Chemical Properties of Alkynes
Master the reactivity of the triple bond. Decode the acidic nature of terminal alkynes, Kucherov's hydration/tautomerism, and cyclic polymerization.
Module Focus: The Dual Personality
Alkynes possess a dual chemical personality. The rich $\pi$ electron cloud makes them susceptible to Electrophilic Addition (though slower than alkenes). However, their defining feature is the extremely high electronegativity of the $sp$-hybridized carbon, which makes the hydrogen attached to a terminal triple bond distinctly acidic. This allows alkynes to undergo unique substitution reactions that alkenes cannot.
1. Acidic Character of Terminal Alkynes
An $sp$-hybridized carbon has 50% s-character, making it highly electronegative. It pulls the electrons of the $C-H$ bond strongly towards itself, allowing the Hydrogen to be removed as a proton ($H^+$) by a strong base.
Terminal alkynes react with Ammoniacal Silver Nitrate ($AgNO_3 + NH_4OH$). The acidic hydrogen is replaced by Silver ($Ag$).
Terminal alkynes react with Ammoniacal Cuprous Chloride ($Cu_2Cl_2 + NH_4OH$). The acidic hydrogen is replaced by Copper ($Cu$).
These tests are exclusively used to distinguish terminal alkynes from internal alkynes and alkenes.
Example: 1-Butyne gives a white/red ppt. 2-Butyne (internal, no acidic hydrogen) gives NO reaction!
2. Electrophilic Addition Reactions
Alkynes undergo electrophilic addition similarly to alkenes. However, because the intermediate vinylic carbocation is very unstable, alkynes are generally less reactive toward electrophiles than alkenes.
- Halogenation ($X_2 / CCl_4$): Proceeds via anti-addition. Forms a tetrahalide. The reddish-brown color of $Br_2$ is discharged (Test for unsaturation).
- Hydrohalogenation ($HX$): Follows Markovnikov's rule twice, resulting in a geminal dihalide (both halogens on the same carbon).
Addition of water requires heavy metal catalysis ($Hg^{2+}$ / $H^+$) because alkynes are less reactive. It proceeds via a highly unstable Enol intermediate.
3. Polymerization
A. Linear Polymerization
Ethyne undergoes linear polymerization under suitable conditions to form polyacetylene (polyethyne). It is a high molecular weight polyene that, interestingly, can conduct electricity under special conditions.
B. Cyclic Polymerization (Aromatization)
When passed through a Red Hot Iron (or Copper) tube at 873 K, three molecules of an alkyne trimerize to form an aromatic ring.
4. Oxidative Cleavage (Ozonolysis)
Unlike alkenes (which yield aldehydes/ketones), the oxidative cleavage of alkynes completely breaks the triple bond, fully oxidizing the carbons into Carboxylic Acids.
Terminal alkynes will yield a carboxylic acid and Carbon Dioxide ($CO_2$) from the terminal carbon.
NEET Grand Test: Alkyne Reactions
15 High-Yield Questions testing tautomerism, distinction tests, and cyclic polymerization products.
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