Complete Guide to Alkanes: Synthesis & Properties
Alkanes, the simplest organic compounds, serve as the framework for all organic molecules. This master guide covers their preparation methods, physical trends, and chemical reactivity for JEE, NEET, and CBSE aspirants.
1. Preparation of Alkanes
Synthesizing alkanes involves several key methods like catalytic hydrogenation of unsaturated hydrocarbons, reduction of alkyl halides, and specialized carbon-coupling reactions like the Wurtz reaction.
Key Synthesis Highlights
- Wurtz Reaction: Coupling of alkyl halides using Sodium in dry ether (best for symmetrical alkanes).
- Decarboxylation: Removal of CO₂ from carboxylic acid salts using Sodalime (NaOH + CaO).
- Kolbe’s Electrolysis: Electrolytic method to prepare even-numbered alkanes at the anode.
2. Physical Properties of Alkanes
Understanding physical trends is crucial for predicting boiling points and solubility, which are frequently asked in competitive exams.
High-Yield Trends
- Boiling Point: Increases with molecular mass; Decreases with branching (due to decreased surface area).
- Solubility: Alkanes are non-polar; soluble in organic solvents, insoluble in water ("Like dissolves like").
- Density: Increases with size but always remains less than water (< 1.0 g/cm³).
3. Chemical Properties of Alkanes
Alkanes are paraffinic (low affinity) but undergo high-energy transformations like free radical substitution and combustion.
Mechanism Focus
- Halogenation: Follows a Free Radical Mechanism. Reactivity: F₂ > Cl₂ > Br₂ > I₂.
- Selectivity: Tertiary hydrogens are most easily replaced (3° > 2° > 1°) due to radical stability.
- Aromatization: Conversion of n-Alkanes (C₆+) into Benzene derivatives using Cr₂O₃/V₂O₅ at high temps.
Complete revision of alkanes done, best for exam preparation
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