One of the earliest commercial plastics was Bakelite^R, formed by the reaction of phenol with a little more than one equivalent of formaldehyde under acidic or basic conditions. Bayer first discovered this reaction in 1872, and practical methods for casting and molding. Bakelite were developed around 1909. Phenol-formaldehyde plastics and resins (also called phenolics) are highly cross-linked because each phenol ring has three sites (two ortho and one para) that can be linked by condensation with formaldehyde. Suggest a general structure for a phenol-formaldehyde resin, and propose a mechanism for its formation under acidic conditions. (Hint: Condensation of phenol with formaldehyde resembles the condensation of phenol with acetone, used in Problem 26-17, to make bisphenol A.)

Phenol on treatment with formaldehyde in the presence of an acidic or a basic catalyst undergoes condensation polymerization to form either a linear or a cross-linked polymer known as phenol-formaldehyde resin or bakelite. Methylene bridges are formed either at ortho or para position or both at ortho and para positions with respect to the phenolic group.

In this process, a large number of monomer molecules combine together usually with the loss of simple molecules like water, alcohol, ammonia, carbon dioxide, hydrogen chloride etc., to form a polymer in which the molecular formula of the repeat unit is not the same as that of the monomer.

Bakelite formation takes place via electrophilic substitution at first followed by condensation reaction.

In mechanism part, protonation of the carbonyl oxygen of formaldehyde takes place first with H⁺ . The protonated form of formaldehyde gets added to the phenol ring where the double bond gets rearranged. Then the species formed reacts with a water molecule to form para-hydroxy benzyl alcohol. Para-hydroxy benzyl alcohol gets protonated in the next step followed by the loss of a water molecule. The product formed after the loss of a water molecule reacts with a phenol where methylene bridges are formed at para position with respect to the phenolic group. Lastly, a water molecule is added and the double bond gets rearranged to form the bakelite.