Alkynes Reagents 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
- markovnikov
-makes ketone
- H shift & double bond to O
Hydroboration of alkyne
- anti-markovnikov
- makes aldehyde on terminal alkyne
- Hydroboration on internal alkynes will give mixture of ketones.
Oxymercuration/demercuration of Alkyne
- markovnikov
- makes ketone
- H shift & double bond to O
Halogenation of Alkyne
- first addition is trans
Ozonolysis of Alkyne
- If R = H , then one product will be CO2
__________________ is the
interconversion of enols and aldehydes/ketones through the
movement of a proton and the shifting of bonding electrons. It is an equilibrium which generally favors the keto form.
tautomerization (keto-enol tautomerization)
Anti or Syn addition?
Catalytic Hydrogenation
Syn
Anti or Syn addition?
Dissolving Metal Reduction
Anti
Addition of Hydrogen Halide to
an alkyne forms a ________
gem dihalide
Markovnikov or Anti-Markovnikov?
Hydrohalogenation
Markovnikov
Markovnikov or Anti-Markovnikov?
Hydration Acid Catalyzed
Markovnikov
- forms enol tautomerization into ketone
Markovnikov or Anti-Markovnikov?
Hydroboration (hydration of alkyne)
Anti-Markovnikov
- forms enol tautomerization into aldehyde
Markovnikov or Anti-Markovnikov?
Oxymercuration (hydration of alkyne)
Markovnikov
- forms enol tautomerization into ketone
The combination of Pd supported on calcium carbonate (CaCO3) that has been treated with a small amount of lead (often Pb(OAc)2 ) is known as ________________
“Lindlar’s catalyst”
Lindlar’s catalyst always gives______ alkenes from alkynes. Alkenes are not hydrogenated
cis
Elimination of Hydrogen Halides
- synthesis of alkynes
- NaNH2 is a strong base and will lead to elimination of HBr.
- Zaitsev’s rule = most substituted alkyne formed
Deprotonation of terminal Alkynes
- Alkynes are relatively acidic [pKa = 25]
- Strong Base NaNH2 will remove the C-H to provide the acetylide anion.
Reactions with Electrophiles
- Deprotonated alkynes are excellent nucleophiles and will react with alkyl halides in SN2 reactions
