13. Hydrogenation reactions

1. 13. Hydrogenation reactions 201450110 이 재 성

2. Index 1. Introduction -Catalyst - Fundamentals of Hydrogenation 2. Homogeneous catalyst -Wilkinson Hydrogenation 3. Heterogeneous catalyst -Lindlar Hydrogenation -Raney nickel Hydrogenation

3. 1. Catalyst. • Catalysis is the increase in the rate of a chemical reaction of one or more reactants due to the participation of an additional substance called a catalyst. • A catalyst is not consumed by the reaction. With a catalyst, less free energy is required to reach the transition state, but the total free energy from reactants to products does not change.

4. 1. Catalyst. - types Catalysts can be heterogeneous or homogeneous, depending on whether a catalyst exists in the same phase as the substrate. a) Homogeneous catalysts : Same phase as the reactants(Typically homogeneous catalysts are dissolved in a solvent with the substrates) b)Heterogeneous catalysts : Different phase than the reactants - The total surface area of solid has an important effect on the reaction rate. The smaller the catalyst particle size, the larger the surface area for a given mass of particles.

5. 1. Catalyst. Catalyzed reactions have a lower activation energy (rate-limiting free-energy of activation) than the corresponding uncatalyzed reaction, resulting in a higher reaction rate at the same temperature.

6. 1. Catalyst. -The first equation (eq. 1, the Arrhenius equation) shows the exponential dependence of rate constant k on activation energyEa and temperature, T. -The negative sign in the exponent means that as Ea increases, the value of k greatly decreases, and that as T increases, k greatly increases.

7. 1. Fundamental of Hydrogenation. • Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalysts. • Catalysts are required for the reaction to be usable; non-catalytic hydrogenation takes place only at very high temperatures.

8. 1. Fundamental of Hydrogenation.

9. 1. Fundamental of Hydrogenation.

10. 1. Fundamental of Hydrogenation. • The ease of hydrogenation is very sensitive to the number of substituents, with monosubstituted alkenes reacting most rapidly, and tetrasubstituted alkenes sometimes being very difficult to reduce.

11. 2. Homogeneous catalyst - Wilkinson catalyst RhCl3(H2O)3 + 4 PPh3 → RhCl(PPh3)3 + OPPh3 + 2 HCl + 2 H2O Wilkinson's catalyst is the common name for chlorotris(triphenylphosphine)rhodium(I), a coordination compound with the formula RhCl(PPh3)3 (Ph = phenyl). 3PPh3 = 3 X 2 = 6 1 Cl = 2 1 Rh(1) = 8 RhCl(PPh3)3 = 16 electrons Square planar structure

12. 2. Homogeneous catalyst - Wilkinson catalyst

13. 2. Homogeneous catalyst - Wilkinson catalyst The rate of hydrogenation depends on (a) presence of a functional group in the vicinity of the C=C bond and (b) degree of substitution of the C=C fragment. Increasing rate -A polar functional group may accelerate catalysis by assisting olefin coordination to Ru -Hydrogenation of internal and branched alkenes is the slowest (cisis faster than trans)

14. 3. Heterogeneous catalyst - Lindlar catalyst : consists of palladium deposited on calcium carbonate or barium sulfate and treated with various forms of lead.

15. 3. Heterogeneous catalyst - Haber-Bosch process :industrial implementation of the reaction of nitrogen gas and hydrogen gas. It is the main industrial procedure to produce ammonia. N2 + 3 H2 → 2 NH3 (ΔH = −92.4 kJ · mol−1) 1. N2(g) → N2 (adsorbed) 2. N2(adsorbed) → 2 N (adsorbed) 3. H2(g) → H2 (adsorbed) 4. H2(adsorbed) → 2 H (adsorbed) 5. N (adsorbed) + 3 H(adsorbed)→ NH3 (adsorbed) 6. NH3(adsorbed) → NH3 (g)

16. 6. Reference • Gary L, Miessler, Donald A. Tarr, 2011, Inorganic Chemistry 3thed, Pearson Education, p572-575 • Organometallic Chemistry – R. C.Mehrotra(2007) • G. Ertl (1983). "Primary steps in catalytic synthesis of ammonia". Journal of Vacuum Science and Technology A1(2): 1247–1253