What is a catalyst? (3 MP)

For a catalysed process... (3 P)

Importance of catalysts... (4 P)

Classes of catalyst: (4 P)

1) Heterogeneous (4 P)

2) Homogeneous (4 P)

3) Bio-catalysts (2 P)

4) Phase transfer (2 P)

Advantages & disadvantages of a homogeneous acid catalyst over a heterogeneous acid catalyst

Catalyst Efficiency: (2 P)

1) Turn over number (TON)

2) Turn over frequency (TOF)

--> (used to compare catalysts)

Reactions in a catalytic converter:

1) CO

2) \(CH_4\)

3) \(NO_x\)

Limitations with catalytic converters

How to change the rate with catalysts: (2 P)

Catalytic converter problems: (3 P)

1) Operating temperature (2 MP)

2) Catalyst poisons

3) Use of precious metals (4 MP)

Metal recovery:

Phytoremediation - use of plants for environmental clean-up

Type of rxn:

1) General acid catalysis

2) Specific acid catalysis

Mechanism of rxn:

1) Specific acid catalysis - \(H^{+}\) transfers before rds, forming an intermediate - 2 slow rds

2) General acid catalysis - \(H^{+}\) transfers in rds (slow) - 1 slow rds

Mechanism of rxn 2:

Specific acid catalysis (less species, generally in RB flask)

Mechanism of rxn 3:

General acid catalysis (more species, generally in enzymatic catalysis)

Mechanism of acid-catalyzed hydrolysis of ester (Specific acid catalysis)

(NOTE: 2 slow steps, both rates increased when catalyzed)

(NOTE difference if no acid catalyst present)

Strenghts of acids in...

Specific acid catalysis

Strenghts of acids in...

General acid catalysis

Henderson–Hasselbalch equation
(for dilute aqueous solutions)

--> large Ka = strong acid
--> small pKa = strong acid (inverse)

Mechanism of proton transfer:

(3 equilibriums)

Rate of proton transfer 1

Rate of proton transfer 2

Rate of proton transfer 3

Rate of proton transfer 4

--> Intramolecular H-bonding: reduces rate

Rate of proton transfer 5

Does NOT affect rate of proton transfer

Proton transfer in polarisable systems

Effect of water on dissociation of Bronsted acids

Ideal conditions mean... (2 P)

Bronsted acid catalysis means...

Bronsted Catalysis Law

Bronsted Catalysis Law for non-ideal conditions, ie. non-aqueous & non-protonic aicds

Bronsted Catalysis Law for non-ideal conditions, ie. non-aqueous & non-protonic aicds 2

Hammett Acidity Function

Can be used for measuing acidities of:

- Lewis acids
- V. concentrated solutions , including superacids (ie, low H2O in system, not being able to transfer H+ throughout rxn)

Hammett Acidity Function in dilute solutions (works v. well too)

Hammet acidity function (\(H_O\)) 2:

Hammet acidity function plot:
(effect of H2O/AH on \(H_O\))

Solvent ("medium") effects on proton transfer

Proton transfer in non-aqueous solvents

--> Acid-base complexes can have enough stability to allow detailed spectroscopic studies & isolation

Reactivity of hydrogen bonded complexes (in NON-aqueous conditions)

--> Hydrogen bonded complexes can act as v. reactive in-situ sources of nucleophiles

Catalytic function of the proton in organic chemistry

- Electronic distortion

Catalytic function of the proton in organic chemistry

- Leaving group stabilisation

Catalytic function of the proton in organic chemistry

- Addition of H+ to electron system (eg \(\pi\) - system)

Definitions:

1) Lewis

2) Lewis acids

3) Lewis bases

Lewis acids

Mechanism of Friedel-Crafts reactions:

Effect of water:

- Weakens overall acidicity, reducing rate

- Changes nature of catalyst --> no longer a catalyst

Mechanism of Friedel-Crafts reactions 2:

Effect of water:

- Weakens overall acidicity, reducing rate

- Changes nature of catalyst --> no longer a catalyst

Heterogeneous Catalysts

- Solid Acids: Inorganic solids possessing Lewis or Bronsted acid sites.

Heterogeneous Catalysts 2:

(NOTE: TS--1 Catalyst is same structure but with Titanium instead of Silicon)

Heterogeneous Catalysts 2:

another version

Immobilised aluminium chloride (\(AlCl_3\))

Product isolation with:

1) \(AlCl_3\)

2) Heterogenous catalyst (solid acids)

Note: Solid acids catalyst can be reused, as opposed to AlCl3 which gets quenched

Heterogeneous Catalysts

- Deactivation (3 P) (eg in Friedel-Crafts reactions)

Heterogeneous Catalysts

- Reactivation (3 P)

Heterogeneous Catalysts

- Problems (2 P)

Heterogeneous Catalysts

- Alkylation mechanism (can get double addition, see shape (size) selectivity)

Heterogeneous Catalysts 2

- Alkylation isomers

Heterogeneous Catalysts

- Shape (size) Selectivity (better w/ solid acids)

Mechanism for catalysed polymerisation of styrene with:

- \(AlCl_3\)

- \H^+\)

Heterogeneous Catalysts

- Solid Bases: Inorganic solids possessing basic sites.

- Michael Reaction mechanism

Heterogeneous Oxidations

(using eg TS-1 - Enichem)

Green oxidations

Things to take care w/ Heterogeneous catalysts (3 P):

Green processing

- The Substrate