Complete Guide to Alkenes: Synthesis & Properties
Alkenes, characterized by the carbon-carbon double bond (C=C), are highly reactive and serve as essential precursors in organic synthesis. This master guide covers their preparation, physical properties, and chemical behavior for JEE, NEET, and Board exams.
1. Preparation of Alkenes
The synthesis of alkenes primarily involves elimination reactions such as dehydration of alcohols and dehydrohalogenation of alkyl halides.
High-Yield Synthesis Points
- Saytzeff’s Rule: In elimination reactions, the more substituted alkene (most stable) is the major product.
- Lindlar’s Catalyst: Pd/CaCO₃ poisoned with PbO converts Alkynes to Cis-Alkenes.
- Birch Reduction: Na/Li in liquid NH₃ converts Alkynes to Trans-Alkenes.
- Dehydration: Order of ease for alcohols: 3° > 2° > 1°.
2. Physical Properties of Alkenes
Alkenes exhibit geometric isomerism (Cis-Trans), which significantly influences their physical constants like boiling points and dipole moments.
Cis vs Trans Trends
- Boiling Point: Cis-isomers usually have higher boiling points due to higher polarity/dipole moments.
- Melting Point: Trans-isomers usually have higher melting points due to better packing in the crystal lattice.
- Solubility: Like alkanes, they are insoluble in water but soluble in organic solvents.
3. Chemical Properties of Alkenes
The C=C bond makes alkenes highly susceptible to **Electrophilic Addition Reactions**. This is the most crucial section for competitive exams.
Must-Know Addition Rules
- Markovnikov’s Rule: The electrophile (usually H⁺) adds to the carbon with more hydrogen atoms.
- Kharasch Effect (Anti-Markovnikov): HBr addition in the presence of peroxides.
- Ozonolysis: Oxidative cleavage of C=C to identify the position of the double bond.
- Hydroboration-Oxidation: Overall syn-addition of water in an anti-Markovnikov fashion.
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