifies Bakelite occurs through step-growth polymerization, where small molecules slowly build onto each other. This gradual building forms larger chains and cross-links, which are essential for creating the characteristic high-strength network structure of Bakelite.

Phenol Reaction Chemistry

Phenol is a unique compound due to its structure—a benzene ring bonded with a hydroxyl (OH) group. This configuration makes phenol an excellent candidate for reactions involving electrophilic substitution. The aromatic ring in phenol is electron-rich, making it susceptible to attack by electrophiles.

The hydroxyl group on phenol activates the benzene ring, increasing its electron density. This highlights phenol’s ability to participate in electrophilic substitution reactions more readily compared to benzene alone. Typically, substitutions occur at the ortho and para positions relative to the hydroxyl group, because these positions are most activated by the OH group’s electron-donating effect.
Consequently, this aspect of phenol reaction chemistry is crucial in several industrial and laboratory syntheses, including the formation of polymers like Bakelite.

Formaldehyde Reaction Chemistry

Formaldehyde is known for its high reactivity, attributed to its simple structure as the smallest aldehyde with a carbonyl group (C=O). Its electron-deficient carbon atom makes it an effective electrophile, which is fundamental in its reactivity with compounds possessing electron-rich centers.

In the context of Bakelite synthesis, formaldehyde's role is particularly crucial. Its electrophilic nature enables it to react with phenol in the aromatic electrophilic substitution process. This reaction results in the formation of methylene bridges linking aromatic units. These bridges are central to building the robust three-dimensional network required in Bakelite's final polymer structure.

Moreover, formaldehyde is versatile in chemistry, often used in other types of reactions such as crosslinking and various forms of polymerization, exhibiting its wide range of applications beyond just the creation of Bakelite.