Organic Chemistry I: Chapter 9 & 10 Mechanism Alkynes

Hydrohalogenation

Reagent(s): HX or Excess HX

Adds What: X or X and X 

Regiochemistry: Mark (unless ROOR then anti Mark)

Steriochemistry: N/A

Elimination of Alkyl dihalide

• alkyl halide goes through two eliminations by a strong base

  • E2 reactions

  • terminal alkyne will be de-protonated —→ quench step is required

    • HBR will make one halide added and excess will result in two halides added to the carbon

Halogenation

Reagent(s): X2 (results in alkene with two added X groups) or XS X2 (results in alkane with four added Xs) 

Adds What: 2 X groups added anti from each other on alkene or 4X added to alkane to make tetrahalide

Hydration 

Reagents: H3 O+/HgSO4 —→ enol —→ taumerization for Ketone

Extra considerations for alkynes:
o H3 O+ alone won’t work – HgSO4 is necessary
o Hydration of an alkyne gives an enol. New mechanism: tautomerization (acidic
conditions).
o No oxymercuration-demercuration (no vinyl carbocation formation, so it’s not
necessary)

Hydroboration-oxidation

Reagents: 1. BH3 2. H2O2, NaOH —→ enol —→ taumerization for aldehyde

Extra considerations for alkynes:
o Hydration of an alkyne gives an enol. New mechanism: tautomerization (basic
conditions).
o BH3 generally gives multiple hydride additions, so we use a bulky dialkylborane (HBR2 )
such as 9-BBN or dicyclohexylborane (Cy2 BH).

Hydrogenation

1. Reagents: metal, Cold Products: alkane

2. Reagents: Na/NH3 Products: trans alkene

3. Reagents: H2 / Lindar Products: cis alkene

Extra consideration for alkynes: single reduction to give cis or trans alkene selectively

Ozonolysis

1. Reagents: O3/H2O  Products: Ketones with O and R group 

2. Extra considerations for alkynes:
   o When the triple bond of an alkyne is fully oxidized, followed by hydrolysis, carboxylic
   acids are produced.
   o When a terminal alkyne undergoes ozonolysis, carbon dioxide is formed.
   

Deprotonation

Reagents: 

 Extra consideration for alkynes:
o Use a strong enough base (pKa of conjugate acid must be greater than 25).
o This SN2 reaction gives us our FIRST C-C BOND-FORMING REACTION!!!!!
o Alkynyl anions are strong bases and strong nucleophiles – this only works well with
primary and methyl alkyl halides.