Preparation and Reactions of Alkynes

Preparation of Alkynes

• Alkynes are prepared using different methods.
• Method 1: Ethanolic KOH reacts with vicinal dihalides (ethylene bromide). This reaction proceeds in two steps, removing HBr in each step.
• Step 1: CH₂BrCH₂Br + KOH/Ethanol → CH₂=CHBr + KBr + H₂O
• Step 2: CH₂=CHBr + KOH/Ethanol → CH≡CH + KBr + H₂O
• Method 2: KOH/EtOH reacts with geminal dihalides (ethylidene chloride).
• Reaction: CH₃CHCl₂ + KOH/EtOH → CH₂=CHCl
• Method 3: Sodamide in liquid ammonia is used for preparing alkynes.
• Example: CH₃CHBrCHBrCH₃ + 2NaNH₂ → HC≡CH + 2NaBr + 2NH₃

Reactions of Terminal Alkynes

• Acidity of terminal alkynes: Terminal alkynes react with NaNH₂ in liquid ammonia to form sodium alkynides.
• Reaction: R-C≡CH + NaNH₂/liq. NH₃ → R-C≡C⁻Na⁺ + NH₃
• Example: Ethane-1-yne reacting with NaNH₂ and liquid ammonia. This forms sodium propynide.
• Reaction: H₂C≡C⁻Na⁺ + CH₃CH₂Br → HC≡CCH₂CH₃ + NaBr
• Metal Derivatives: Acetylene and terminal alkynes form metal derivatives with silver and copper in ammonical solutions.
• Example: CH₃CH₂C≡CH + AgNO₃/NH₄OH → CH₃CH₂C≡CAg + NH₄NO₃ + H₂O
• Example: 2CH₃C≡CH + Cu₂Cl₂ + 2NH₄OH → (CH₃C≡C)₂Cu + 2NH₄Cl + 2H₂O

Addition of H₂/Pt

• Alkynes can be reduced to alkanes using H₂ and a platinum catalyst.
• Excess or 1 mole of Hydrogen affects the reaction outcome.
• This reaction helps differentiate between terminal and non-terminal alkynes (no reaction for non-terminal).

Addition of Cl₂ or Br₂

• Alkynes react with Cl₂ or Br₂/CCl₄ adding to the double bond.
• The reaction produces dibromo and tetrachloro alkanes.
• The product varies depending on the reaction conditions, and concentration of reacting compounds.

Addition of HBr or HCl

• Alkynes react with HBr or HCl.
• The reaction produces vinyl or ethylidene bromide.
• This addition follows Markovnikov's rule.

Conjugated Dienes

• Alkenes with more than one double bond are called dienes, trienes, or tetraenes.
• Classes of dienes include cumulated, isolated, and conjugated.

Conjugation

• Conjugation describes alternating single and double bonds in a molecule. This creates greater stability than isolated dienes.

Preparation of Alkadienes

• 1,3-Butadiene is prepared from butane-1,4-diol using H₃PO₄ at 270°C.

Reactions of Conjugated Dienes

• Conjugated dienes show abnormal addition reactions.
• They undergo 1,4-addition at high temperature and 1,2-addition at low temperature.

Stability of Conjugated Dienes

• Conjugated dienes are more stable than isolated dienes due to allylic cation resonance stabilization.

Diels-Alder Reaction

• A reaction between a diene (containing two double bonds) and a dienophile (containing a double bond) to form a ring structure.
• This cycloaddition reaction is favoured by electron-withdrawing substituents in the dienophile.

Examples of Dienophiles

• Maleic anhydride, benzoquinone, and acrylonitrile are common dienophiles.

Explore the various methods for preparing alkynes, including the use of ethanolic KOH and sodamide in liquid ammonia. Understand the reactions of terminal alkynes and their acidity, along with the formation of sodium alkynides. Test your knowledge on these essential concepts in organic chemistry.