Concepts & Applications Full

Define a catalyst

A catalyst speeds up the rate of reaction by reducing the energy barrier, without impacting the position of equilibrium. A catalyst is not consumed during the reaction.

What is the Arrhenius equation?

k = Ae^(-Ea/RT)

What is the linearised Arrhenius equation?

lnk = lnA - Ea/RT

What is A in the Arrhenius equation?

A describes the collisions: the frequency of collision and the steric dependence for a successful collision

Convert the Eyring equation to include entropy and enthalpy

Linearise the Eyring equation

ln(k/T) = ln(k_bolt/h) - ΔH^‡/RT + ΔS^‡/R

What is the significance of the ^‡ symbol in the Eyring equation?

It shows that we are discussing the entropy and enthalpy of activation, which is used to inform about the transition state.

The equation does not describe the overall reaction entropy/enthalpy.

What do you plot to give a linear Eyring equation graph?

What is the gradient and intercept?

y axis: ln(k/T)

x axis: 1/T

Gradient: -ΔH^‡/R

Intercept: ln(k_bolt/h) + ΔS^‡/R

What do ΔH^‡ and ΔS^‡ in the Eyring equation tell you about the mechanism of a reaction?

A large, positive ΔS^‡ suggests an increase in disorder in the transition state, hence a dissociative mechanism.

A positive ΔH^‡ suggests that there is bond breaking involved. To improve a catalyst you should aim to have a lower ΔH^‡.

What is the equation for TOF?

What is the drawback of TOF?

number of cycles (turnovers) / number of centres x time

It doesn’t consider temperature or concentrations, so you have to be careful when comparing the values.

What is the equation for TON?

What is the benefit of TON over TOF?

moles of product / moles of catalyst

TON accounts for deactivation, gives a more realistic idea of the maximum yield

What is the equation for selectivity?

(quantity of desired products / amount of reactants consumed) x 100

What is the equation for efficiency?

(Mw(desired products) / Mw(all products of the reaction)) x 100

What is the equation for catalytic rate?

What are the units?

converted mol of substate / volume or mass of catalyst x time

molL^-1h^-1 or molkg^-1h^-1

What are the three types of catalyst?

Heterogeneous

Homogeneous

Enzymatic

What are the drawbacks of a heterogeneous catalyst?

• Degradation over time due to leaching, poisoning or sintering

• Can be difficult to control shape or size

• Selectivity is often low

• Activity depends on surface area

What are the benefits of a homogeneous catalyst?

• Often selective and modifiable

• Work at low temperatures and concentrations

• Easier to obtain mechanistic information

• Because the activity is linked to catalyst concentration, its easier to control

What are the drawbacks of a homogeneous catalyst?

• Low thermal stability so often used under 200 degrees

• Difficult to separate (expensive)

What are the benefits of an enzymamtic catalyst?

• Highly specific

• Work at low temperatures and concentrations

• Enzymes can stabilise the transition state

What are the drawbacks of an enzymatic catalyst?

• Can be deactivated by pH change/ extreme temperatures

Explain the impact of using water as a solvent on the rate of catalysis (acid/base)

Protons in hydrogen bonded systems move very rapidly because of their low mass.

This means that proton transfer in acid-base catalysis is very fast.

What is the equation for pH and proton concentration?

pH = -log[H⁺]

What is the first step in a specific acid catalysed process?

The acid protonates a reactant.

This is in equilibrium, so the concentration of the conjugate acid becomes finite.

What is the catalyst in a specific acid catalysis process?

Protonated solvent (which fully transfers the proton the one of the reactants before the rate determining step)

How does the pH impact the rate of reaction in a specific acid catalysed reaction?

The pH ([H⁺]) impacts the position of equilibrium with the protonated reagent.

The reaction rate is then proportional to the concentration of this protonated reagent ([RH⁺]).

Therefore, rate ∝ pH

Define specific acid catalysis

When the substrate (reactant) is protonated by a strong acid prior to the rate determining step.

Define general acid catalysis

When every species that’s able to donate a proton contributes to the reaction rate (not just H₃O⁺)

Write the equation for general acid catalysis rate

rate = k_H[H⁺][AH] + k₀[AH][sol] + k_’0[A^-][sol] + k_’OH[A^-][OH^-]

How is general acid catalysis different to specific acid catalysis?

The proton donor in general acid catalysis is now the undissociated reagent.

This proton transfer is now slow, so is the rate limiting step

Describe this graph and the reasons for its shape

Linear regions below pH 2.5 and above 10.

This is where specific acid catalysis and specific base hydrolysis dominate, so rate is proportional to [H⁺].

The flat region in the middle is due to solvent catalysed reaction - this is flat because the solvent concentration doesn’t change.

How do you spot general acid catalysis vs specific acid catalysis?

The rate of reaction for specific acid catalysis will not change under varying concentrations of buffer, because rate is only dependent on pH.

Changing [buffer] in general acid catalysis will have an impact on rate because you are changing [HA].

Draw how an enzyme can use general acid catalysis to aid ester hydrolysis by water

Draw how an enzyme can use general base catalysis to aid ester hydrolysis by water

Define mirco, meso and macropores

Misopore < 2 nm

Mesopore 2-50 nm

Macropore > 50 nm

Draw a zeolite surface structure and clearly label the Bronsted acid site

What happens to a zeolite surface when you heat it?

Water is lost, and the Bronsted acid site becomes a Lewis acid site.

The Lewis acid site has an empty orbital to accept an electron pair from a Lewis base to form a Lewis adduct.

What are the two ways that a zeolite can produce a carbenium ion from hydrocarbons?

1) The Bronsted acid site can donate a proton to an alkene

2) The Lewis acid site can abstract H^- from an alkane

Give a general description of how zeolites cause catalytic cracking.

The zeolite either adds H+ to an alkene or removes H- from an alkane to give a carbenium ion.

The beta bond to the carbenium ion centre breaks heteolytically.

This gives an alkene and a carbenium ion.

Describe the two processes that a carbenium ion can undergo and why this is important.

It can undergo hydride shift and methyl shift to rearrange.

Methyl shift is slower than hydride shift

This allows the cation to rearrange to give the most stable products (tertiary carbocations).

What are the three types of size selectivity? Give examples of each

1) Reactant selectivity: only linear alkanes are narrow enough to enter a pore so they are the only ones to react

2) Product selectivity: Only the 1,4-dimethyl product is narrow enough to leave the pore

3) Transition State selectivity: The 1,3,5-trisubstituted transiton state is too large for the pore, so only the 1,2,4 product is formed

What does the Ziegler Natta catalyst do?

Alkene polymerisation

Draw the mechanism for alkene polymerisation using a metallocene.

What is the evidence which supports this mechanism?

If you add fully deuterated ethene, there is no deuterium scrambling: The chain is (CD₂CD₂)_n(CH₂CH₂)_n

Give the words beneath the boxes

Isotactic

Syndiotactic

Atactic

How does the choice of transition metal in an alkene polymerisation catalyst impact the rate?

Late transition metals (RHS) are more electron rich so exhibit more backbonding to an alkene.

This makes the alkene more electron rich.

The migrating group is R^-, so increased electron density on the alkene makes the migration slower

What does Mw/Mn tell you about polymer chain length?

High Mw/Mn = wider range of chain lengths

Low Mw/Mn suggests good selectivity (equal chain lengths)

What are the three methods of alkene polymerisation termination?

1) β-H transfer & alkene dissociation

2) Adding H2 (gives M-H and alkane)

3) Adding HX (gives M-X and alkane)

Draw a diagram to depict associative displacement

Draw a mechanism to show chain walking

Name this type of Metathesis

Ring closing metathesis

Name this type of Metathesis

Cross metathesis

Name this type of Metathesis

Ring opening metathesis polymerisation (ROMP)

Name this type of Metathesis

Acyclic diene metathesis polymerisation (ADMET)

Name this type of Metathesis

Enyne metathesis (EYM)

Name this type of Metathesis

Ring-opening cross metathesis (ROCM)

Name this process of H₂ activation

Oxidative addition

Name this process of H₂ activation

Hydrogenolysis

Name this process of H₂ activation

Heterolytic cleavage

What factors of an alkene/alkyne impact the rate of hydrogenation with Wilkinson’s catalyst?

Alkynes bind faster due to the higher electron density of the triple bond.

Electron donating groups (CN, OH) increase the rate of binding (& therefore hydrogenation), while electron withdrawing groups (R) decrease it

Describe transfer hydrogenation.

Draw the mechanism.

When a hydrogen donor (alcohol, amine) gives two protons to the catalyst, which are then transferred to another molecule like a ketone to reduce it to an alcohol

What is alkene hydroformylation?

Alkene + CO + H₂ —> aldehyde

Draw the mechanism for hydroformylation of an alkene using Co(CO)₄(H)

How can changing the loading of a metal on a support change the selectivity of hydrogenation?

Using single Pd atoms on a graphene support reduces the amount of full hydrogenation of a diene.

This is because only one alkene site can bind and react at a time

Describe the difference in bonding between physisorption and chemisorption.

Draw a diagram to depict this.

Physisorption: Van de Waals

Chemisorption: Chemical bonds

What is a monolayer?

When one layer of molecules physisorb onto a catalyst surface

Describe and explain the entropy change that accompanies physisorption

ΔS < 0

For physisorption to be spontaneous, ΔG must be negative.

Therefore, ΔH must be negative (exothermic)

What volume does 1 mole of gas occupy at STP?

22,400 cm³

What are the 3 assumptions made in the Langmuir Isotherm?

1) The surface has a fixed number of identical sites

2) ΔH_ads is independent of coverage

3) Adsorbates do not interact

What is b in the Langmuir isotherm?

What is the equation for b

b is the equilibrium constant of binding of A to the M surface

b = [A-M] / [A][M]

What is the Langmuir Isotherm equation?

where θ is surface coverage

b is the binding equilibrium constant

P is the pressure

What is the linearised Langmuir isotherm equation?

What is the gradient and intercept?

P/n = 1/bn_m + (1/n_m)P

Because θ = n/n_m

Where n_m is the number of sites on the monolayer surface

Plotting P/n against P gives the gradient 1/n_m

The intercept is 1/bn_m

What improvements does the Brunauer-Emmett-Teller (BET) Adsorption Isotherm make on the Langmuir Adsorption isotherm?

What is the limitation of the BET isotherm?

• Allows for multilayer adsorption

• Accounts for different enthalpies of adsorption of multilayer and monolayers

Limitation: Only valid for a limited pressure range

What do you plot when using the BET isotherm?

What is the gradient?

What is the intercept?

Plot P / V(P₀-P) vs P/P₀

Gradient = (c-1) / V_mc

Intercept = 1/V_mc

What is the equation for the rate of surface decomposition?

Rate = kθ

Rate = k(bP / (1+bP))

Discuss how changing the pressure impacts the rate using the rate equation:

If low pressure: bP << 1, so rate = kbP

Therefore, rate is proportional to pressure at low pressures

If high pressure: bP >> 1, so rate = k

Zero order reaction, rate is constant with increasing pressure because the surface is fully occupied

What is a Langmuir-Hinshelwood type reaction?

Give the rate equation

A and B both adsorb, react while adsorbed then desorb as the product.

Rate = kθ_Aθ_B

What is a negative order reaction?

When the rate of reaction relies upon two adsorbed species reacting, so increasing the concentration of one reduces the rate by inhibiting the other from adsorbing.

What is the equation for θ_A in a Langmuir-Hinshelwood reaction?

What does this tell you about A and B surface coverage?

θ_A = b_AP_A / ( 1 + b_AP_A + b_BP_B)

A and B surface coverage are inversely proportional to each other - the binding of one inhibits the other by occupying a binding site.

Using this equation, describe the two limits of a Langmuir Hinshelwood reaction

Limit 1:

Low pressure of A and B: rate = kb_AP_Ab_BP_B

Rate is first order with respect to the pressures of A and B

Limit 2:

Low pressure of A, high pressure of B: rate = kb_AP_A / (b_BP_B)

Rate is first order with respect to A, and negative first order with respect to B

What is an Eley-Rideal type reaction?

Give the rate equation

A adsorbs and B reacts with adsorbed A

rate = kθ_A[B]

Draw a graph plotting rate against θA for both Langmuir-Hinshelwood and Eley-Rideal reactions.

Briefly describe the shape of these plots.

Name a method of activation of the C terminus

Converting to an acid chloride

What is the drawback of C-terminus activation as an acid chloride

Reacts with Boc group to form oxazolone, which tautomerises to give a mixture of products

What is an alternative to activating the C-terminus?

Using a coupling reagent

Give two examples of coupling reagents

DCC and HATU (+Base)

How does DCC activate a peptide? Draw the structure of the intermediate

Produces a good leaving group on the C-terminus

How does HATU activate a peptide? Draw the structure of the intermediate

Adds a good leaving group

What is a protecting group for amines

carbamates

What is a protecting group for carboxylic acids

esters

What are the 3 protecting groups for hydroxyl groups

alkyl ethers

silyl ethers

esters

Give an example of a carbamate n-terminus protecting group and draw its structure

Boc

What are the reagents for adding a boc protecting group

Boc2O and a base (eg. DiPEA)

What are the reagents for removing Boc

Acid (TFA)

What is an alternative to Boc? Draw the structure

Fmoc

What is the deciding factor between Boc and Fmoc?

Boc is stable in basic conditions but removed with acid. Fmoc is stable in acidic conditions but removed with base

Draw the deprotection mechanism for Fmoc

Give 4 examples of ester c-terminus protecting groups and draw their structures

Methyl ester

Benzyl ester

Allyl ester

Tert-butyl ester

What are the reagents for the removal of a methyl ester PG?

NaOH, H2O

What are the reagents for the removal of a benzyl ester PG?

H2, Pd/C

What are the reagents for the removal of an allyl ester PG?

Pd(PPh3)4

What are the reagents for the removal of a tert-butyl ester PG?

TFA