Alkynes

Full Notes

Alkynes are hydrocarbons that contain a carbon–carbon triple bond (C≡C), making them part of the unsaturated hydrocarbons family.

They are also known as acetylenes, with ethyne (C₂H₂) being the simplest member.

Nomenclature and Isomerism

Alkynes are named following IUPAC rules, ending with the suffix “-yne” to indicate the presence of a triple bond.

Straight-chain alkynes are named based on the longest carbon chain that includes the triple bond. The triple bond is assigned the lowest possible number.

Isomerism: They exhibit both position isomerism (e.g., but-1-yne and but-2-yne) and chain isomerism due to different arrangements of the carbon skeleton.

Structure of Triple Bond

The triple bond in alkynes consists of one sigma (σ) bond and two pi (π) bonds.

Orbital picture: Each carbon in the triple bond is sp hybridised.

The two sp orbitals form σ-bonds (one with hydrogen or another carbon and one with the other carbon of the triple bond).

The unhybridised p-orbitals form two π-bonds perpendicular to each other.

Linear Geometry: The bond angle between the atoms involved in the triple bond is 180°.

Preparation of Alkynes

Alkynes can be synthesised from several precursor compounds through elimination or substitution reactions.

From Calcium Carbide (CaC₂):

Industrial method for producing ethyne.

From Vicinal Dihalides:

Vicinal dihalides undergo double dehydrohalogenation.

A strong base like sodamide (NaNH₂) is used.

Properties of Alkynes

Alkynes share physical properties with other hydrocarbons but have distinct chemical properties due to their triple bond and terminal hydrogen acidity.

Physical Properties

• Non-polar and insoluble in water.
• Soluble in organic solvents.
• Boiling points increase with molecular mass.
• Slightly higher boiling points than alkenes and alkanes due to stronger dispersion forces.

Acidic Character of Alkynes

Terminal alkynes (with a hydrogen attached to the triple bonded carbon) are weakly acidic. They can react with sodium metal to form sodium acetylide:

This property distinguishes terminal alkynes from internal alkynes.

Addition Reactions

Triple bonds undergo electrophilic addition reactions similar to alkenes but in two steps (to go from triple → double → single bond):

Addition of Dihydrogen (H₂)

Catalyst: Pt/Pd/Ni
Reaction: Alkynes are hydrogenated stepwise.
One mole of H₂ → alkene (cis if Lindlar’s catalyst is used).
Two moles of H₂ → alkane.

HC≡CH + H₂ → CH₂=CH₂ → CH₃–CH₃ (Ethyne to ethene to ethane)

Addition of Halogens (X₂)

Reagent: Br₂, Cl₂
Medium: Inert (e.g., CCl₄)
Results in dihalo- and tetrahalo-alkanes.
Used as a test for unsaturation (Br₂/CCl₄ decolourisation).

HC≡CH + 2Br₂ → CHBr₂–CHBr₂ (1,1,2,2-tetrabromoethane)

Addition of Hydrogen Halides (HX)

Two molecules of HCl, HBr, or HI can add across triple bonds. Markovnikov rule applies – halogen bonds to the more substituted carbon.
One molecule gives vinyl halide and second gives gem-dihalide.

H–C≡C–H + HBr → CH₂=CHBr → CHBr₂–CH₃ (Forms 1,1-dibromoethane)

Addition of Water (Hydration)

Reagent: H₂O with Hg²⁺/H⁺ catalyst at 333 K
Markovnikov addition of –OH. Initially forms an enol, which rearranges to a ketone or aldehyde (keto–enol tautomerism).

HC≡CH + H₂O → CH₂=CHOH → CH₃–CHO (ethanal)

Polymerisation Reactions

Alkynes can undergo addition polymerisation.

Linear Polymerisation of Ethyne (Acetylene):

Under suitable conditions (heat, pressure, catalysts), ethyne polymerises to form polyacetylene (or polyethyne), a high molecular weight polymer containing repeating units of (–CH=CH–CH=CH–).

It can conduct electricity under specific conditions due to a conjugated π-bond system.

Cyclic Polymerisation of Ethyne

Process: When ethyne (acetylene) is passed through a red hot iron tube at 873 K, a cyclic polymerisation reaction takes place.
Result: Three molecules of ethyne combine to form benzene (C₆H₆).

Reaction: 3 CH≡CH —(873 K, red hot Fe tube)→ C₆H₆ (benzene)

This is a key method to convert aliphatic compounds into aromatic compounds.

Summary

• Alkynes contain a C≡C triple bond with sp hybridisation and linear geometry.
• IUPAC names use the suffix -yne and the triple bond gets the lowest possible number.
• Common preparations include from CaC₂ and via double dehydrohalogenation of vicinal dihalides.
• Terminal alkynes are weakly acidic and form metal acetylides.
• Typical reactions are electrophilic additions of H₂, X₂, HX and hydration leading to carbonyl compounds.
• Alkynes undergo polymerisation including cyclic trimerisation to benzene.