Chapter 6: 6.8 Hydrogenation

Question 1

Describe:

Hydrogenation reactions

Answer 1

Add hydrogen atoms across multiple bonds (both alkenes and alkynes) to reduce them

Question 2

Making more bonds with hydrogen is…

Answer 2

Reducing

Question 3

Alkenes can be hydrogenated to make…

Answer 3

Alkanes

Question 4

Alkanes can be hydrogenated to make…

Answer 4

• Alkanes
• Alkenes (cis or trans)

Question 5

What are the types of hydrogenation reactions that you should know?

Answer 5

1. Reduction of alkenes and alkynes to make alkanes with H2 and Pd/C
2. Reduction of alkynes to make cis alkenes with H2 and Lindlar’s catalyst
3. Reduction of alkynes to make trans alkenes with meat/liquid ammonia (NH3)

Question 6

Describe:

Full Hydrogenation Reactions

Answer 6

Reactions with hydrogen gas (H2) and a metal catalyst (usually Pd/C or Pt or Ni) leads to syn addition of the hydrogen atoms for the stereochemical outcome

Question 7

Describe:

Mechanism of Full Hydrogenation reaction

Answer 7

The hydrogens bind to a metal surface before being added to the multiple bond
* Thus they go on the same side (syn)

Question 8

To pause at the alkene, what type of hydrogenation reactions must occur?

Answer 8

Partial Hydrogenation Reactions
1. Reduction of alkynes to make cis alkenes with H2 and Lindlar’s catalyst
2. Reduction of alkynes to make trans alkenes with metal/liquid ammonia (NH3)

Question 9

Define and Describe:

Lindlar’s catalyst

Answer 9

Pd catalyst that has been poisoned with Pb (lead)
* Makes cis alkenes from alkynes