Alkynes Flashcards
- Alkynes can be reduced to alkanes using H₂ and a metal catalyst (Pd, Pt, or Ni).
- The reaction proceeds via syn addition of hydrogen atoms, similar to alkene hydrogenation.
- To stop at the alkene intermediate, a less active catalyst (Lindlar’s catalyst) is used. This catalyst allows for syn addition of hydrogen to form a cis-alkene.
Catalytic Hydrogenation
- Alkynes can be reduced to trans-alkenes using sodium (Na) or lithium (Li) metal, in liquid ammonia (NH₃).
- The reaction proceeds via a radical anion mechanism, with anti addition of electrons followed by protonation.
Dissolving Metal Reduction
- Alkynes undergo hydrohalogenation to form vinyl halides.
- Follows Markovnikov’s rule.
The reaction proceeds via a carbocation intermediate, which can undergo rearrangements if a more stable carbocation can be formed. - A second equivalent of HX can add to form a geminal dihalide (two halogens on the same carbon).
Hydrohalogenation of Alkynes
- Alkynes can be hydrated in the presence of an acid catalyst and water to form an enol intermediate, which tautomerizes to a ketone.
- The reaction follows Markovnikov’s rule, with the hydroxyl group adding to the carbon with fewer hydrogen atoms.
- Tautomerization is the rapid equilibrium between an enol and a ketone, favored towards the ketone form due to greater C=O bond strength
Acid-catalyzed hydration
(hydration rxn of alkyne)
- Alkynes can undergo hydroboration-oxidation to form an enol intermediate, which tautomerizes to an aldehyde (for terminal alkynes) or a ketone (for internal alkynes).
- Follows anti-Markovnikov regioselectivity, with the hydroxyl group adding to the less substituted carbon.
- The enol intermediate is formed via syn addition of boron and hydrogen, followed by oxidation and tautomerization.
Hydroboration-oxidation
(hydration reaction of alkyne)
- Alkynes undergo halogenation to form vicinal dihalides (two halogens on adjacent carbons).
- The reaction proceeds via a cyclic halonium ion intermediate, similar to alkene halogenation.
- A second equivalent of X₂ can add to form a tetrahalide.
Halogenation of Alkynes
Reaction with O₃ followed by H₂O
- Alkynes undergo ozonolysis to form carboxylic acids.
- Terminal alkynes produce CO₂ as a byproduct.
- The reaction proceeds via an ozonide intermediate, which is cleaved by water to form carboxylic acids.
Ozonolysis of Alkynes
Catalytic Hydrogenation
-Can also use Pt, Ni, etc. as hydrogenation catalysts. These will also reduce alkynes to alkanes.
Partial Hydrogenation
- Lindlar catalyst is a “poisoned” palladium catalyst for the partial hydrogenation of alkynes. It contains palladium (Pd), lead (Pb), and quinoline, and is selective for the cis-alkene. Sometimes also see “Pd,CaCO3”.
Dissolving Metal Reduction
-Sodium (Na) is a very strong reducing agent, and will reduce alkynes (but not alkenes). Selective for the trans-alkene. The NH3 provides the hydrogens in the final product.
Hydrohalogenation
Hydration acid catalyzed
- makes ketone
Hydroboration of alkyne
- makes aldehyde (still keto)
- Hydroboration on internal alkynes will give mixture of ketones.
Oxymercuration of alkyne
- makes ketone
Halogenation of alkyne
- first addition is trans
Ozonolysis of alkyne
