Identifying Alkyne Reactions

Addition of Hydrogen Halides

Electrophilic addition of HX

• reagent is some halide (HX - like HBr or HCl)

• when substituents are different → regio selectivity and follows Mark’s Rule → no stereo selectivity → only one product

• when substituents are same → no regioselectivity when 1st HX is added → 2 intermediates → use Mark’s rule to add 2nd HX → racemic mixture

Addition of Halogens

Electrophilic addition of X2

• Reagent is any X2 (usually Br2 or Cl2)

• Intermediate is a bromonium or chloronium

• halide attacks on the opposite side giving anti-addition resulting in a trans-alkene intermediate (stereo selectivity is present)

• Excess X2 is added again through anti addition to give an alkane with 4 bromines and 2 R groups

Acid Catalyzed Hydration

Addtion of water/ Hydration

• reagents are HgSO4/H2SO4/H+ and H2O → follows Mark’s rule producing an enol → tautomerizes into a ketone and only a ketone

• tautomerizing occurs because the intramolecular forces that cause it happens quicker than the intermediate can react with more boron

• intramolecular forces happen quicker than intermolecular ones

Catalytic Hydrogenation 

Alkyne reduction

• Reagents are H2 and a metal catalyst

• 2 sets of H2 get added to both sides of the bond, reducing it to an alkane

Catalytic hydrogenation with Lindar’s 

Catalyst determines if it becomes an E or Z alkene

• Reagents H2 and metal catalyst result in a Z alkene

• Reagents Na metal/ NH3 result in an E alkene

Ozonolysis

Oxidative cleavage

• reagents are O3 and water

• all pi bonds and triple bonds are split, O is added to end of the split double bond

• split pi bond - ends kept as they are

• split triple bound - end without substituent is attached to an OH

• terminal alkenes - one of the products will be formic acid, which turns into CO2

Hydroboration 

Hydration reaction

• 1) reagent is either HB(SIA)2/9-BBN/BH3

• 2) reagent is always H2O2, NaOH

• H and Br attachment follows anti-Mark’s rule (regio selectivity)

• adds through syn-addition (H and Br add to same side)

• which means the OH with be on the more substituted side rather than the H → enol func group

• tautomerizes into ketone if internal and aldehye if terminal