Alkyne Reaction and Free Radical Halogenation Reactions

Catalytic Hydrogenation (Catalytic Reduction)

What’s added: 4 H atoms

Stereoselectivity: Anti

Rearrangement: Not possible

Note: You may see Pt used as well. This is just the catalyst and does not change the outcome of the products.

Reduction to Cis-Alkene

What’s added: 2 H atoms

Stereoselectivity: Syn

Rearrangement: Not possible

Reduction to Trans-Alkene

What’s added: 2 H atoms

Stereoselectivity: Anti

Rearrangement: Not possible

Hydrohalogenation with HBr (Terminal Alkyne)

What’s added: 1 H atom and 1 halogen atom (can be F, Br, I, or Cl) per equivalent of HX

Regioselectivity: Markovnikov

Intermediate: Carbocation

Rearrangement: Not possible

Hydrohalogenation with HBr (Internal Alkyne)

What’s added: 1 H atom and 1 halogen atom (can be Cl or Br) per equivalent of HX

Regioselectivity: Markovnikov

Intermediate: Carbocation

Rearrangement: Not possible

Halogenation with Br2

What’s added: 2 halogen atoms (can be F, Br, I, or Cl) per equivalent of X2

Stereoselectivity: Anti

Intermediate: Bromonium ion

Rearrangement: Not possible

Hydration of an Internal Alkyne

What’s added: 1 O atom

Rearrangement: Not possible

Do know that this reaction produces enols, which then tautomerize to form ketones.

Hydration of a Terminal Alkyne (Markovnikov)

What’s added: 1 O atom

Regioselectivity: Markovnikov

Rearrangement: Not possible

Hydration of a Terminal Alkyne (Anti-Markovnikov)

What’s added: 1 O atom

Regioselectivity: Anti-Markovnikov

Rearrangement: Not possible

SN2 Addition of an Acetylide Ion to an Alkyl Halide

What’s added: additional C atoms (-R of alkyl halide)

Intermediate: Acetylide Ion

Rearrangement: Not possible

SN2 Addition of an Acetylide Ion to a Ketone

What’s added: 1 alkyl group

Intermediate: Acetylide Ion

Rearrangement: Not possible

SN2 Addition of an Acetylide Ion to an Epoxide

What’s added: 2-hydroxylpropane (from epoxide)

Intermediate: Acetylide Ion

Rearrangement: Not possible

Ozonolysis/Oxidative Cleavage on an Internal Alkyne

What’s added: 4 O atoms and 2 H atoms

Know that the reaction cuts the triple bond in half. An O replaces two of the bonds as C=O and the third lone bond becomes a bond to -OH.

Ozonolysis/Oxidative Cleavage on a Terminal Alkyne

What’s added: 4 O atoms and 1 H atom

Know that the reaction cuts the triple bond in half. On the internal side, an O replaces two of the bonds as C=O and the third lone bond becomes a bond to -OH. On the terminal side, two oxygens O replace all the bonds on carbon, forming the most oxidized form of carbon: CO2.

Alkyne Formation from Double Elimination of a Vicinal Dihalide

Free Radical Halogenation using Bromine (more selective)

What’s added: 1 Br atom

Regioselectivity: Most Substituted Product

Intermediate: Radical Intermediate

Rearrangement: Not possible

Free Radical Halogenation using Chlorine (less selective)

What’s added: 1 Cl atom

Intermediate: Radical Intermediate

Rearrangement: Not possible

Allylic/Benzylic Bromination

What’s added: 1 Br atom

Intermediate: Allylic Radical Intermediate

Rearrangement: Not possible

Note: this reaction results in the formation of allylic radical intermediates which resonate and thus allow for the formation of multiple products.