Alkynes: Preparation, Acidic Character & Polymerisation – Complete Guide
Alkynes are unsaturated hydrocarbons containing at least one carbon-carbon triple bond (C≡C), characterized by the general formula CnH2n−2. The simplest member is ethyne (commonly called acetylene, C2H2), widely utilized in oxy-acetylene welding torches.
1. Electronic Structure of the Triple Bond
- In ethyne (HC≡CH), both carbon atoms are sp hybridized.
- Each carbon forms one σ bond with hydrogen (sp−s overlap) and one σ bond with the adjacent carbon (sp−sp overlap), resulting in a strictly linear geometry with bond angle 180°.
- The two unhybridized 2p orbitals (2py and 2pz) on each carbon overlap mutually at right angles to form two π bonds, creating a continuous cylindrical electron cloud encasing the internuclear C−C axis.
- C≡C bond length is 120 pm (substantially shorter than alkene 134 pm and alkane 154 pm), and the bond dissociation enthalpy is an immense 823 kJ mol−1.
2. Methods of Preparation of Alkynes
A. From Calcium Carbide
Industrial manufacture involves heating quicklime with coke in an electric arc furnace, followed by hydrolysis of calcium carbide:
CaC2 + 2 H2O → Ca(OH)2 + HC≡CH (Ethyne)
B. From Vicinal Dihalides
Vicinal dihalides undergo stepwise dehydrohalogenation. The first elimination is achieved with alcoholic KOH, while the second (more difficult) elimination requires the stronger base sodamide (NaNH2):
3. Acidic Character of Terminal Alkynes
A hydrogen atom attached to a triply-bonded carbon atom in an alkyne exhibits distinctly acidic properties. This is because the carbon is sp hybridized with 50% s-character (compared to 33.3% in sp2 alkenes and 25% in sp3 alkanes).
The high s-character holds the C−H bonding electrons tightly towards the carbon nucleus, facilitating release of the terminal proton (H+).
Relative Acidity: HC≡CH > CH2=CH2 > CH3−CH3 • HC≡CH > CH3−C≡CH >> CH3−C≡C−CH3 (non-acidic).
Reactions Demonstrating Terminal Alkyne Acidity
- Reaction with Sodium metal:
2 HC≡CH + 2 Na → 2 HC≡C−Na+ (Sodium acetylide) + H2 ↑
- Reaction with Sodamide (NaNH2):
CH3−C≡CH + NaNH2 → CH3−C≡C−Na+ + NH3
- Precipitation with Tollens' Reagent (Distinction Test):
HC≡CH + 2 [Ag(NH3)2]OH → Ag−C≡C−Ag ↓ (White precipitate) + 4 NH3 + 2 H2O
- Precipitation with Ammoniacal Cuprous Chloride:
HC≡CH + 2 [Cu(NH3)2]Cl → Cu−C≡C−Cu ↓ (Red precipitate) + 2 NH4Cl + 2 NH3
JEE Diagnostic: Internal alkynes (like but-2-yne CH3−C≡C−CH3) have no terminal hydrogen and give NO precipitate with Tollens' or Cu2Cl2 reagents.
4. Addition Reactions & Kucherov's Hydration
Kucherov's Hydration Reaction (High-Yield NEET Concept)
Alkynes add water in the presence of 1% HgSO4 and 40% dilute H2SO4 at 333 K to form unstable enols that tautomerise into carbonyl compounds:
- From Ethyne: Gives Acetaldehyde (the only alkyne yielding an aldehyde):
HC≡CH + H2O → (Hg2+/H+, 333 K) → [CH2=CH−OH] ↔ CH3−CHO (Ethanal)
- From Propyne: Markovnikov addition yields Acetone:
CH3−C≡CH + H2O → (Hg2+/H+) → [CH3−C(OH)=CH2] ↔ CH3−CO−CH3 (Propanone)
5. Polymerisation Reactions
A. Linear Polymerisation
Ethyne undergoes controlled linear polymerisation under special conditions to yield polyacetylene (polyethyne) – a repeating conjugated chain [−CH=CH−CH=CH−]n. Doped polyacetylene films conduct electricity like metals, forming the basis of conducting organic polymers.
B. Cyclic Polymerisation (Aromatic Trimerisation)
When ethyne is passed through a red-hot iron or quartz tube at 873 K, three molecules polymerise cyclically to form Benzene:
Similarly, passing propyne yields 1,3,5-trimethylbenzene (Mesitylene).
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