lecture 9

• Catalytic Hydrogenation


  -uses hydrogen gas and a metal catalyst to reduce functional groups (especially alkenes)

  -Metal dispersed to increase surface area, can be recovered and reused via filtration
• Catalytic hydrogenation- metal usage

  -Most common metals: Pd and Pt

  -Rh and Ru also used

  -Ni rarely used as it requires higher temps and longer reaction times
• Catalytic Hydrogenation- heterogenous process

  -Happens on the surface of the metal

  -hydrogen absorbed onto metal surface

  -alkene coordinates to surface

  -One hydrogen delivered followed by next
• Catalytic Hydrogenation- syn selectivity


  -Both hydrogens delivered to same face of the double bond
• Catalytic Hydrogenations- unwanted products


  -unwanted isomerization

  -commercial- vegetable oil to solid fats good, trans fats bad
• Catalytic Hydrogenation- Practical Considerations

  -Balloon filled with H2 gas

  -Reaction very clean and high yielding

  -Catalyst easily recovered

  -higher pressure requires= Parr hydrogenator
• Alkynes to alkenes

  -Pd/Pt catalyst

  -catalyst 'poisoned' with lead, reaction stops at alkenes

  -Lindlar's catalyst

  -lead reduces reactivity of Pd, reduces more reactive alkenes but not less reactive alkenes

  -syn addition for cis-alkene
• Transfer hydrogenation

  -Other sources of hydrogen than hydrogen gas

  -cyclohexane, hydrazine, or formic acid can also act as hydrogen donors