Animated Solution for Chemistry - Organic Chemistry: Preparation of bakelite proceeds via reactions

Bakelite is one of the oldest and most famous synthetic polymers. It is a thermosetting plastic formed by the condensation reaction of two simple monomers: phenol (${\text{C}}_6{\text{H}}_5\text{OH}$) and formaldehyde ($\text{HCHO}$).

To understand how these two molecules combine to form a rigid, infusible solid, we need to break down the reaction mechanism into two distinct phases.

Electrophilic Aromatic Substitution

The first step of the reaction occurs in the presence of an acid or base catalyst. Let's consider the acid-catalyzed pathway. The acid protonates the oxygen atom of formaldehyde, making the carbon atom highly electrophilic.

Phenol, on the other hand, has a hydroxyl group ($-\text{OH}$) attached to the benzene ring. This group is strongly activating and directs incoming electrophiles to the ortho and para positions due to resonance (+R effect).

When the protonated formaldehyde attacks the electron-rich phenol ring, an electrophilic aromatic substitution takes place. The result is the formation of $o$-hydroxymethylphenol and $p$-hydroxymethylphenol. These molecules serve as the crucial intermediates for the next stage.

Dehydration and Polymerization

Now that we have our intermediates, the actual polymerization begins. The hydroxymethyl group ($-{\text{CH}}_2\text{OH}$) of one intermediate molecule reacts with the hydrogen atom at the ortho or para position of another phenol ring.

During this condensation process, the $-\text{OH}$ from the hydroxymethyl group and the $-\text{H}$ from the ring combine to form a molecule of water (${\text{H}}_2\text{O}$), which is eliminated from the system. Because water is lost, this step is fundamentally a dehydration reaction.

As this dehydration continues, the phenol rings become linked together by methylene ($-{\text{CH}}_2-$) bridges. Initially, this forms a linear polymer known as Novolac.

When Novolac is further heated with an excess of formaldehyde, the cross-linking process accelerates. Methylene bridges form not just in a linear chain, but across different chains, creating a massive, three-dimensional network. This highly cross-linked, rigid structure is what we call Bakelite.

By analyzing the mechanism, we can clearly see that the preparation of Bakelite involves two primary types of reactions:

1. Electrophilic substitution to form the hydroxymethylphenol intermediates. 2. Dehydration to link these intermediates into a polymer chain.

Therefore, the correct description of the reactions involved is Electrophilic substitution and dehydration.