Alkynes and Their Synthesis

Alkynes

• Hydrocarbons containing a triple bond
• General formula is CnH2n-2
• Named using IUPAC nomenclature, ending in '-yne'
• Examples:
  • Ethyne or acetylene (HCCH)
  • 1-Penten-4-yne (CHCCH2CH2CH3)
  • 3-Penten-1-yne (CHCCHCH2CH3)
  • 4,4-Dimethyl-1-pentyne (H3CCCH2CCHC(CH3)3)

Synthesis of Alkynes

• From other alkynes with fewer carbons: Can be achieved by adding a carbon to an existing alkyne.

• By elimination reactions:

  • Double dehydrohalogenation: This involves removing two hydrogen halides (HX) from a dihaloalkane.
    • Vicinal dihalides: These are dihalides where the halogens are on adjacent carbons.
      • Example: 1,2-dibromoethane (CH2BrCH2Br) can be converted to ethyne (HCCH) using alcoholic KOH or Na/liq NH3.
    • Geminal dihalides: These are dihalides where the halogens are on the same carbon.
      • Example: ethylidene chloride (CH3CHCl2) can be converted to ethyne (HCCH) using 2 Na/liq NH3.
  • Double dehalogenation of tetrahalogenated compounds:
    • Using Zn/acetic acid or NaI/acetone to remove four halogens.

Reactions of Alkynes

• Hydrogenation:

  • Addition of hydrogen (H2) to the triple bond.
  • Can be controlled to produce different products:
    • Lindlar's catalyst (Pd/CaCO3): This catalyst produces a syn addition, resulting in a cis (Z)-alkene.
    • Pd catalyst: This catalyst produces an anti addition, resulting in a trans (E)-alkene.
    • Na/NH3: This catalyst produces an anti addition, resulting in a trans (E)-alkene.

• Halogenation:

  • Addition of halogens (like Br2) to the triple bond.
  • Occurs in two steps:
    • First step: addition of one halogen molecule to form a trans dihaloalkene.
    • Second step: addition of another halogen molecule to form a tetrahaloalkane.
  • This type of reaction is useful for adding bromine to molecules, but the reaction can also be used to create other dihaloalkanes.

• Reaction with Sodium Amide (NaNH2):

  • Forms sodium acetylide (HC≡CNa)
  • Sodium acetylide is a strong nucleophile and can be used to alkylate alkynes
  • Sodium acetylide can react with primary alkyl halides to form new alkynes with longer carbon chains.
  • This is a useful reaction for creating alkynes with specific structures.

Substitution Reactions of Alkenes

• When alkenes react with Br2 at high temperatures (around 500°C), a substitution reaction occurs at the α carbon via a free radical mechanism.
  • This reaction results in the replacement of a hydrogen atom on the α carbon with a bromine atom.
• However, if the reaction is carried out at room temperature in non-polar solvents like CCl4, the bromine adds to the double bond via an ionic mechanism.
  • This reaction results in the formation of a dibromoalkane.
• The reaction of alkenes with Br2 is temperature dependent, leading to different products depending on the conditions.