Catalytic Hydrogenation of Alkenes and Alkynes

Catalytic hydrogenation of alkenes and alkynes is a crucial process in various industries, particularly in refining petroleum and producing margarine. The reaction's kinetics require catalysts like nickel, palladium, and platinum, and the process involves the adsorption of alkene and hydrogen onto the catalyst surface, followed by hydrogen transfer and reductive elimination to form alkanes. This tutorial further explored the mechanisms of homogeneous catalysts, emphasizing the importance of ligand dissociation and reactant coordination order, and ultimately highlighting the intricate interplay of factors contributing to catalyst effectiveness.

Hydrogenation Difficulty

• 00:00:06 Hydrogenating alkenes is thermodynamically favorable but kinetically challenging, requiring catalysis. Early solutions involved nickel surfaces under extreme conditions, leading to the development of Raney nickel and other supported metal catalysts, primarily used in the petroleum and food industries due to the stability of saturated hydrocarbons compared to unsaturated ones.

Heterogeneous Catalysis

• 00:01:55 The mechanism of heterogeneous hydrogenation involves alkene and hydrogen molecule diffusion towards a metal surface, forming a pi bond and M-H bonds respectively. This process includes alkene adsorption, metal-alkyl complex formation, and reductive elimination, with alkynes hydrogenated via a similar, albeit faster, double hydrogenation mechanism.

Homogeneous Catalysis

• 00:04:00 Homogeneous hydrogenation uses catalysts like rhodium, iridium, or ruthenium complexes, which are soluble and function via a similar mechanism to heterogeneous catalysis, but with additional considerations. The order of reactant coordination is crucial, with 'hydrogen first' catalysts like Wilkinson's catalyst displaying high chemoselectivity, preferentially saturating less-substituted double bonds, with the exception of ethylene.

Wilkinson's Catalyst

• 00:05:46 Wilkinson's catalyst is a 16-electron rhodium species that reacts under mild conditions and displays excellent chemoselectivity. Its mechanism involves oxidative addition of hydrogen, alkene coordination, hydrometalation, isomerization, and reductive elimination. The catalyst's inability to hydrogenate ethylene efficiently is due to the formation of an off-cycle species that inhibits further catalysis.

Olefin First Catalysts

• 00:09:16 'Olefin first' catalysts follow a similar mechanism to 'hydrogen first' catalysts but with a different order of steps. Hydrometalation can precede H2 oxidative addition, as exemplified by a catalyst with a rhodium center, a carbonyl ligand, and triphenylphosphines. The catalyst coordinates with an alkene, followed by oxidative addition of hydrogen and reductive elimination, regenerating the catalytic intermediate.