Addition Reactions of Alkynes

Elimination

• Geminal/vicinal dihalide → alkyne in two E2 steps

• Excess NaNH2 (dissolved in NH3) + H2O

• Needs a strong base for the second deprotonation step

• Base must have a conjugate acid with a pka above 25

Formation of alkynide ion

Deprotonation of a terminal alkyne using NaNH2 or a base with a less acidic CA than pKa = 25

Catalytic hydrogenation

• Alkyne → alkane

• H2 + Pt catalyst

• Syn addition of H, cis alkene intermediate

Catalytic hydrogenation w/ poisoned catalyst

• Alkyne → cis alkene

• H2 and Lindlar’s or Ni2B catalyst

• Syn addition of H, does not catalyze the formation of an alkane

Dissolving metal reduction

• Alkyne → trans alkene

• N2 + NH3 (l)

• Anti addition of H

Hydrohalogenation

• Alkyne → alkene w/ halogen or geminal dihalide

• HX (for alkene and halogen) or excess HX (for geminal dihalide)

• Markovnikov addition

Hydrohalogenation with ROOR and HBr

• Anti-Markovnikov hydrohalogenation

Halogenation

• Internal alkyne → anti/syn alkene with 2 halogens or alkane with 4 halogens (tetrahalide)

• X2 + CCl4 or excess X2 + CCl4

• Anti-alkene is major, syn is minor

SN2 with alkynide ions

• Building alkynes with terminal alkynes or acetylene, which are converted to alkynide

• If adding two alkyl groups, it must be done in two separate steps

• NaNH2 + RX + RX

Acid-catalyzed hydration

• Alkyne → ketone

• H2SO4, H2O, HgSO4

• Markovnikov addition

• Keto-enol tautomerization converts enol to ketone

Keto-enol tautomerization

• Part of acid-catalyzed hydration

• Catalyzed by an acid

• Pi bond attacks acidic proton, oxygen resonates to a C=O, water attacks OH to form a ketone

Hydroboration-oxidation

• Alkyne → aldehyde

• 1. BH3 + THF 2. NaOH + H2O2

• Anti-Markovnikov addition

• Base-catalyzed tautomerization converts enolate to aldehyde

Base-catalyzed tautomerization

• Part of hydroboration-oxidation

• Catalyzed by a base

• -OH attacks ROH, oxygen resonates to a C=O, carboanion attacks water to form an aldehyde

Summarize hydration regioselectivity for terminal alkynes

Ketone = Markovnikov, aldehyde = anti-Markovnikov