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Description
Flashcards by Sulivan González, updated more than 1 year ago
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Created by Sulivan González
over 8 years ago
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| Question | Answer |
| What is a catalyst? (3 MP) | It increases the rate at which a chemical rxn approaches equilibrium, without itself being consumed or changed in the process |
| For a catalysed process... (3 P) | 1) The rxn rate constant obeys the Arrhenius Eq. & provides lower energy rxn pathways 2) It does not circumvent normal thermodynamic predictions 3) It does not alter equilibrium constants |
| Importance of catalysts... (4 P) | 1) 90% chemical processes use catalysts 2) Changes in catalyst influence greatly rates and selectivities of rxns 3) Reactor design associated w/ optimizing performance of catalyst 4) Catalytic Cycle: reactants + catalyst --> complex complex --> products + catalyst |
| Classes of catalyst: (4 P) 1) Heterogeneous (4 P) 2) Homogeneous (4 P) 3) Bio-catalysts (2 P) 4) Phase transfer (2 P) | 1) Range of different active sites / Active site immobilised on solid support / Tuneable selectivity / Easily separated 2) Organometallic complexes widely used/ More active than heterogeneous / High selectivity / Difficult to separate 3) Enzymes, bugs / Highly selective 4) Reagent soluble in separate phase to substrate / use PTC to transfer reagent into organic |
| 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) | 1) No. of rxns a single site can achieve before becoming inactive - max turnovers (assumes 100% yield) = equivalents rectant/equivalents catalyst - actual turnovers = max turnovers x yield 2) No. of rxns per site per unit time - TOF = actual turnovers/time for rxn (hour) |
| 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) | 1) Increasing the surface area (eg. smashing the catalyst), allowing more active sites - especially true for heterogeneous catalysts 2) Changing shape of catalyst (eg making surface rougher) - can create defects on surface at which catalysis can occur, increasing activity |
| Catalytic converter problems: (3 P) 1) Operating temperature (2 MP) 2) Catalyst poisons 3) Use of precious metals (4 MP) | 1) Inactive when cold, activity destroyed if too hot 2) Catalyst poisons (eg Zn in cars etc.) 3) Problems of stability, dispersion, lifetime and elemental sustainability |
| Metal recovery: Phytoremediation - use of plants for environmental clean-up | |
| Type of rxn: 1) General acid catalysis 2) Specific acid catalysis | 1) Rxn catalysed by any acid species (Bronsted and Lewis), generally HA 2) Rxn catalysed by one acidic species only, generally \(H_3O^{+}\) |
| 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 | Specific acid MUST be strong acid |
| Strenghts of acids in... General acid catalysis | General acid CAN be weaker acid, but stronger means more efficient rxn |
| Henderson–Hasselbalch equation (for dilute aqueous solutions) --> large Ka = strong acid --> small pKa = strong acid (inverse) | pH is the standard method for measuring acidity in AQUEOUS systems |
| Mechanism of proton transfer: (3 equilibriums) | |
| Rate of proton transfer 1 | Proton transfer in org. systems can be slower than in aq. The C-H bond breaking process is slow due to bond length & bond angle change |
| Rate of proton transfer 2 | - Nevertheless, diffusion of \(H^{+}\) in water is much faster than diffusion of other ions - There is rapid proton transfer in hydrogen bonded systems in water |
| Rate of proton transfer 3 | |
| Rate of proton transfer 4 --> Intramolecular H-bonding: reduces rate | |
| Rate of proton transfer 5 | rate of proton transfer much slower When electronic reorganisation is required leading to change in bond lengths/angles (eg keto-enol tautomerism) |
| Does NOT affect rate of proton transfer | Electrostatic interactions, eg having \(Na^{+}\) \(Cl^{-}\) in solution (NOTE: but may affect the rxn itself) |
| Proton transfer in polarisable systems | |
| Effect of water on dissociation of Bronsted acids | Order of acidities based on amount of water required for complete dissociation to \(H_5O_2^{+}\) --> more water required = more acidic |
| Ideal conditions mean... (2 P) | - Low acid concentrations (< 1M) - Full dissociation of acid |
| Bronsted acid catalysis means... | - Catalyst source of \(H^{+}\), typically \(H_5O_2^{+}\) in water under ideal conditions, which somehow lowers \(E_a)\) - Substrates are protonated - Rates of rxn increase w/ increase in acidity (slope = 1), up to diffusion control. |
| Bronsted Catalysis Law | Under ideal conditions (dilute aq. solutions), stronger acids increase the rate, but to the limit of diffusion control |
| Bronsted Catalysis Law for non-ideal conditions, ie. non-aqueous & non-protonic aicds | PUT MORE... NON-AQ COND, NON-PROT ACIDS ETC. |
| 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: | - Can replace the pH in concentrated solutions - Measures tendency of a solution to protonate a NEUTRAL base - Superacidity at \(H_O\) \(\geq\) 12 |
| Hammet acidity function plot: (effect of H2O/AH on \(H_O\)) | |
| Solvent ("medium") effects on proton transfer | Do rxn in solvent that helps catalysis (eg hydrolysis of esters in aqueous, to move H+ throughout) |
| 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 | 1) Capability of coordinating with unshared electron pairs 2) Have a vacant orbital which permits coordination of molecules w/ unshared electron pairs 3) Have unshared electron pairs available for donation |
| Lewis acids | - \(M^(n+)\) or similar coordinating to site of high e- density - Non-protonic (eg \(AlCl_3\)), & bulkier (than \(H^+\)), so rxns more subject to steric limitations - Active intermediate is complex with Lewis base, ie. \(BH^+\) - Water can change nature of Lewis acid catalysts - Rate/kinetic plot can be > 1, ie. rates can more than double on doubling the quantity of Lewis acid |
| 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. | - Balance of acid site type (eg through heating) & strength critical for catalyst selectivity - Porosity & high surface area) v. important as het. catalysts are diffusion limited and need to move in & out of micropores |
| 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 | 1) - Quench with water, neutralise HCl, \(Al(OH)_3\) formed, organic product extracted 2) Simply filter --> In both cases, distil to produce the pure product |
| 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) | Want to replace Cr(VI) with a heterogeneous process employing \(O_2\) or \(H_2O_2\) |
| Green oxidations | |
| Things to take care w/ Heterogeneous catalysts (3 P): | - Stability of catalyst - Leaching & product contamination - Selectivity |
| Green processing - The Substrate | - Better use a renewable resource - Care w/ oxygen content of bio-renewables - Care w/ physical state of bio-resource - Care w/ type of reactor |
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