Alkynes: Structure, Bonding, and Nomenclature

Alkynes are hydrocarbons containing a carbon-carbon triple bond, characterized by linear geometry and sp hybridization. They undergo addition reactions (similar to alkenes) and possess unique acidic properties at terminal positions, allowing for substitution reactions not seen in alkanes or alkenes. Key exam topics include preparation via elimination, partial hydrogenation to cis-alkenes using the Lindlar catalyst, Markovnikov hydration to form ketones, and the use of terminal alkyne acidity to form explosive acetylides for identification.

Thematic Breakdown

1. Structure & Bonding (⚠️ High-Yield)

• Triple bond composition: 1 σ bond + 2 π bonds.

• Hybridization: sp (50% s-character).

• Geometry: Linear, bond angle = 180°.

• Electronegativity: sp carbon > sp² > sp³ due to higher s-character.

• Molecular formula: CₙH₂ₙ₋₂ (four fewer H than alkanes → 2 degrees of unsaturation).

Common Pitfall: Students often forget that the triple bond contributes TWO π bonds. Exam questions may ask how many π bonds are in a molecule like 2-butyne (answer: 2 π bonds, 1 σ bond).

2. Classification & Isomerism

• Isomerism begins at C₄H₆ (butyne).

• Types: Chain, positional, and functional isomerism with dienes (e.g., C₄H₆ = 1-butyne or 1,3-butadiene).

3. Physical Properties

• Low m.p./b.p. (nonpolar, only London forces).

• m.p./b.p. increase with molecular weight.

• Insoluble in water; soluble in organic solvents (like alkanes/alkenes).

4. Preparation of Alkynes (⚠️ High-Yield)

A. From Calcium Carbide (Industrial, now outdated)

CaC2+2H2O→Ca(OH)2+HC≡CHCaC2​+2H2​O→Ca(OH)2​+HC≡CH

B. From Methane Pyrolysis (Modern)

2CH4→1500∘C,0.1secHC≡CH+3H22CH4​1500∘C,0.1sec​HC≡CH+3H2​

C. Double Dehydrohalogenation (Exam Favorite)

Vicinal or geminal dihalide → two successive E2 eliminations using strong base (e.g., NaNH₂, KOH).

Example:

CH3CHBrCH2Br→1. KOHCH3C≡CHCH3​CHBrCH2​Br1. KOH​CH3​C≡CH

Common Pitfall: Two eliminations are required. Many students stop at the alkene intermediate. The reaction requires 2 equivalents of strong base.

5. Reactions of Alkynes (⚠️ High-Yield)

a) Addition Reactions (General)

Alkynes add two equivalents of H₂, X₂, or HX (Markovnikov).

b) Partial Hydrogenation → cis-Alkene

• Lindlar catalyst (Pd/CaCO₃ + Pb acetate + quinoline) → syn addition → cis alkene.

• Na/NH₃(l) → trans alkene.

Rule of Three:

• Definition: Catalyst poisoned to stop at alkene stage.

• Example: 2-Butyne + Lindlar Pd → cis-2-butene.

• Pitfall: Using normal Pt/Pd gives alkane (over-reduction).

c) Addition of HX (Markovnikov)

CH3C≡CH+HCl→CH3CCl=CH2→+HClCH3CCl2CH3CH3​C≡CH+HCl→CH3​CCl=CH2​+HCl​CH3​CCl2​CH3​

• First equivalent → alkene (Markovnikov).

• Second equivalent → geminal dihalide.

d) Hydration (⚠️ Very High-Yield for Exams)

• Reagents: HgSO₄, H₂SO₄, H₂O.

• Product: Ketone (not enol – tautomerizes).

• Mechanism: Enol intermediate → keto tautomer.

Example:
Propyne → Acetone (CH₃COCH₃) — not propanal.

Mnemonic: "Hydration of HC≡CH makes H3C–CHO (acetaldehyde) – but internal alkynes give ketones."

e) Oxidation (Ozonolysis)

• Cleaves triple bond → carboxylic acids (terminal) or two carboxylic acids (internal).

Example: CH₃(CH₂)₃C≡CH + O₃ then H₂O → CH₃(CH₂)₃COOH + HCOOH (or CO₂).

6. Acidity of Terminal Alkynes (⚠️ Unique to Alkynes)

• Reason: sp-hybridized C–H bond is relatively acidic (pKₐ ≈ 25).

• Strong base required to deprotonate (e.g., NaNH₂, LiNH₂, not NaOH).

• Reaction with metals:

  2RC≡CH+2Na→2RC≡C−Na++H22RC≡CH+2Na→2RC≡C−Na++H2​

Rule of Three:

• Definition: Terminal alkynes act as weak acids (stronger than NH₃, weaker than H₂O).

• Example: Propyne + NaNH₂ → sodium propynide + NH₃.

• Pitfall: Students try to use NaOH – No reaction because H₂O is a stronger acid than terminal alkyne.

Identification Tests (⚠️ Exam Classic)

• Ammoniacal AgNO₃ → white precipitate (silver acetylide).

• Ammoniacal Cu₂Cl₂ → red precipitate (copper acetylide).

Mnemonic: "Ag = Alkyne (white), Cu = Copper-red."

DANGER: Acetylides are explosive when dry – exams test this safety fact.

Quick-Reference Table (⚠️ Memorize)

Self-Check Questions

1. Why is the terminal hydrogen of propyne more acidic than the hydrogens in propene or propane?
   → *sp hybridization gives 50% s-character, increasing electronegativity and stabilizing the conjugate base.*

2. A student reacts 2-butyne with H₂/Lindlar Pd. What is the product, and what stereochemistry results?
   → *cis-2-butene (syn addition).*

3. What would you observe if you bubbled 1-butyne through ammoniacal AgNO₃ solution? Why doesn’t 2-butyne react the same way?
   → *White precipitate (silver acetylide). 2-butyne is internal – no acidic H.*

4. Complete the reaction: CH₃C≡CH + excess HCl → ?
   → *CH₃CCl₂CH₃ (2,2-dichloropropane) via Markovnikov addition of two equivalents.*

5. What is the molecular formula of an alkyne with 7 carbons? How many degrees of unsaturation does it have?
   → *C₇H₁₂ (2 degrees from triple bond + any rings).*