Bakelite: phenol–formaldehyde resin template

Protonated formaldehyde attacks phenol at the para (or ortho) position, rearomatisation gives a hydroxymethylphenol, loss of water gives a benzylic cation and a second phenol bonds to it: a methylene bridge. Repeated at the 2, 4 and 6 positions this builds the cross-linked Bakelite network.

Mechanism step by step

Acid-catalysed step growth: hydroxymethylation of phenol, then CH₂ bridges between rings, then a cross-linked network

1. Hydroxymethylation – The OH lone pair pushes through the ring: the para carbon attacks protonated formaldehyde
2. Rearomatisation – Loss of H⁺ restores the ring: 4-hydroxymethylphenol
3. Protonation – Acid protonates the CH₂OH group
4. Loss of water – A benzylic cation, stabilised by the para OH (a protonated quinone methide)
5. Methylene bridge – A second phenol attacks the cation through its ortho carbon (the OH-bearing carbon takes the + charge)
6. Rearomatisation – Ar–CH₂–Ar: the methylene-bridged dimer

Key points

• Phenol is activated at three positions (2, 4 and 6), so the chains branch and cross-link into a rigid 3D network: a thermoset that chars instead of melting.
• Acid with excess phenol gives a fusible novolac, cured with hexamine; base with excess HCHO gives a resol (hydroxymethylphenols from the phenoxide) that cures on heating.
• Step-growth condensation: each CH₂ bridge costs one molecule of water.
• Bakelite (Baekeland, 1907) was the first fully synthetic plastic: electrical fittings, laminates and adhesives.

Drawn with 160 atoms, 21 arrows, 14 shapes – every one editable in MolDraw.

Sources

• Odian, Principles of Polymerization, 4th ed., Wiley 2004, ch. 2 Textbook
• Young & Lovell, Introduction to Polymers, 3rd ed., CRC Press 2011 Textbook