-Naveen KadianM.Pharm. First Year

Department of Pharma Chemistry

K.L.E.’s College Of Pharmacy

Introduction Basis Importance Classification

• Homogeneous catalysis Mechanism Example

• Heterogeneous catalysis Mechanism Examples

Promoters Catalytic Poisoning Autocatalysis Enzyme catalysis

• Enzymes References

Catalyst: -The substances that alter the rate of a reaction but itself remains chemically unchanged at the end of the reaction is called a Catalyst.

The process is called Catalysis.

WITHOUT A CATALYST WITH A CATALYST

A catalyst lower the activation barrier for a transformation, by introducing a new reaction pathway

Examples of CatalystsExamples of Catalysts

Metals Ni, Pt hydrogenation reactions

Fe Haber Process Rh, Pdcatalytic converters

Oxides Al2O3 dehydration reactions

V2O5 Contact Process

Format FINELY DIVIDED increases the surface areaprovides more collision sites

IN A SUPPORT MEDIUM maximises surface area and reduces costs

There are two types of catalytic processes: -1. Homogeneous catalysis2. Heterogeneous catalysis These two processes have industrial importance.

There is another mechanism involving catalysis i.e. enzyme catalysis which possess biological importance.

Homogeneous CatalysisHomogeneous CatalysisAction • catalyst and reactants are in the same phase

• the catalyst is evenly distributed throughout.

• reaction proceeds through an intermediate species of lower energy

• there is usually more than one reaction step

• transition metal ions are often involved - oxidation state changes

Example

Acids Esterificaton

Conc. H2SO4 catalyses the reaction between acids and alcohols

CH3COOH + C2H5OH CH3COOC2H5 + H2O

NB Catalysts have NO EFFECT ON THE POSITION OF EQUILIBRIUM

but they do affect the rate at which equilibrium is reached

Transition metal compounds participate in homogeneous catalysis as they have the ability to change their oxidation states.

1. Reaction between iron(III) and vanadium(III)

The reaction is catalysed by Cu2+

step 1 Cu2+ + V3+ ——> Cu+ + V4+

step 2 Fe3+ + Cu+ ——> Fe2+ + Cu2+

overall Fe3+ + V3+ ——> Fe2+ + V4+

Catalyst is in different physical phase from the reactants.

It is also called Contact catalysis. It possesses great industrial importance.

It works in three steps: -- Adsorption- Reaction- Desorption

Heterogeneous CatalysisHeterogeneous Catalysis

Adsorption (STEP 1)Incoming species lands on an active site and forms bonds with the catalyst. It may use some of the bonding electrons in the molecules thus weakening them and making a subsequent reaction easier.

Heterogeneous CatalysisHeterogeneous Catalysis

Adsorption (STEP 1)Incoming species lands on an active site and forms bonds with the catalyst. It may use some of the bonding electrons in the molecules thus weakening them and making a subsequent reaction easier.Reaction (STEPS 2 and 3)Adsorbed gases may be held on the surface in just the right orientation for a reaction to occur.This increases the chances of favourable collisions taking place.

Heterogeneous CatalysisHeterogeneous Catalysis

Desorption (STEP 4)There is a re-arrangement of electrons and the products are then released from the active sites

Adsorption (STEP 1)Incoming species lands on an active site and forms bonds with the catalyst. It may use some of the bonding electrons in the molecules thus weakening them and making a subsequent reaction easier.

Reaction (STEPS 2 and 3)Adsorbed gases may be held on the surface in just the right orientation for a reaction to occur.This increases the chances of favourable collisions taking place.

Heterogeneous CatalysisHeterogeneous Catalysis

Desorption (STEP 4)There is a re-arrangement of electrons and the products are then released from the active sites

Adsorption (STEP 1)Incoming species lands on an active site and forms bonds with the catalyst. It may use some of the bonding electrons in the molecules thus weakening them and making a subsequent reaction easier.

Reaction (STEPS 2 and 3)Adsorbed gases may be held on the surface in just the right orientation for a reaction to occur.This increases the chances of favourable collisions taking place.

Examples of Heterogeneous Catalysis: -

1. Gas Phase2SO2 + O2 + [Pt] 2SO3 + [Pt]

2. Liquid PhaseH2O2 + [Pt] 2 H2O + O2 + [Pt]

3. Solid Phase2KClO3 + [MnO2] 2KCl + 3O2 + [MnO2]

A substance which, though itself not a catalyst, promotes the activity of a catalyst is called a Promoter.

Example: -

N2 + 3H2 2NH3

Explanation of Promotion Action

1. Change of Lattice Space: The lattice spacing of the catalyst is changed thus enhancing the spacing between the catalyst particles. The adsorbed molecules of the reactant are further weakened and cleaved. This makes the reaction go faster.

2. Increase in peaks and cracks: Promoters increase the peaks and cracks on the surface of the catalyst thereby increasing the concentration of reactant molecules and hence the rate of reaction.

A substance which destroys the activity of the catalyst to accelerate a reaction, is called a poison and the process is called Catalytic Poisoning.

Example: -2SO2 + O2 + [Pt] 2SO3

This is poisoned by As2O3

Explanation of Catalytic Poisoning

1. The poison is adsorbed on the catalyst surface in preference to the reactants.

2. The catalyst may combine chemically with the impurity.

Fe + H2S FeS + H2

When one of the products of a reaction itself acts as a catalyst for that reaction the phenomenon is called autocatalysis.

Examples of autocatalysis: -(a) Hydrolysis of an ester

CH3COC2H5 + H2O CH3COOH + C2H5OH

Here CH3COOH is acting as a catalyst.

When a catalyst reduces the rate of reaction, it is called a Negative catalyst or Inhibitor.

A negative catalyst is used to slow down or stop altogether an unwanted reaction.

Example: -4CHCl3 + 3O2 4COCl2 + 2H2O + 2Cl2

Chloroform (anaesthetic) on oxidation by air forms carbonyl chloride (poisonous).Ethanol when added to chloroform acts as a negative catalyst.

Explanation of Negative Catalysis: -

1.By poisoning a catalyst.2.By breaking a chain reaction.

Cl2 Cl. + Cl. H2 + Cl. HCl + H. H. + Cl2 HCl + Cl.

NCl3 breaks the chain of reactions by absorbing Cl. and the reaction stops.

NCl3 + Cl. ½ N2 + 2Cl2

Importance of catalysis

Many major industrial chemicals are prepared with the aid of catalysts. Many fine chemicals are also made with the aid of catalysts.

– Reduce cost of production– Lead to better selectivity and less

waste

Activity is affected by ...

temperature - it increases until the protein is denatured

substrate concentration - reaches a maximum when all sites are blocked

pH - many catalysts are amino acids which can be protonated

being poisoned - when the active sites become “clogged” with unwanted

ENZYMESActionAction enzymes are extremely effective biologically active catalysts

they are homogeneous catalysts, reacting in solution with body fluidsonly one type of molecule will fit the active site “lock and key”

mechanismmakes enzymes very specific as to what they catalyse.

MECHANISM OF ENZYME ACTIONMECHANISM OF ENZYME ACTION

AA B B C C

AA Only species with the correct shape can enter the active site in the enzyme

BB Once in position, the substrate can react with a lower activation

energy

CC The new products do not have the correct shape to fit so the

complex breaks up

ENZYMESENZYMES

ANIMATED ACTIONANIMATED ACTION

AA Only species with the correct shape can enter the active site in the enzyme

BB Once in position, the substrate can react with a lower activation energy

CC The new products do not have the correct shape to fit so the complex breaks up

The catalysis brought about by enzymes is known as Enzyme catalysis.

Examples-1. Inversion of cane sugar

C12H22O11+H2oinvertase

C6H12O6+ C6H12O6GLUCOSE FRUCTOSE

C12H22O11ZYMASE +C2H5OH CO2

ETHANOL

2. Conversion of glucose to ethanol

3. Hydrolysis of urea

NH2C

O

NH2 + H2urease

NH3+ co2