Alkenes & Alkynes – IIT‑JEE (Main + Advanced) Study Notes
Goal of the notes – Give you a quick‑reference that you can skim before an exam and also a deep dive for revision.
Use the worked example to see how the theory translates into a solution.
The revision summary is a bullet‑point “cheat sheet” at the end.
1. Simple Explanation
| Feature | Alkenes | Alkynes |
|---|---|---|
| General formula | (R_2C=CR_2) (C=C) | (R-C\equiv C-R) (C≡C) |
| Hybridisation | sp(^2) (planar, 120°) | sp (linear, 180°) |
| Bond strength | ~ 612 kJ mol(^{-1}) | ~ 839 kJ mol(^{-1}) |
| Reactivity (addition) | High (electrophilic addition) | Slightly lower (but still reactive) |
| Stereochemistry | cis/trans ↔ E/Z | No cis/trans; but E/Z for internal alkynes (substituent ordering) |
| Common uses | Polyethylene, polystyrene, many intermediates | Polymers (acetylene polymer), acetylene in welding, alkyne chemistry in organic synthesis (Sonogashira, Glaser, etc.) |
Key point – The presence of a C=C or C≡C bond means the molecule is unsaturated and can undergo addition reactions that break the multiple bond and form new single bonds.
2. Key Points (Bullet‑Form)
2.1 Nomenclature
| Rule | Alkene | Alkyne |
|---|---|---|
| Longest chain | Contains the double bond | Contains the triple bond |
| Suffix | –ene | –yne |
| Numbering | Start from end giving lowest locant to double bond | Same, lowest locant to triple bond |
| Substituents | Listed alphabetically, with locants, separated by commas | Same |
| E/Z | For internal alkenes with 2 different substituents on each C | For internal alkynes (rare) |
| Cis/trans | For simple alkenes (CH(_2) groups on same side) | Not applicable |
2.2 Classification
| Alkene | Alkane | Alkyne |
|---|---|---|
| Terminal – one H attached to sp(^2) C | Terminal – one H attached to sp C | |
| Internal – two alkyl groups attached to each sp(^2) C | Internal – two alkyl groups attached to each sp C | |
| Conjugated – alternating double bonds (1,3‑butadiene, etc.) |
2.3 Synthesis (Common Routes)
| From | Alken