59
*
HYDROGENATION
• Concerned with two forms of hydrogenation: heterogeneous (catalyst insoluble) and
homogeneous (catalyst soluble)
Heterogeneous Catalysis
• Catalyst insoluble in reaction medium
• Reactions take place on catalyst surface
• Rate of reaction and selectivity dependant on active sites on surface
•Active sites are the part of the catalyst substrate and hydrogen can adsorb on
• By blocking or poisoning active sites the reactivity of a catalyst is reduced and
the selectivity increased
• Good poisons are metal cations, halides, sulfides, amines and phosphines
• Reaction is a surface phenomenon and not fully understood
H H H H
H*
H*
H*
H*
*
*H
H*
*
*H
H
H
H
• catalyst surface
• adsorption of H2
• H2 disassociation / activation
• alkene adsorption
• alkene activation
• hydrogenation
• predominantly syn
• Differences in catalyst arise due to ability of each metal to bind to various substrates and the
different modes of binding
•Order of Reactivity of Various Metals
Pt = C=O >> C=C > {H} > Ar
Pd = C=C > {H} > C=O > Ar
Ru = C=O > C=C > Ar > {H}
{H} = hydrogenolysis
C–X → C–H
•Order of Alkene Reactivity
R
R
R
R R R
R
R R
R
R
R
> > > >
•Note: many other factors involved (eg. the release of ring-strain)
• Co-ordination of alkene on catalyst can lead to double bond isomerisation
• Possibility of migration related to the degree of reversibility of co-ordination
• Pd allows migration presumably via reversible co-ordination
• Pt essentially binds irreversibly resulting in no isomerisation
