Alkynes

Reductions of Alkynes

a) 1) H2 2) Pd/Pt

b) 1) H2 2) Lindlars Catalyst

c) 1) Na^• , NH3(I)

- Cannot reduce terminal alkynes

Can be cis or trans

Hydrohalogen of Alkynes

H-X

Excess is 2 equiv

Scope: X = Cl, Br, I

Stoichiometry: 1 eq hx yields vinyl halide (1 bonded to C) , 2 equiv HX yields germinal halide (2 bonded to same C)

Stereochemistry for Formation of vinyl halides: Anti addition of H and X

Regiochemistry: Markovnikov addition of 1st X, 2nd X adds to same carbon

Mechanism: rate = k(alkyne)(H-X)²

Hydration of Alkynes

a) Reagents

Step 1) H2O, HgSO4 catalyst

Step 2) H2SO4 (or H3O+) for alkynes

b) Regioselectivity

For terminal alkynes, Markovnikov

addition of water occurs

c) keto/enol tautomerization

-C=C DB less stable than C=O DB therefore the enol form is less stable than the keto form

Hydroboration/Oxidation

1) Disiamylborane or 9-BBN, THF

2) H2O2, NaOH

-same products as hydration if internal alkyne

b) Anti-Markovnikov addition of OH if terminal alkyne

c) tautomerization is base-catalyzed

Halogenation of Alkynes

1 or 2 equiv X-X

X2 = Cl2, Br2

-One equiv yield trans vinyl dihalide

-Two equiv yields tetrahalide

Oxidative Cleavage of Alkynes

1) O3

2) H2O

Internal and Terminal

Alkylation of Alkynide/Acetylide Anions

Deprotonation of terminal alkynes followed by SN2

Base: any base who’s conjugate acid has a pKa higher than 25

NaH

NaH2

H3C-Li

Only useful with methyl or primary alkyl halides (otherwise E2 major)

Organic Synthesis (Retrosynthetic Analysis)

Identity FG in product

Identify the immediate precursor/reagent to the product.

Repeat. If you get stuck, start from the forward direction and meet in the middle.