Industrial Chemistry
Industrial chemistry applies the principles of organic and inorganic chemistry to manufacture commercially valuable products on the bulk scale. The PMDC MDCAT 2026 syllabus expects familiarity with polymers, dyes, and adhesives — their classifications, key examples, and the chemistry behind them. Most years see 1-2 MCQs from this chapter.
Polymers
A polymer is a giant molecule (macromolecule) built up by repeated linking of smaller units called monomers. The process is polymerisation. Polymers may be natural (cellulose, proteins, starch) or synthetic (polythene, nylon, PET).
Two main mechanisms
Monomers contain a C=C double bond and join together without loss of any small molecule. The mechanism is free-radical (or ionic). The empirical formula of the polymer matches that of the monomer.
- Polythene (PE) — from ethene (CH2=CH2); LDPE for bags, HDPE for bottles.
- Polypropylene (PP) — from propene; ropes, food containers.
- PVC — from vinyl chloride (CH2=CHCl); pipes, insulation.
- Polystyrene (PS) — from styrene; foam packaging, disposable cups.
- PTFE (Teflon) — from tetrafluoroethene; non-stick coatings.
Two monomers (each with two reactive groups) combine with the elimination of a small molecule, usually H2O, HCl or NH3. Empirical formula of the polymer differs from that of the monomers.
- Nylon-6,6 — hexamethylenediamine + adipic acid; eliminates H2O. Linked by amide (−CONH−) bonds. Used in fibres, fabrics.
- PET / Terylene / Dacron — ethylene glycol + terephthalic acid; eliminates H2O. Linked by ester (−COO−) bonds. Used in bottles and synthetic fibres.
- Bakelite — phenol + formaldehyde; eliminates H2O. Cross-linked thermoset; electrical fittings.
Thermoplastics vs thermosetting
- Thermoplastics soften on heating and harden on cooling, repeatedly. Linear or lightly branched. Examples: PE, PP, PVC, PS.
- Thermosetting harden permanently on heating because of chemical cross-linking. Cannot be remoulded. Examples: Bakelite, urea-formaldehyde, melamine, epoxy resins.
Dyes
A dye is a coloured organic compound that imparts colour to fabric, hair, leather, paper or food. To be a useful dye it must (1) absorb visible light, (2) be water-soluble or otherwise dispersible, and (3) bind firmly to the substrate.
Chromophore and auxochrome
- Chromophore — the unsaturated group that gives the molecule its colour by absorbing visible light. Examples: −N=N− (azo), >C=O, −NO2, −C=C−.
- Auxochrome — an electron-donating group that intensifies the colour and helps fix the dye to the fabric. Examples: −OH, −NH2, −NHR, −SO3H.
Classification by structure
Contain one or more −N=N− groups. Largest class — cover the full visible spectrum. Made by diazotisation of an aromatic primary amine followed by coupling with a phenol or amine. Methyl orange and Congo red are common azo dyes.
Three aryl rings linked to a central carbon. Examples: malachite green, crystal violet, fuchsin. Used as biological stains and in inks.
Water-insoluble dyes (typically indigo and anthraquinone derivatives). Reduced with sodium dithionite to a soluble "leuco" form, applied to the fabric, then air-oxidised back to the insoluble coloured form trapped within the fibre. Indigo (used for blue jeans) is the classic example.
Adhesives
An adhesive is a substance that bonds two surfaces by adhesion (to the surfaces) and cohesion (within itself). Adhesives may be natural or synthetic.
Natural adhesives
- Animal glues (collagen-based), starch and dextrin pastes, casein, natural rubber latex.
- Used in paper, woodworking and traditional binding applications.
Synthetic adhesives
Two-component adhesive: an epoxide resin (e.g. bisphenol-A diglycidyl ether) plus a hardener (a polyamine). Cross-link to form a strong, water-resistant thermoset bond. Used in metal, glass and ceramic bonding.
Methyl or ethyl cyanoacrylate. Polymerise almost instantly through anionic addition initiated by traces of moisture (OH−) on the surfaces being joined. Bond strong but brittle.
White wood glue / school glue. Emulsion polymer of vinyl acetate; sets by water evaporation and chain entanglement.
Worked MCQs
Five MCQs that capture the high-yield testing patterns for industrial chemistry. Read every explanation — the deeper concept lives there.
Q1. Polythene is obtained from which monomer?
Polythene (PE) is the addition polymer of ethene: n CH2=CH2 → (−CH2−CH2−)n. Propene gives polypropylene; ethyne is a different monomer altogether.
Q2. Nylon-6,6 is best classified as a:
Nylon-6,6 is made from hexamethylenediamine and adipic acid with elimination of water for every amide bond formed — a textbook condensation polymerisation.
Q3. The chromophore in an azo dye is:
The diazo (−N=N−) group is the colour-bearing chromophore in azo dyes. Hydroxyl and amino groups serve as auxochromes that intensify and shift the absorption.
Q4. Bakelite is a:
Bakelite forms by condensation of phenol and formaldehyde with elimination of water. Cross-linking creates a rigid 3D network that cannot be remoulded — a classic thermoset.
Q5. Cyanoacrylate "super glue" sets rapidly because of:
Trace water (OH−) on the surfaces initiates anionic addition polymerisation of the cyanoacrylate monomer in seconds. No solvent loss is needed; that is why super glue bonds skin almost instantly.
Quick Recap
- Polymerisation: addition (no by-product, monomer has C=C) vs condensation (loses small molecule, two reactive groups per monomer).
- Addition polymers: PE, PP, PVC, PS, PTFE.
- Condensation polymers: Nylon-6,6 (amide), Terylene/PET (ester), Bakelite (phenol-formaldehyde).
- Thermoplastics soften on heating; thermosets are cross-linked and do not.
- Dye colour comes from chromophores (e.g. −N=N−); auxochromes (−OH, −NH2) deepen it.
- Dye classes: azo (largest), triphenylmethane, vat (e.g. indigo).
- Synthetic adhesives: epoxy (two-pack thermoset), cyanoacrylate (instant), PVA (white wood glue).