L8: Intro to catalytic cycles

Question 1

Organometallic catalysis: homogenous or heterogenous? Typical substrate in industry.

Answer 1

• Homogenous catalysts
• Most organic processes in industry use simple feedstocks as substrates (e.g. ethene, propene or methanol)

Question 2

Catalyst: basic definition

Answer 2

• Substance that increases rate of a chemical reaction without being consumed in that reaction

Question 3

General principles of organometallic catalysis (x2)

Answer 3

1. Accelerate rate: providing alternate reaction pathway with lower activation energy; original reaction was still thermodynamically viable
2. Alter product selectivity: this is done by accelerating just one of two or more competing reactions

Question 4

Catalysts can’t alter overall thermodynamics of a reaction; Discuss, compare feasibility of catalysis at different DeltaG values

Answer 4

• DeltaG is a state function; overall free energy change for a reaction is independent of the path the reaction takes from reactant to product
• A catalyst increases the rate but doesn’t alter final position of thermodynamic equation (k unchanged)
• Hence, for a positive DeltaG, catalysis feasible. For slightly positive DeltaG, catalysis still feasible, but % conversion of A to B is always low. For negative DeltaG, it is unlikely that catalysis alone would make the process economically viable

Question 5

Benefits of heterogenous catalysis

Answer 5

• Ease of product separation
• Ease of catalyst recovery

Question 6

Benefits of heterogenous catalysts

Answer 6

• Higher activities can often be achieved
• Selectivity often superior
• Catalyst more easily modified and studied

Question 7

How might the benefits of both types of catalysts be harnessed in future? (2 methods)

Answer 7

• Anchoring hom. catalysts to insoluble polymer supports
• Water soluble version where catalyst remains in aq. phase and catalysis occurs at the aqueous/organic phase boundary -> biphasic system

Question 8

Basic activity of an homogenous organometallic catalyst

Answer 8

1. Brings reactant in close proximity (by coordination to metal centre)
2. Activates the reactant
   e.g. H₂ is ‘activated towards nucleophilic attack by rupture of H-H bond
   e.g. An alkene is activated towards nucleophilic attack by coordination

Question 9

Tolman’s rule, testing practically

Answer 9

• Organometallic reaction sequences generally proceed by elementary steps which involve IMs having 18 or 16 valence electrons
• Plausible reactions according to this rule must then be tested experimentally; validity tested via kinetics, spectroscopic detection of IMs, trapping etc.

Question 10

4 key elementary steps in catalysis

Answer 10

1. Ligand dissociation/association
2. Oxidative addition/reductive elimination
3. Intramolecular H or R migration
4. Nucleophilic attack on coordinated pi-ligands

Question 11

Quantifying catalyst efficiency

Answer 11

• TON: Turnover number, no. passes through catalytic cycle before the catalyst becomes deactivated (moles product/moles catalyst used)
• TOF: Turnover frequency, no. passes through CC per unit of time (rate that product forms in mols per second/ mols catalyst present)

Question 12

Consequence of high TOF/TON; how do side reactions affect activity?

Answer 12

• High TON means a long-lived catalyst but not necessarily highly active
• High TOF = highly active
• Decomposition or ‘poisoning’ by side reactions likely to cause deactivation