Alkynes in Organic Chemistry

Alkynes are hydrocarbons containing at least one carbon–carbon triple bond (C≡C). They are unsaturated and belong to the homologous series with general formula C_nH_2n−2. The simplest alkyne is ethyne (acetylene, C₂H₂).Contents

Nomenclature

• Common names: Based on acetylene. Example: propyne ≈ methylacetylene.
• IUPAC names: Replace “-ane” with “-yne” in the parent hydrocarbon name.
• Numbering: Number from the end nearer to the triple bond to give it the lowest locant.
• Functional priority: When multiple functionalities exist, apply IUPAC priority rules; the triple bond takes a position number, e.g., but-2-yne.

Structure	IUPAC Name	Common Name
HC≡CH	ethyne	acetylene
CH3–C≡CH	prop-1-yne	methylacetylene (propyne)
CH3–C≡C–CH3	but-2-yne	dimethylacetylene

Structure & Hybridization

Each carbon of the C≡C bond is sp-hybridized (50% s-character), producing a linear geometry (180°). The triple bond comprises one σ bond and two mutually perpendicular π bonds.

Concept tip: More s-character → electrons are held closer to the nucleus → higher acidity of the attached hydrogen (see terminal alkynes below).

Physical Properties

• Generally non-polar; insoluble in water but soluble in non-polar solvents.
• Lower members (C₂–C₄) are gases; higher members are liquids/solids.
• Burn with a sooty flame due to higher carbon content.

Methods of Preparation

1) Dehydrohalogenation of Dihalides

Vicinal or geminal dihalides give alkynes upon double dehydrohalogenation using alcoholic KOH (first elimination) followed by a strong base like NaNH₂ (second elimination).

E.g.  CH3–CHBr–CH2Br  --(alc. KOH, then NaNH2)-->  CH3–C≡CH  +  2HBr

2) Hydrolysis of Calcium Carbide

CaC2  +  2H2O  →  C2H2  +  Ca(OH)2

Industrial route to ethyne (acetylene).

Chemical Reactions

Addition Reactions

• Hydrogenation:
  • Partial (Lindlar’s catalyst) → cis-alkene.
  • Dissolving metal reduction (Na/NH₃ (l)) → trans-alkene.
  • Complete (excess H₂, Pd/C) → alkane.
• Halogenation (X₂): One equivalent → dihaloalkene; excess → tetrahaloalkane.
• Hydrohalogenation (HX): Markovnikov addition; excess HX leads to gem-dihalides.
• Hydration (H₂SO₄/HgSO₄): Gives an enol that tautomerizes to a ketone (aldehyde for some terminal cases via anti-Markovnikov with hydroboration–oxidation).
• Ozonolysis: Cleaves the triple bond to yield carboxylic acids (and/or CO₂ if terminal).

Acidity of Terminal Alkynes

Terminal alkynes (RC≡C–H) are weakly acidic. Strong bases (e.g., NaNH₂, BuLi) deprotonate them to form acetylide anions, which undergo nucleophilic substitution with primary alkyl halides to extend carbon chains.

RC≡C–H  +  NaNH2  →  RC≡C:⁻ Na⁺  +  NH3

RC≡C:⁻  +  R'–X  →  RC≡C–R'  +  X⁻

Uses of Alkynes

• Ethyne + O₂ in oxy-acetylene torch → high-temperature flame for welding/cutting.
• Feedstock for solvents, plastics, synthetic fibers, and fine chemicals.

Worked Example: Addition of HCl to Ethyne

HC≡CH  +  HCl  →  CH2=CHCl   (1 eq.)
CH2=CHCl  +  HCl  →  CH3–CHCl2 (excess) 

Explanation: Stepwise electrophilic addition across the π bonds; Markovnikov orientation in each step forms first vinyl chloride then gem-dichloride.

Practice: MCQ, Fill-ups & Mini-Quiz

MCQ (with answers & explanations)

1. The general formula of acyclic alkynes is:
   1. C_nH_2n+2
   2. C_nH_2n
   3. C_nH_2n−2
   4. C_nH_2n−6
   Answer & whyOption (c). Each triple bond reduces two hydrogens compared to alkanes.
2. The hybridization of carbon atoms in a C≡C bond is:
   1. sp³
   2. sp²
   3. sp
   4. none
   Answer & whyOption (c). One σ + two π bonds require linear sp-hybridized carbons.
3. Selective formation of a cis-alkene from an alkyne uses:
   1. Na/NH₃ (l)
   2. H₂/Lindlar’s catalyst
   3. HBr (excess)
   4. O₃/H₂O
   Answer & whyOption (b). Lindlar’s catalyst gives syn-addition → cis-alkene.
4. Terminal alkynes are more acidic than alkenes because:
   1. They are more polar
   2. sp carbanion has higher s-character
   3. They form stronger σ bonds
   4. Resonance stabilization
   Answer & whyOption (b). Greater s-character stabilizes negative charge closer to the nucleus.
5. Hydroboration–oxidation of a terminal alkyne generally gives:
   1. Gem-dihalide
   2. Aldehyde (via anti-Markovnikov hydration)
   3. Ketone (Markovnikov)
   4. Carboxylic acid directly
   Answer & whyOption (b). BH₃ addition followed by H₂O₂/OH⁻ gives an enol → aldehyde.

Fill in the Blanks (answers in [ ])

1. In alkynes, each C of the triple bond is [sp] hybridized.
2. The flame of burning alkynes is typically [sooty] due to high carbon content.
3. Deprotonation of RC≡C–H with NaNH₂ gives an [acetylide anion].
4. Hydration of a terminal alkyne with H₂SO₄/HgSO₄ yields a vinyl alcohol that tautomerizes to a [ketone].
5. Calcium carbide on hydrolysis produces [ethyne].

Mini-Quiz (3 quick questions)

1. T/F: Dissolving metal reduction of an alkyne gives a cis-alkene. Show answerFalse. Na/NH₃ (l) gives a trans-alkene.
2. 1-liner: Name the reagent set that converts 1-butyne → 1,1-dichlorobutane. Show answerExcess HCl (hydrohalogenation, Markovnikov on the formed alkene gives gem-dichloride).
3. Predict: Ozonolysis of 2-butyne gives _____. Show answerTwo equivalents of acetic acid (CH₃COOH).

FAQs

What is the general formula of alkynes?

C_nH_2n−2 for acyclic members (n ≥ 2). Why are terminal alkynes acidic?

The conjugate base is stabilized by the high s-character of sp hybrid orbitals holding negative charge closer to the nucleus. How can I differentiate an alkyne from an alkene in the lab?

Alkynes typically decolorize bromine in CCl₄; terminal alkynes form precipitates with ammoniacal AgNO₃ (silver acetylides). How to get a trans-alkene selectively from an alkyne?

Use dissolving metal conditions (Na/NH₃ (l)) for anti-addition.

Internal (CHEMASH)

• Alkanes: Basics & Properties
• Alkenes: Reactions & Mechanisms
• Reaction Mechanisms (Guide)

External (Authoritative)

• NIST Chemistry WebBook
• Organic-Chemistry Portal

Quick Revision

• Formula: C_nH_2n−2
• Hybridization: sp (linear, 180°)
• Key Preps: double dehydrohalogenation; CaC₂ hydrolysis
• Selective Partial Hydrogenation: Lindlar (cis), Na/NH₃ (trans)
• Terminal Alkyne: weakly acidic → acetylide formation → C–C bond formation