Thermodynamics and mechanism of hydrogenation
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Hydrogenation is a strongly exothermic reaction. In the hydrogenation of vegetable oils and fatty acids, for example, the heat released is about 25 kcal per mole (105 kJ/mol), sufficient to raise the temperature of the oil by 1.6-1.7 °C per iodine number drop. The mechanism of metal-catalyzed hydrogenation of alkenes and alkynes has been the extensively studied. First of all isotope labeling using deuterium confirms the regiochemistry of the addition:
RCH=CH2 + D2 → RCHDCH2D
• Heterogeneous catalysis
On solids, the accepted mechanism today is called the Horiuti-Polanyi mechanism.
1. Binding of the unsaturated bond, and hydrogen dissociation into atomic hydrogen onto the catalyst
2. Addition of one atom of hydrogen; this step is reversible
3. Addition of the second atom; effectively irreversible under hydrogenating conditions
• Homogeneous catalysis
In many homogeneous hydrogenation processes, the metal binds to both components to give an intermediate alkene-metal(H)2 complex. The general sequence of reactions is assumed to be as follows or a related sequence of steps:
* binding of the hydrogen to give a dihydride complex ("oxidative addition"):
LnM + H2 → LnMH2
* binding of alkene:
LnM(η2H2) + CH2=CHR → Ln-1MH2(CH2=CHR) + L
* transfer of one hydrogen atom from the metal to carbon (migratory insertion)
Ln-1MH2(CH2=CHR) → Ln-1M(H)(CH2-CH2R)
* transfer of the second hydrogen atom from the metal to the alkyl group with simultaneous dissociation of the alkane ("reductive elimination")
Ln-1M(H)(CH2-CH2R) → Ln-1M + CH3-CH2R
Preceding the oxidative addition of H2 is the formation of a dihydrogen complex.
RCH=CH2 + D2 → RCHDCH2D
• Heterogeneous catalysis
On solids, the accepted mechanism today is called the Horiuti-Polanyi mechanism.
1. Binding of the unsaturated bond, and hydrogen dissociation into atomic hydrogen onto the catalyst
2. Addition of one atom of hydrogen; this step is reversible
3. Addition of the second atom; effectively irreversible under hydrogenating conditions
• Homogeneous catalysis
In many homogeneous hydrogenation processes, the metal binds to both components to give an intermediate alkene-metal(H)2 complex. The general sequence of reactions is assumed to be as follows or a related sequence of steps:
* binding of the hydrogen to give a dihydride complex ("oxidative addition"):
LnM + H2 → LnMH2
* binding of alkene:
LnM(η2H2) + CH2=CHR → Ln-1MH2(CH2=CHR) + L
* transfer of one hydrogen atom from the metal to carbon (migratory insertion)
Ln-1MH2(CH2=CHR) → Ln-1M(H)(CH2-CH2R)
* transfer of the second hydrogen atom from the metal to the alkyl group with simultaneous dissociation of the alkane ("reductive elimination")
Ln-1M(H)(CH2-CH2R) → Ln-1M + CH3-CH2R
Preceding the oxidative addition of H2 is the formation of a dihydrogen complex.
