Some Basic Principles of Organic Chemistry
Hybridisation, IUPAC naming, isomerism, electron movement, inductive/resonance/hyperconjugation.
IUPAC nomenclature
Naming alkanes, functional groups, substituents.
Isomerism
Structural, stereo, geometric, optical.
Inductive, resonance and hyperconjugation
Effects on stability and reactivity.
Polymer: large molecule made by linking many small molecules (monomers).
Polymerization: the process of forming polymers.
Types based on source:
- Natural: rubber, cellulose, starch, proteins, nucleic acids.
- Synthetic: plastic, nylon, polyester.
- Semi-synthetic: modified natural — rayon (from cellulose), gun-cotton.
Types based on structure:
- Linear: straight chains. Examples: HDPE, PVC, nylon.
- Branched: main chain + side chains. Example: LDPE.
- Cross-linked / 3D network: chains connected by covalent bonds. Example: Bakelite, vulcanized rubber.
Types based on thermal behavior:
- Thermoplastic: soften on heating, harden on cooling. Reversible. Examples: polyethene, polystyrene, PVC.
- Thermosetting: set permanently on heating. Cannot be remoulded. Examples: Bakelite, urea-formaldehyde, melamine.
Types based on mechanism:
- Addition polymers: monomers add without losing atoms. Need unsaturated (C=C) monomers.
- Condensation polymers: monomers combine with loss of small molecules (usually water or NH₃).
ADDITION POLYMERS (chain polymerization):
| Polymer | Monomer | Uses |
|---|---|---|
| Polyethylene (PE) | Ethene (CH₂=CH₂) | Bags, bottles, films |
| Polypropylene (PP) | Propene | Carpets, packaging |
| Polystyrene (PS) | Styrene (C₆H₅CH=CH₂) | Disposable cups, insulation |
| PVC | Vinyl chloride (CH₂=CHCl) | Pipes, raincoats, cables |
| Teflon (PTFE) | Tetrafluoroethene (CF₂=CF₂) | Non-stick coating, gaskets |
| PMMA (acrylic) | Methyl methacrylate | Bulletproof glass, displays |
| Polyacrylonitrile | Acrylonitrile (CH₂=CHCN) | Orlon (synthetic wool, sweaters) |
Mechanism: initiated by free radical (peroxide), cationic (acid), or anionic (base).
CONDENSATION POLYMERS:
| Polymer | Monomers | Uses |
|---|---|---|
| Nylon-6,6 | Hexamethylenediamine + Adipic acid | Textiles, ropes, tyre cords |
| Nylon-6 | Caprolactam | Tyre cord, hosiery |
| Terylene / Dacron (PET) | Ethylene glycol + Terephthalic acid | Synthetic fibers, water bottles |
| Bakelite | Phenol + Formaldehyde | Electrical switches, handles |
| Melamine | Melamine + Formaldehyde | Unbreakable crockery |
Nylon-6,6 structure: repeating unit has 6 carbons in each monomer (hence the numbering 6,6).
NATURAL POLYMERS
Carbohydrates:
- Starch (α-1,4 glucose; α-1,6 branches in amylopectin).
- Cellulose (β-1,4 glucose; linear; structural in plants).
- Glycogen (animal storage; like amylopectin but more branched).
Proteins: linear chains of amino acids linked by peptide bonds.
Nucleic acids: DNA, RNA — chains of nucleotides.
Rubber:
- Natural rubber: cis-1,4-polyisoprene from Hevea brasiliensis tree latex.
- Vulcanization (1839, Goodyear): heated with sulfur → cross-links → stronger, more elastic.
- Synthetic rubber: neoprene (from chloroprene), Buna-N (acrylonitrile + butadiene), SBR (styrene + butadiene — used in tyres).
BIODEGRADABLE POLYMERS (emerging):
- PHBV (poly-β-hydroxybutyrate-co-β-hydroxyvalerate): produced by bacteria, degrades in soil/marine environments.
- Polylactic acid (PLA): from corn starch / sugarcane. Used in compostable bags, sutures, 3D printing.
- PGA (polyglycolic acid): dissolvable surgical sutures.
These are research/industry priorities for reducing plastic pollution.
Molecular weight of polymers is not a single value — distribution.
- Number average molecular weight (M_n) = Σ N_i M_i / Σ N_i (counts all molecules equally).
- Weight average molecular weight (M_w) = Σ N_i M_i² / Σ N_i M_i (weighted by mass).
- Polydispersity index (PDI) = M_w / M_n. PDI = 1 means uniform; commercial polymers have PDI = 2-20.
Common JEE question types:
- Identify monomer from polymer (or vice versa).
- Distinguish addition vs condensation.
- Classify as thermoplastic vs thermosetting.
- Recognize natural vs synthetic.