Lesson 10: Alkynes - H2/ Lindlar Catalyst, Synthesis Problems

Lecture 10 Alkynes - H2/ Lindlar Catalyst, Synthesis Problems

1. Hydrogenation of Alkynes

A. Hydrogenation with H2/Pd or Pt (Full Reduction to Alkanes)

• Mechanism:

  • Step 1: Adsorption: The alkyne and hydrogen molecules adsorb onto the surface of the palladium or platinum catalyst.

  • Step 2: Addition: Hydrogen atoms add to the carbons of the triple bond in a syn addition manner, reducing the alkyne to an alkene.

  • Step 3: Further Hydrogenation: Another set of hydrogen atoms adds to the carbons of the double bond, fully reducing the alkyne to an alkane.

• Example:

  • Ethyne (acetylene) reacts with H2 in the presence of a Pd/C catalyst to form ethane.

B. Partial Hydrogenation with H2/Lindlar's Catalyst (Reduction to Cis-Alkenes)

• Mechanism:

  • Step 1: Adsorption: The alkyne and hydrogen molecules adsorb onto the surface of Lindlar's catalyst (Pd poisoned with lead acetate and quinoline).

  • Step 2: Addition: Hydrogen atoms add to the carbons of the triple bond in a syn addition manner, reducing the alkyne to a cis-alkene.

  • Step 3: Termination: The reaction stops at the alkene stage due to the poisoned catalyst preventing further hydrogenation.

• Example:

  • Ethyne (acetylene) reacts with H2 in the presence of Lindlar's catalyst to form cis-ethylene.

2. Hydroboration-Oxidation of Alkynes

• Reagents: 1) BH3 or disiamylborane, 2) H2O2, NaOH

• Mechanism:

  • Step 1: Hydroboration: BH3 or disiamylborane adds to the alkyne in a syn manner, forming a vinylic borane intermediate.

  • Step 2: Oxidation: The vinylic borane is oxidized by hydrogen peroxide in a basic medium to form an enol intermediate.

  • Step 3: Tautomerization: The enol tautomerizes to form a carbonyl compound (aldehyde for terminal alkynes, ketone for internal alkynes).

• Example:

  • Terminal alkyne (e.g., 1-hexyne) reacts with disiamylborane followed by H2O2/NaOH to form hexanal.

3. Ozonolysis of Alkynes

• Reagents: O3, followed by H2O or Zn/AcOH

• Mechanism:

  • Step 1: Formation of Ozonide: Ozone reacts with the alkyne to form an ozonide intermediate.

  • Step 2: Cleavage: The ozonide cleaves upon hydrolysis to form two carboxylic acids.

• Example:

  • 2-butyne reacts with ozone followed by hydrolysis to form two molecules of acetic acid.

4. Alkylation of Terminal Alkynes

• Reagents: NaNH2, RX (alkyl halide)

• Mechanism:

  • Step 1: Deprotonation: Sodium amide (NaNH2) deprotonates the terminal alkyne, forming an acetylide anion.

  • Step 2: Nucleophilic Substitution: The acetylide anion acts as a nucleophile, attacking the alkyl halide (RX), leading to the formation of a new carbon-carbon bond.

• Example:

  • Terminal alkyne (e.g., ethyne) reacts with NaNH2 and ethyl bromide (EtBr) to form 1-butyne.

Solving Alkyne Reaction Problems:

1. Identify the Reagents and Conditions:

   • Determine if the reaction is hydrogenation, hydroboration-oxidation, ozonolysis, or alkylation based on the given reagents and conditions.

2. Predict the Intermediate and Final Products:

   • For hydrogenation, decide if it stops at the alkene stage or goes to the alkane.

   • For hydroboration-oxidation, identify the enol intermediate and the final carbonyl compound.

   • For ozonolysis, predict the cleavage products (carboxylic acids).

   • For alkylation, determine the new carbon-carbon bond formed.

3. Consider Regioselectivity and Stereochemistry:

   • In hydroboration-oxidation, ensure anti-Markovnikov addition.

   • In hydrogenation with Lindlar’s catalyst, ensure the cis-alkene is formed.

   • For alkylation, ensure the correct substitution occurs without rearrangement.