Alkene Hydrogenation

Question 1

Why not hydrogenation

Answer 1

is thermodynamically favorable but no reaction pathway exists

Question 2

Wilkinson’s Catalytic Cycle

Answer 2

1. activation
2. hydrogen oxidative addition
3. alkene coordination
4. hydride migration
5. reductive elimination

Question 3

Hydrogenation catalyst requirements

Answer 3

form stable dihydride complexes

transition between n and n+2 oxidation states

vary coordination from 3-6

vary electron count from 14 to 18

Question 4

Wilkinson’s Active complex

Answer 4

dissociation of phosphine ligand

evidence

1. activity increases with more bulky phosphines
2. activity decreases with excess phosphine

Question 5

Oxidative Addition of hydrogen process

Answer 5

1. hydrogen coordinates haptically
2. π backbonding into anti bonding orbital weakens and breaks H-H bond
3. metal donates electron pair forming cis hydride ligands and oxidising metal by 2

Question 6

Hydrogenation Stereochemistry

Answer 6

Addition of hydride is syn coplanar, produces only one enantiomer/meso isomer

1. 4 member transition of migration puts hydride on inside
2. no rotation of alpha carbon preserves conformation
3. reductive elimination occurs by overlap with cis hydride causing it to be added to same side

Question 7

Why hydrogenation efficient

Answer 7

The reductive step is non-reversible

there is no feasible oxidative addition of alkane