Mastering Alkynes: Synthesis, Acidity & Reactions
Alkynes (unsaturated hydrocarbons with a triple bond) are unique due to their high s-character and nucleophilic nature. This guide simplifies their preparation and chemical properties for JEE, NEET, and CBSE Class 11/12 students.
1. Preparation of Alkynes
Alkynes are commonly synthesized through double elimination from vicinal dihalides or industrial methods involving inorganic carbides.
Key Synthesis Pathways
- Calcium Carbide Method: Reacting $CaC_2$ with $H_2O$ is the standard method for preparing Acetylene (Ethyne).
- Dehydrohalogenation: Treatment of vicinal dihalides with Alcoholic KOH followed by NaNH₂ (stronger base).
- Kolbe’s Electrolysis: Electrolyzing aqueous solutions of potassium salts of maleic or fumaric acid.
2. Physical Properties of Alkynes
Alkynes are non-polar molecules with slightly higher boiling points than corresponding alkanes and alkenes due to the more compact triple bond.
Critical Property: Acidity
- Terminal Acidity: Hydrogens attached to sp-hybridized carbons are acidic. Terminal alkynes react with Na or $NaNH_2$.
- Acidity Order: $Alkyne > Alkene > Alkane$ (due to 50% s-character in sp hybridization).
- Solubility: Insoluble in water but highly soluble in ether, benzene, and carbon tetrachloride.
3. Chemical Properties of Alkynes
Chemical behavior is dominated by electrophilic addition reactions and specific tests for terminal triple bonds.
Mechanism & Reactions
- Kucheroff's Reaction: Hydration of Alkynes using $Hg^{2+}/H_2SO_4$ to yield Carbonyls (via tautomerism).
- Cyclic Polymerization: Ethyne passed through a red-hot iron tube forms Benzene.
- Tollen’s & Fehling’s Test: Used to distinguish terminal alkynes (white/red ppt) from internal alkynes.
- Ozonolysis: Cleaves triple bonds to form Carboxylic Acids (unlike alkenes which form Aldehydes/Ketones).
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