Chapter 7.8: Reduction of Alkynes

Question 1

What are the reagents of dissolving-metal reduction of an alkyne?

Answer 1

Na°/NH3

Question 2

What is the purpose of a Lindlar catalyst?

Answer 2

 Stops a catalytic reduction after the addition of one mole of hydrogen

Question 3

Describe the mechanism of hydroboration-protonolysis of an alkyne

Answer 3

o Internal alkynes react with borane to give a trialkenylborane
o Treating a trialkenylborane with a carboxylic acid results in stereoselective replacement of boron by hydrogen: a cis-alkenyl group bonded to a boron is converted to a cis-alkene

Question 4

Define Lindlar catalyst

Answer 4

finely powdered palladium metal deposited on solid calcium carbonate that has been specially modified with lead salts. Its particular use is as a catalyst for the reduction of an alkyne to a cis alkene

Question 5

Describe the mechanism of catalytic reduction of an alkyne

Answer 5

o Treatment of an alkyne with H2 in the presence of a transition metal catalyst, most commonly palladium, platinum, or nickel, results in the addition of two moles of H2 to the alkyne and its conversion to an alkane

Question 6

What is a finely powdered palladium metal deposited on solid calcium carbonate that has been specially modified with lead salts. Its particular use is as a catalyst for the reduction of an alkyne to a cis alkene?

Answer 6

Lindlar catalyst

Question 7

What is the stereochemistry of dissolving-metal reduction of an alkyne

Answer 7

o Stereochemistry: anti  trans or E product

Question 8

Describe the mechanism of dissolving-metal reduction of an alkyne

Answer 8

 Step 1: A one-electron reduction of the alkyne gives an alkenyl radical anion (an anion containing an unpaired electron on one carbon and a negative charge on an adjacent carbon)
 Step 2: Add a proton
 Step 3: A one-electron reduction of the alkenyl radical gives and alkenyl anion
 Step 4: Add a proton