CHEMCA
EXAM MASTER REVIEW SHEET
Hydrocarbons: Alkynes
1. Structure & Acidic Nature
Unsaturated hydrocarbons with $C \equiv C$ (Hybridization: $sp$, Bond Angle: $180^\circ$).
Terminal alkynes are acidic due to high s-character (50%) of $sp$ carbon.
$HC \equiv CH > CH_2=CH_2 > CH_3-CH_3$
Only terminal alkynes ($R-C \equiv C-H$) react with $NaNH_2$ or Ammoniacal $AgNO_3$.
Important for identification tests!
2. Methods of Preparation
From Calcium Carbide
$CaC_2 + 2H_2O \rightarrow Ca(OH)_2 + C_2H_2$
• Industrial method for Ethyne.
Dehydrohalogenation
From Vicinal/Geminal Dihalides.
1. Alc. $KOH$ (removes first $HX$).
2. $NaNH_2$ (stronger base required for second $HX$).
3. Addition Reactions
• Lindlar's Catalyst ($Pd/BaSO_4 + \text{Quinoline}$): Results in CIS-Alkene.
• Birch Reduction ($Na/\text{liq. } NH_3$): Results in TRANS-Alkene.
Reagent: $HgSO_4 / \text{dil. } H_2SO_4$.
Alkynes $\rightarrow$ Enol $\xrightarrow{\text{Tautomerize}}$ Carbonyls.
*Ethyne gives Acetaldehyde; Others give Ketones.*
4. Tests for Terminal Alkynes
| Test Reagent | Observation | Product Formed |
|---|---|---|
| Ammoniacal $AgNO_3$ (Tollens') | White Precipitate | Silver Acetylide ($R-C \equiv C-Ag$) |
| Ammoniacal $Cu_2Cl_2$ | Red Precipitate | Copper Acetylide ($R-C \equiv C-Cu$) |
5. Cyclic & Linear Polymerization
Cyclic Trimerization
3 moles of Ethyne passed through Red Hot Iron/Copper Tube at 873K.
$3 C_2H_2 \xrightarrow{Fe, \Delta} \text{Benzene } (C_6H_6)$
Linear Polymerization
Ethyne passed through $Cu_2Cl_2/NH_4Cl$ gives Vinyl Acetylene.
Starting material for Synthetic Rubber (Neoprene).
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