Organic chemistry I

ALKYNES

Alkynes are a homologous series of unsaturated hydrocarbons containing at least one carbon–carbon triple bond (C≡C). They have the general formula CnH2n-2. Alkynes are more reactive than both alkanes and alkenes due to the high electron density in the triple bond.

Classification of alkynes

Name Molecular Formula Structural Formula General Properties
Ethyne C₂H₂ H–C≡C–H Colourless gas; burns with sooty flame; highly reactive.
Propyne C₃H₄ CH₃–C≡CH Flammable; undergoes addition reactions.
1-Butyne C₄H₆ CH₃–CH₂–C≡CH Terminal alkyne; forms salts with strong bases.
2-Butyne C₄H₆ CH₃–C≡C–CH₃ Internal alkyne; less acidic.
1-Pentyne C₅H₈ CH₃–CH₂–CH₂–C≡CH Undergoes hydrogenation and halogenation.
2-Pentyne C₅H₈ CH₃–CH₂–C≡C–CH₃ Less reactive than terminal alkynes.
1-Hexyne C₆H₁₀ CH₃–(CH₂)₃–C≡CH Undergoes polymerisation.
2-Hexyne C₆H₁₀ CH₃–CH₂–C≡C–CH₂–CH₃ Internal; lower acidity.
1-Heptyne C₇H₁₂ CH₃–(CH₂)₄–C≡CH Flammable, reactive.
1-Octyne C₈H₁₄ CH₃–(CH₂)₅–C≡CH Undergoes addition reactions.
General Properties of Alkynes
  1. Unsaturated hydrocarbons with a triple bond.
  2. Very reactive due to high electron density in the triple bond.
  3. Undergo addition reactions more readily than alkenes.
  4. Terminal alkynes are weakly acidic.
  5. Burn with a very smoky flame due to high carbon content.
  6. Soluble in organic solvents but insoluble in water.

ETHYNE (Acetylene)

Ethyne is the first and simplest member of the alkyne family. It is a colourless, odourless, highly flammable gas.

Structure of Ethyne
H – C ≡ C – H
Lab. Preparation of Ethyne

Ethyne is prepared by reacting calcium carbide (CaC₂) with water.

CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂
Physical Properties of Ethyne
Chemical Properties of Ethyne
  1. Combustion

    Ethyne burns with a very luminous and sooty flame because of its high carbon content. Under sufficient oxygen, it undergoes complete combustion, but in limited air, it burns incompletely. 

    C₂H₂ + O₂ → CO₂ + H₂O       (incomplete; sooty flame)
2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O   (complete combustion)
  2. Addition Reactions

    (a) Hydrogenation

    Ethyne readily adds hydrogen across its triple bond to form ethene, and on further hydrogenation forms ethane. The complete hydrogenation of ethyne in the presence of nickel as catalyst yields ethane. This is typical of unsaturated hydrocarbons. 

    C₂H₂ + H₂ → C₂H₄ → C₂H₆
    (b) Halogenation

    Ethyne reacts with halogens such as bromine to form dihalo- and tetrahalo-derivatives in the presence of iron (III) chloride as catalyst. Ethyne reacts with chlorine to form 1,2 dichloroethene. Further chlorination attacks the double bond to form 1,1,2,2 tetrachloroethane. Similarly, The complete addition of bromine with ethyne yields 1,1,2,2 tetrabromoethane. The reaction causes the decolorization of bromine water. 

    C₂H₂ + Br₂ → C₂H₂Br₂ → C₂H₂Br₄
    (c) Addition of Hydrogen Halides

    Hydrogen halides add to ethyne in two steps, forming haloalkenes at first and then dihaloalkanes. This follows Markovnikov's rule in most cases. The reaction of Ethyne with Hydrogen bromide at 100°C yields bromoethene. Further reaction yields 1,2- dibromoethane. Hence, the complete addition of ethyne with hydrogen halides produces 1,2-dihaloalkanes 

    C₂H₂ + HBr → CH₂=CHBr → CH₃–CHBr₂
    (d) Addition of Water (Hydration)

    In the presence of suitable catalysts, ethyne reacts with aqueous tetraoxosulphate (VI) acid at 60°C to produce ethanol and then ethanal. This is an important industrial method for producing aldehydes. 

    C₂H₂ + H₂O → CH₃–COH → CH₃–COH (ethanal)
    (e) Reaction with KMnO₄

    Ethyne reacts with acidified potassium permanganate, leading to oxidation and discoloration of the purple KMnO₄ solution. An alkaline solution turns green. This reaction yields ethanedioc acid or carbon(IV) oxide. This reaction is used as a test for unsaturation. 

                 
C₂H₂ + [O] → COOH-COOH
5C₂H₂ + 8KMnO₄ + 12H₂SO₄ → 10CO₂ + 8MnSO₄ + 4K₂SO₄ + 12H₂O
  3. Substitution in Terminal Alkynes

    Because ethyne has an acidic hydrogen atom at the terminal carbon, it reacts with sodium metal to form sodium ethynide and hydrogen gas. This property distinguishes terminal alkynes from alkenes and alkanes. 

    C₂H₂ + Na → C₂HNa + ½H₂
  4. Polymerisation

    Under suitable conditions, ethyne polymerizes to form polyacetylene, an important conducting polymer. This reaction involves the linking of many ethyne molecules. In the presence of organo nickel as catalyst, ethyne polymerizes to form benzene.

    3C₂H₂ → C₆H₆ (Benzene)

    nC₂H₂ → (C₂H₂)ₙ   (polyacetylene)
Uses of Ethyne
Test for Ethene and Ethyne
Test Ethene Ethyne
Reaction with Ammoniacal CuCl No reaction Forms copper acetylide (red precipitate)
Reaction with AgNO₃ (Ammoniacal) No reaction Forms silver acetylide (white precipitate)
Acidity Not acidic Terminal alkynes react with sodium metal, releasing H₂
Reaction with Bromine water Occurs more rapidly Reaction is slow

Summary