Hydrocarbons
Alkanes, Alkenes & Alkynes
Target Audience: KEAM & IIT‑JEE aspirants
Format: Markdown – systematic, exam‑oriented, with derivations, key formulae, pitfalls & a hard practice problem.
1. Overview
| Class | General Formula | Bonding | Reactivity | Key Characteristics |
|---|---|---|---|---|
| Alkanes | (C_nH_{2n+2}) | σ‑only (single bonds) | Radical substitution (halogenation) | Saturated, non‑polar, low boiling points |
| Alkenes | (C_nH_{2n}) | σ + π (double bond) | Addition (halogenation, hydrogenation, hydration, polymerisation) | Unsaturated, planar, cis‑/trans isomerism |
| Alkynes | (C_nH_{2n-2}) | σ + 2π (triple bond) | Addition (halogenation, hydrogenation, hydration) | Highly unsaturated, linear, E/Z not applicable |
Rule of thumb: For a saturated hydrocarbon,
Catoms double the number ofHatoms plus two. A double bond removes two H atoms, a triple bond removes four.
2. Systematic Nomenclature (IUPAC)
- Longest Carbon Chain → Parent hydrocarbon
- Numbering → Lowest set of locants – for double/triple bonds the lowest possible numbers are preferred.
- Substituents → Prefixes (methyl, ethyl, etc.) with locants.
- Alkenes/Alkynes
- Prefix “-ene” for double, “-yne” for triple.
- If both present → use “-diyne”, “-diene”, etc.
- Stereochemistry
- Alkenes: cis/trans or E/Z (based on Cahn‑Ingold‑Prelog).
- Alkynes: N/A (linear symmetry).
Tricky point: When a