Hydrocarbons and Polymers
~9 min read
- Hydrocarbons: Compounds of C and H. Alkanes (CₙH₂ₙ₊₂), Alkenes (CₙH₂ₙ), Alkynes (CₙH₂ₙ₋₂).
- Saturated vs unsaturated: Saturated — only single bonds (alkanes). Unsaturated — double or triple bonds (alkenes, alkynes).
- Polymers: Long chains from small monomers. Natural (rubber, cellulose, protein). Synthetic (PE, PVC, nylon, polyester).
Hydrocarbons are the carbon-hydrogen backbone of organic chemistry; polymers extend them into modern plastics and fibres. CDS/OTA tests homologous series and common polymers.
Three Hydrocarbon Series
| Series | Formula | Bond | Examples |
|---|---|---|---|
| Alkanes | CₙH₂ₙ₊₂ | Single C-C | Methane CH₄, Ethane C₂H₆, Propane C₃H₈, Butane C₄H₁₀ |
| Alkenes | CₙH₂ₙ | Double C=C | Ethene C₂H₄ (ethylene), Propene C₃H₆ |
| Alkynes | CₙH₂ₙ₋₂ | Triple C≡C | Ethyne C₂H₂ (acetylene) |
Members of one series with same general formula differ by -CH₂- (homologous series). They share chemical character.
Important Compounds
- Methane (CH₄): Main component of natural gas, biogas (~50-70%). Greenhouse gas; marsh gas. Lab — CH₃COONa + NaOH.
- Ethanol (C₂H₅OH): Beverage alcohol, sanitiser, solvent, fuel additive (E10, E20). Made by fermentation of sugar.
- Acetic acid (CH₃COOH): Vinegar (5-8%). Glacial acetic acid is pure (mp 17 °C).
- Acetylene (C₂H₂): Oxyacetylene torch (~3300 °C); used to ripen fruits artificially (calcium carbide trick is unsafe and banned).
- Methanol (CH₃OH): Wood spirit; highly toxic.
Polymers
| Polymer | Monomer | Type | Use |
|---|---|---|---|
| Polyethene (PE) | Ethene | Addition | Polythene bags, bottles |
| Polypropene (PP) | Propene | Addition | Plastic chairs, ropes, packaging |
| PVC | Vinyl chloride | Addition | Pipes, cables, raincoats |
| Polystyrene | Styrene | Addition | Disposable cups, packaging foam |
| Teflon (PTFE) | Tetrafluoroethene | Addition | Non-stick cookware |
| Nylon | Diamine + diacid | Condensation | Ropes, fabrics, brushes, parachutes |
| Polyester (Terylene) | Ethylene glycol + terephthalic acid | Condensation | Clothes, PET bottles |
| Bakelite | Phenol + formaldehyde | Thermoset | Electric switches, handles |
Natural polymers — cellulose (plant cell wall), starch (food storage), proteins, natural rubber (isoprene), DNA.
CDS/OTA PYQ Examples
Q: Main component of natural gas is:
(a) Methane (b) Ethane (c) Butane (d) Propane
Answer: (a) Methane (CH₄).
Q: PVC monomer is:
(a) Ethene (b) Vinyl chloride (c) Styrene (d) Tetrafluoroethene
Answer: (b) Vinyl chloride.
Q: Vinegar contains:
(a) Acetic acid (b) Citric acid (c) Lactic acid (d) Formic acid
Answer: (a) Acetic acid (5-8%).
Q: Non-stick coating on cookware is:
(a) PVC (b) PE (c) Teflon (PTFE) (d) Nylon
Answer: (c) Teflon.
Q: Nylon is a:
(a) Natural fibre (b) Synthetic addition polymer (c) Synthetic condensation polymer (d) Metallic compound
Answer: (c) Condensation polymer of diamine and diacid.
Drill Hydrocarbons and Polymers for CDS/OTA
CDS/OTA-pattern items on Hydrocarbons and Polymers with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why are alkanes called saturated hydrocarbons?
All C-C bonds are single; each C has the maximum (saturated) number of H atoms it can hold. No more H can be added.
What is the difference between addition and condensation polymers?
Addition — monomers join without losing any atoms (PE, PVC). Condensation — small molecules (usually water) are lost as monomers join (nylon, polyester).
Why is plastic an environmental problem?
Most synthetic polymers are non-biodegradable; they persist for centuries, choke landfills and oceans, and break down into microplastics that enter food chains.