Reactions of Alkynes

Question 1

Acetylide Addition
(Reactants (including conditions/catalysts), Products, Intermediates, Regioselectivity, Stereoselectivity)

Answer 1

Reactants: 1. Alkyne, NaNH3, NH3 2. Haloalkane
Catalysts/Conditions: None
Products: Alkane added after alkyne
Intermediate: acetylide anion
Regioselectivity: None
Stereoselectivity: None

Question 2

Double Dehydrohalogenation
(Reactants (including conditions/catalysts), Products, Intermediates, Regioselectivity, Stereoselectivity)

Answer 2

Reactants: 1. Dihalide, NaNH3 (2 eq)
Catalysts/Conditions: NH3
Products: Alkyne
Intermediate: Alkene 
Regioselectivity: None
Stereoselectivity: None

Question 3

Halogenation of Alkynes

Reactants (including conditions/catalysts), Products, Intermediates, Regioselectivity, Stereoselectivity

Answer 3

Reactants: Alkyne, X2 (2eq)
Catalysts/Conditions: DSM or CH2Cl2 (inert solvent)
Products: Tetrahalide
Intermediate: Cyclic Bridge
Regioselectivity: None
Stereoselectivity: None

Question 4

Hydrohalogenation of Alkynes

Reactants (including conditions/catalysts), Products, Intermediates, Regioselectivity, Stereoselectivity

Answer 4

Reactants: Alkyne, hydrohalide (2eq)
Catalysts/Conditions: None
Products: HXs added to same C
Intermediate: Carbocation but dont worry about rearrangements
Regioselectivity: Xs are both Mark
Stereoselectivity: None

Question 5

Oxymercuration-Demercuration of Alkynes

Reactants (including conditions/catalysts), Products, Intermediates, Regioselectivity, Stereoselectivity

Answer 5

Reactants: Alkyne, HgSO4, H2O
Catalysts/Conditions: Acid Catalyst (ex H2SO4)
Products: Ketone
Intermediate: Enol
Regioselectivity: Not end C (makes ketone)
Stereoselectivity: None

Question 6

Hydroboration Oxidation of Alkynes

Reactants (including conditions/catalysts), Products, Intermediates, Regioselectivity, Stereoselectivity

Answer 6

Reactants: 1. Alkyne, (Sia)2BH, THF 2. NaOH, H2O2
Catalysts/Conditions: None
Products: Aldehyde
Intermediate: Alcohol (Tautomerization)
Regioselectivity: O is Anti Mark
Stereoselectivity: None

Question 7

Catalytic Reductions of Alkynes

Reactants (including conditions/catalysts), Products, Intermediates, Regioselectivity, Stereoselectivity

Answer 7

Reactants: Alkyne, H2
Catalysts/Conditions: Pd or Pt or Ni
Products: Alkane (cant stop at alkene)
Intermediate: none
Regioselectivity: None
Stereoselectivity:None

Question 8

Lindlars (Reduction)

Reactants (including conditions/catalysts), Products, Intermediates, Regioselectivity, Stereoselectivity

Answer 8

Reactants: Alkyne, H2
Catalysts/Conditions: Lindlar's Catalyst
Products: Z-Alkene
Intermediate: None
Regioselectivity: None
Stereoselectivity: Z-Alkene!

Question 9

Na/NH3 (Reduction)

Reactants (including conditions/catalysts), Products, Intermediates, Regioselectivity, Stereoselectivity

Answer 9

Reactants: Alkyne, Na^0, NH3
Catalysts/Conditions: None
Products: E-Alkene
Intermediate: None
Regioselectivity: None
Stereoselectivity: E-Alkene

Question 10

If you are making a terminal alkyne using double dehydrohalogenation, what extra step do you need?

Answer 10

H2O as a work-up step to add the H back to the acetylide anion.

Question 11

Do you need to worry about carbocation rearrangements with alkynes?

Answer 11

No.

Question 12

What are tautomers? Give an example.

Answer 12

Structural isomers that differ in the position of one H+ and one double bond. An “enol” and a “keto” are tautomers.