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CHEMICAL REACTIONS:

BRINGING MOLECULES TO LIFE1

A chemical reaction is a process that leads to

the transformation of one set of chemical

substances to another.

Classically, chemical reactions encompass

changes that strictly involve the motion of

electrons in the breaking (for reactants) and

forming (for products) of chemical bonds

between atoms.

2

A reaction could involve:

✓ ions,

✓ compounds,

✓ molecules of a single element.

3

The substances (or substance) initially involved in a

chemical reaction are called reactants or reagents.

Substances which are formed after chemical reaction are

called products.

Products usually have properties different from the

reactants. The chemical reaction accompanies by

chemical change; pure hydrogen gas (H2) and pure

oxygen gas (O2) form water liquid (H2O); an iron (Fe)

from steel garbage reacts with gaseous oxygen (O2)

from air and forms iron trioxide (Fe2O3) (rust).

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CHEMICAL VS. PHYSICAL CHANGE

What is the chemical change?

In the chemical change the reacting substances form

new entities that have different formulas and

different properties. New properties may involve a

change in colour or formation of gas (bubbles), or a

solid product.

When a silver tarnishes, the bright silver metal (Ag)

reacts with sulphur (S) to become the blackish

substance we call tarnish (AgS).

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PHYSICAL CHANGE

In the physical change the appearance of a substance

is altered but not its composition. When water

becomes a gas, or freezes to a solid it is still a water.

If we smash a rock or tear a piece of paper, only size

of the material changes.

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When a refrigerator or air conditioner cools

the air, there is no reaction between the air

molecules. When you melt an ice cube, it is a

physical change. The change in temperature is a

physical change.

When you put bleach in the washing machine to

clean your clothes, a chemical change breaks up

your stains.

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QUIZ. CHEMICAL VS. PHYSICAL

CHANGES

8

Among the presented below changes

choose chemical: rusting a nail, pouring a

milk, souring a milk, boiling a water,

fermenting grapes, dissolving Alka-Seltzer

tablet in a glass of water.

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CHEMICAL EQUATION

A chemical equation is a symbolic representation of a

chemical reaction. Atomic symbols are used to

represent the elements that take part in a reaction.

Numbers are used to denote the ratios of reactants

and products applied in the reaction.

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Chemical reaction vs. chemical equation

A chemical reaction is the process that occurs when one

or more substances are changed into one or more new

substances.

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For example:

➢Sodium chloride (table salt) dissociates in water to form

sodium and chlorine ions.

➢Methane combusts in oxygen to form carbon dioxide,

heat and water.

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Whereas a chemical equation is the symbolic

representation of a chemical reaction where the

reactant entities are given on the left hand side and

the product entities on the right hand side.

A + B →C + D

ReactantsProducts

12

The tip of the arrow points in the direction in which

the reaction proceeds.

Usually the arrow is from reactants to products.

A double arrow (↔) pointing in opposite directions is

used for equilibrium reactions.

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1.

A) CH3COOH → CH3COO- + H+

B) CH3COOH ↔ CH3COO- + H+

C) CH3COOH ← CH3COO- + H+

2.

A) NaOH + HCl ← NaCl + H2O

B) NaOH + HCl ↔NaCl + H2O

C) NaOH + HCl → NaCl + H2O

13

Find the answer containing the proper

direction of tips in chemical reaction: BALANCING CHEMICAL REACTIONS

Equations should be balanced according to the

stoichiometry, the number of atoms of each

species should be the same on both sides of the

equation.

It is according to the mass action law

(Guldberg-Waage, law). 14

Two important principles apply when we write balanced

equations for reactions between species in a solution:

the balanced equation only includes the species that

participate in forming of products

the total charge must be the same on both sides of a

balanced equation.

The total charge can be zero or non-zero, the charge of

both the reactant and product sides of the equation is

the same. 15 16

For example, using for the above chemical reactions:

2 H2(g) + O2(g) → 2 H2O(ℓ)

NaCl(s) + H2O → Na+(aq) + Cl- (aq)

K2SO4+ BaCl2 → 2KCl + BaSO4

2K+ + SO4-2 +Ba2++ 2Cl-→ 2K+ +BaSO4+ 2Cl-

SO4-2 +Ba2+→ BaSO4

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15 16

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WRITE THE CHEMICAL REACTION IN THE IONIC

FORM AND CANCEL SOLUTES WHICH DO NOT

REACT

AgNO3(aq) + NaCl(aq) → AgCl + NaNO3(aq)

Ag++ Cl-→AgCl↓

Pb(NO3)2(aq) + K2SO4(aq) → PbSO4 + 2 KNO3(aq)

Ba(NO3)2(aq)+ Na2SO4→ BaSO4 + 2 NaNO3

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Pb2+ + SO42-→ PbSO4 ↓

ADDITIONAL WORDS CONNECTING WITH CHEMICAL

REACTION

chemical reaction rate - the speed at which a

chemical reaction occurs (see lecture on kinetics)

activation energy - the external energy that has to

be added in order for a chemical reaction to occur

(see lecture on kinetics)

catalyst - a substance that helps a chemical reaction

to occur (lowers the activation energy), but does not

participate in the reaction itself 18

Law of Conservation of Mass - states that matter

(mass) is neither created nor destroyed in a chemical

reaction. (thermodynamics)

Thus number of reactant atoms = number of product

atoms in a chemical reaction

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Chemical reactions happen at a characteristic reaction

rate at a given temperature and chemical concentration

(kinetics). Rapid reactions are often described as

spontaneous, requiring no input of extra energy other

than thermal energy.

Non-spontaneous reactions run so slowly that they are

considered to require the input of some type of

additional energy (extra heat, light or electricity) in

order to proceed to completion (chemical equilibrium)

at human time scales.20

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More elaborate reactions are represented by

reaction schemes, which in addition to starting

materials and products show important intermediates

or transition states.

2FeCl3 + 6 NH4OH →2Fe(OH)3 + 6NH4Cl

↓ temp

Fe2O3

21 22

Some relatively minor additions to the reaction can be

indicated above the reaction arrow. Examples of such

additions are: water, heat, illumination, a catalyst, etc.

2CO + 2NO N2 + 2CO2

Similarly, some minor products can be placed below

the arrow, often with a minus sign.

C2H5OH + HCOOH HCOOC2H5- H2O

Heat,

catalyst

TYPES OF THE REACTION

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An elementary reaction is the smallest division into

which a chemical reaction can be decomposed to.

It has no intermediate products.

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Elementary reaction

Reactant →product

This is a sample reaction coordinate of an elementary reaction.

Note that there is one transition state and no intermediates.

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The actual sequence of the individual

elementary reaction is known as reaction

mechanism.

Example overall reaction is:

HCOOH → CO + H2O

Mechanism

Step 1: HCOOH + H+ → HCOOH2+ (fast)

Step 2: HCOOH2+ → H2O + HCO+ (slow)

Step 3: HCO+ → CO + H+ (fast)

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ELEMENTARY REACTION VS SERIES OF

REACTIONS

Single reactions often happen as part of a

larger series of reactions.

An elementary reaction is assumed to

occur in a single step and to pass through

a single transition state.

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Complex reaction

Reactant → intermediates → products

This is a sample reaction coordinate of a complex reaction.

Note that it involves an intermediate and multiple transition

states. Again, a complex reaction can be explained in terms

of elementary reactions.http://chemwiki.ucdavis.edu/Physical_Chemistry/Kinetics/Rate_Laws/Reactions/Elementary_Re

actions

ELEMENTARY REACTIONS

An unimolecular elementary reaction is the type of

reaction in which a molecule A dissociates or

isomerises to form the product(s)

➢ N2O4(aq)→ 2 NO2(aq)

Whereas in a bimolecular elementary reaction, two

atoms, molecules, ions A and B, react together to

form the product(s)

➢ HF (aq) + NH3(aq)→ F-(aq) + NH4

+(aq)

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QUIZ

Choose the correct statements.

a. An elementary step has 0 intermediates.

b. An elementary step has 1 intermediate.

c. An elementary step has 2 intermediates.

d. An elementary step has 0 transition states.

e. An elementary step has 1 transition state.

f. An elementary step has 2 transition states.

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QUIZ

Which of the presented above reaction is/are

unimolecular and which is/are bimolecular?

A) O3 → O2 + O

B) O3 + NO→ NO2 + O2

C)

D)

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CH2

CH2CH2

C C

CH3H

H H

CHAIN REACTION

A chain reaction is a sequence of reactions where a

reactive product or by-product causes additional

reactions to take place.

Chain reactions are one way in which systems which

are in thermodynamic non-equilibrium can release

energy or increase entropy in order to reach a state

of higher entropy. 31 32

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33http://en.wikipedia.org/wiki/Proton%E2%80%93proton_chain_reaction 34

QUIZZES

QUESTION 1:

Chemical reactions create and break down...

A) proton orbitals; B) chemical bonds; C) atomic

nuclei; D) none of the above

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o QUESTION 2:

Reactions start with products and create reactants.

A) True; b) False

True or false

QUESTION 3:

All elements can bond with all other elements.

36

QUESTION 4:

During reactions, both the molecules and state of matter

can change.

QUESTION 5:

Equilibrium values are only for molecules in the same

state of matter.

Examples: Liquids reacting with liquids or solids

reacting with solids.

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37

QUESTION 6:

Stoichiometry is a long word that describes...

A) The numer of atoms in the reactant equalling the numer of

atoms in the products.

B) The proces of breaking down waxes into fatty acids

C) The proces of electrons jumping from one orbital to another

D) None of the above

QUESTION 7:

Catalysts ___________ reactions.

A) speed up; B)slow down; C)neither choice

QUESTION 8:

Reactions usually require energy to occur.

True or false

VARIOUS TYPES OF CHEMICAL REACTIONS

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Representation of four basic chemical reactions

types: synthesis, decomposition, single replacement

and double replacement.

SYNTHESIS (COMBINATION) REACTION

Direct Combination or Synthesis Reaction

In a synthesis reaction two or more chemical species

combine to form a more complex product.

A + B → AB

The combination of iron and sulfur to form iron (II)

sulfide is an example of a synthesis reaction:

8 Fe + S8 → 8 FeS

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EXAMPLES OF SYNTHESIS REACTION

2 CO(g) + O2(g) → 2CO2(g)

3 H2(g) + N2(g) → 2 NH3(g)

4 Al(s) + 3 O2(g) → 2 Al2O3(s)

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DECOMPOSITION (ANALYSIS) REACTION

Chemical Decomposition or Analysis Reaction

In a decomposition reaction a compound is broken

into smaller chemical species.

AB → A + B

The electrolysis of water into oxygen and hydrogen gas

is an example of a decomposition reaction:

2 H2O → 2 H2 + O2

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EXAMPLES OF DECOMPOSITION

2 KCl(s) → 2 K(s) + Cl2(g)

2Cl2O5(g) → 2Cl2(g) + 5 O2(g)

2SO3(g)→ 2SO2(g) + O2(g)

Mg(NO3)2 →Mg(NO2)2 +O2(g)

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SINGLE DISPLACEMENT REACTION

Single Displacement or Substitution Reaction

A substitution or single displacement reaction is

characterized by one element being displaced from a

compound by another element.

A + BC → AC + B

An example of a substitution reaction occurs when

zinc combines with hydrochloric acid. The zinc replaces

the hydrogen:

Zn + 2 HCl → ZnCl2 + H244

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SINGLE DISPLACEMENT REACTION

Fe + CuSO4→ FeSO4 + Cu

2AgNO3 + Zn → Zn(NO3)2 + 2Ag

Mg + 2HCl → MgCl2 + H2

2NaI + Cl2→ NaCl + I2

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METATHESIS

Metathesis or Double Displacement Reaction

In a double displacement or metathesis reaction two

compounds or ions exchange bonds in order to form

different compounds.

AB + CD → AD + CB

An example of a double displacement reaction occurs

between sodium chloride and silver nitrate to form

sodium nitrate and silver chloride.

NaCl(aq) + AgNO3(aq) → NaNO3(aq) + AgCl(s)46

DOUBLE DISPLACEMENT REACTION

2NaOH + CuSO4→ Cu(OH)2 + Na2SO4

2 KCl + H2SO4→ K2SO4 + 2HCl

Na2CO3 + FeSO4 → FeCO3 + Na2SO4

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ACID-BASE REACTION, NEUTRALIZATION

Acid-Base Reaction

An acid-base reaction is type of double displacement

reaction that occurs between an acid and a base.

The H+ ions from the acid reacts with the OH- ions

from the base, they form water and an ionic salt:

HA + BOH → H2O + BA

The reaction between hydrobromic acid (HBr) and

sodium hydroxide is an example of an acid-base

reaction:

HBr + NaOH → NaBr + H2O 48

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RED-OX REACTION

Oxidation-Reduction or Redox Reaction

In a redox reaction the oxidation numbers of atoms

are changed. Redox reactions may involve the transfer

of electrons between chemical species.

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OXIDATION-REDUCTION REACTIONS

The species that loses electrons is said to be oxidized.

The species that gains electrons is said to be reduced.

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E. g. reaction between hydrochloric acid and zinc metal,

where the Zn atoms lose electrons and are oxidized to

form Zn2+ ions:

Zn(s) → Zn2+(aq) + 2e-

The H+ ions of the HCl gain electrons and are reduced to

H atoms, which combine to form H2 molecules:

2H+(aq) + 2e- → H2(g)

The overall equation for the reaction becomes:

Zn(s) + 2H+(aq) → Zn2+(aq) + H2(g)

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ZN + 2HCL → ZNCL2 +H2

The presented above reaction is the reaction of

(more than one answer is true):

1. single replacement

2. double replacement

3. neutralization

4. synthesis

5. red-ox

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COMBUSTION

A combustion reaction is a type of redox reaction in

which a combustible material combines with an oxidizer

to form oxidized products and generate heat

(exothermic reaction).

Usually in a combustion reaction oxygen combines with

another compound to form carbon dioxide and water.

An example of a combustion reaction is the burning of

naphthalene:

C10H8 + 12 O2 → 10 CO2 + 4 H2O + Q54

COMBUSTION REACTIONS

combustion of methane

CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(g)

burning of benzene

C6H6 + 15 O2 → 6 CO2 + 3 H2O

combustion of ethane

2 C2H6 + 7 O2 → 4 CO2 + 6 H2O

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QUESTION 9:

Which of these is a type of reaction?

A) combustion; B) substitution; c) combination; d) all

of the above

Question 10

Which types of reaction are presented in the

following example:

1) 2Mg + O2 → 2MgO

2) 2FeO + C → 2Fe + CO2

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OTHER REACTIONS

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Isomerization

In an isomerization reaction, the structural arrangement

of a compound is changed but its net atomic

composition remains the same.

OP

OH

OH

OH O

OH OP

OH

OH

OHOH

O

glucose-6

-phosphate

fructose-6

-phosphate

C6H10O10PC6H10O10P

58

N N

hv

hv'/d

N

N

trans azobenzol cis-azobenzol

59 60

http://chemwiki.ucdavis.edu/Organic_Chemistry/Organic_Chemistry_With_a_Biologi

cal_Emphasis/Chapter_13%3A_Reactions_with_stabilized_carbanion_intermediate

s_I/Section_13.2%3A_Isomerization_reactions

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HYDROLYSIS REACTION

Hydrolysis is a type of decomposition reaction where one reactant is water.

The general formula of a hydrolysis reaction is:

AB + H2O → AH + BOH

In organic hydrolysis reactions water combines and e.g. an ester. This reaction follows the general formula:

RCO-OR' + H2O→ RCO-OH + R'-OH

the dash denotes the covalent bond that is broken during the reaction. 61 62

CH3COONa + H2O → CH3COOH + Na+ + OH-

O

R X+ OH2

R

O

OH+ XH

acyl halide carboxylic acid

hydrolysis

Complexation

In complexation reactions, several ligands react with a metal

atom (coordinative centre) to form a coordination complex. This

is achieved by providing lone pairs of the ligand into empty

orbitals of the metal atom and forming dipolar bonds

(coordinate covalent bonds). The ligands are Lewis bases, they

can be both ions and neutral molecules, such as, chloride ions,

carbon monoxide, ammonia or water.

Cu2+ + 4 NH3 → Cu(NH3)42+

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Empty valence orbital

Lone electron pair of nitrogen64

A complex of iron and benzene A heam structure

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65Cisplatin PtCl2(NH3)2

REACTIONS IN WATER OR AQUEOUS SOLUTION

Several types of reactions occur in water.

When water is the solvent for a reaction. It is said to

occur in aqueous solution, which is denoted by the

abbreviation (aq) following the name of a chemical

species.

Three important types of reactions in water are

precipitation, acid-base, and oxidation-

reduction. 68

PRECIPITATION REACTIONS

In a precipitation reaction, an anion and a cation

contact each other and an insoluble ionic compound

precipitated out of solution.

For example, when aqueous solutions of silver nitrate,

AgNO3, and salt, NaCl, are mixed, the Ag+ and Cl-

combine to yield a white precipitate of silver chloride,

AgCl:

Ag+(aq) + Cl-(aq) → AgCl(s)

Ba2+ (aq) + SO42-(aq) → BaSO4(s)

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ACID-BASE REACTIONS

H+(aq) + OH-(aq) → H2O

For example, when hydrochloric acid, HCl, and sodium

hydroxide, NaOH, are mixed, the H+ reacts with the

OH- to form water

HCl(aq) +NaOH(aq) →NaCl(aq) + H2O

HCl acts as an acid by donating H+ ions or protons

and NaOH acts as a base, furnishing OH- ions.

CH3COOH(aq)+ NaOH(aq)→ CH3COONa(aq) + H2O

Hydrogen ions from acetic acid reacts with hydroxide

ions from sodium hydroxide react to produce water

and salt. 70

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BIOCHEMICAL REACTIONS

71 72

Biochemical reactions are all chemical reactions

(processes) which take place within the living body at

cellular level. They could be catabolic (exothermic) or

anabolic (endothermic).

e.g. photosynthesis or respiration

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All biochemical reactions are mainly controlled by

enzymes. These proteins can specifically catalyse a

single reaction, so that reactions can be controlled very

precisely.

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http://www.ck12.org/na/Biochemical-Reactions-

4/lesson/user%3Aa2JyYWRidXJuQGxvdmV0dC5vcmc./Biochemical-Reactions-%253A%253Aof%253A%253A-

The-Chemistry-of-Life/

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ENZYME AT WORK

75 76

MICHAELIS–MENTEN KINETICS IS ONE OF THE

SIMPLEST AND BEST-KNOWN MODELS OF ENZYME

KINETICS

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Max velocity of

reaction

From this point

increase of

concentation does

not influence on

rate

KM = concentration of

substrate at which the enzyme

operates at the velocity equal

to ½ Vmax

MICHAELIS CONSTANT, KM

Where [E] – enzyme concentration

[S] –concentration of substrate

[ES] – concentration of enzyme-substrate complex

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79

KM = Michaelis constant = is the concentration of substance

that is required for an enzyme to operate at one-half maximum

velocity.

The value of KM tells us about the strength of the bond

between substrate and enzyme at its active site.

KM for different enzymes varies from as low as 10-8M to around

1.0 M.

Some enzymes require low concentration of substrate to

operate at half their maximum velocity, whereas some require a

much higher concentration.

.

KM

When KM is small the substrate binds strongly to the

active site and the substance is converted readily to

product at low concentration.

A large KM value tells us that the enzyme binds weakly

to its substrate and needs a high concentration of

substrate to effectively catalyse the formation of

product. 80

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Enzyme KM (M)

Carbonic anhydrase 2.6 × 10−2

Chymotripsin 1.5 × 10−2

Fumarase 5.0 × 10−6

Pepsin 3.0 × 10−4

Ribonuclease 7.9 × 10−3

Tyrosyl-tRNA synthetase 9.0 × 10−4

82

The factors that affect KM are:

•pH

•temperature

•ionic strengths

•the nature of the substrate

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83

http://www.worthington-biochem.com/introbiochem/tempEffects.html

84

The optimum pH value will vary greatly from one

enzyme to another

Table: pH for Optimum Activity

Enzyme pH Optimum

Lipase (pancreas) 8.0

Lipase (stomach) 4.0 - 5.0

Lipase (castor oil) 4.7

Pepsin 1.5 - 1.6

Trypsin 7.8 - 8.7

Urease 7.0

Invertase 4.5

Maltase 6.1 - 6.8

Amylase (pancreas) 6.7 - 7.0

Amylase (malt) 4.6 - 5.2

Catalase 7.0

85

Effect of ionic strength of buffer on enzyme activity.

https://www.researchgate.net/profile/Rachna_Goswa

mi2

BREAKING AND FORMING OF COVALENT

BONDS

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REDISTRIBUTION OF VALENCE ELECTRONS

Moving of valence electrons during chemical reactions:

✓ A pair of valence electrons move together remaining as a pair

✓ A pair of valence electrons splits up and they then move

separately, as single utilities

This first process is called heterolytic bond cleavage (fission) ,

whereas the latter one- homolytic.

Greek –lytic =lysis = split

Hetero = different

Homo = the same

Heterolytic = different way of splitting

Homolytic = the same way of splitting87

HETEROLYTIC VS HOMOLYTIC BOND

CLEAVAGE

Heterolytic cleavage occurs when the valence electrons

are distributed differently (unequally) between two

atoms once a bond joining the atoms has been broken.

Homolytic cleavage occurs when the valence electrons

are distributed in the same way (equally) between the

two atoms when the bond is broken.

88

HETEROLYTIC REACTION

Process of bond cleavage (for reactant) and formation of the new

one (for products) involves valence electron pair movement.

For heterolytic reaction should be described:

1. a source of a valence electron pair

2. an atom that can accept, and subsequently share, the electron

pair

89

NUCLEOPHILE

A nucleophile – an electron-rich species, which has a valence

electron pair that can be donated to form covalent bond

Groups of nucleophiles:

➢ molecules or ions with lone pairs of electrons

➢ molecules with double (or triple) bonds

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91

http://www.chem.memphis.edu/parrill/chem3311/2003/NucElec.pdf

ELECTROPHILE

An electrophile is an electron-poor (electron-deficient) species,

which can accept a complete electron pair and share it with

the nucleophile as a covalent bond.

For the electrophile to be able to accept a complete electron

pair and form a covalent bond the electrophile must possess

(or able to possess) an empty orbital.

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95

http://www.chem.memphis.edu/parrill/chem3311/2003/NucElec.pdf

OXIDATION AND REDUCTION AS EXAMPLE

OF HETEROLYTIC CLEAVAGE

97

Nucleophile donates an electron pair to an electrophile.

So the former entity loses a pair of electrons, while the

latter simultaneously gains a pair of electrons.

Nucleophile Fe(S)→Fe2+(aq) + 2e

Electrophile H2O(l) + ½ O2(aq) + 2e → 2 OH-( aq)

Fe(s) + H2O(l) + ½ O2(aq)→ Fe(OH)2(aq)

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98

Electrons always flow from nucleophile to electrophile:

Example 1:

Using the curved arrows shown below, label each reactant as a nucleophile or electrophile.

IDENTIFY THE NUCLEOPHILES AND

ELECTROPHILES IN EACH MECHANISM STEP

SHOWN BELOW.

99

HOMOLYTIC REACTIONS

Involve equal splitting of valence electron pair between two

previously joined atoms.

The product of homolytic bond cleavage is a special chemical

species called free radical. These species are characterized by

having one unpaired electron in its valence shell.

The unpaired electrons make a free radical:

➢ extremely reactive

➢ unstable

➢ extremely dangerous. 100 101

-CH2 -

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RADICAL REACTIONS

feature three distinct stages:

➢ Initiation

➢ Propagation

➢ Termination

102

INITIATION

The stage in a homolytic reaction during which free-radical

species are first formed.

103

One of the best ways of forming free radicals is by irradiating

with electromagnetic radiation in the ultraviolet region of the

electromagnetic spectrum. The energy of this part of light

spectrum is sufficient to break certain covalent bond

(e.g. Cl-Cl in Cl2).

INITIATIONS

When methane and chlorine are mixed in the dark,

nothing happened, the two compounds fail to react.

These two species react via free-radical reaction which

can only occur when there is sunlight to initiate free-

radical formation.

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102 103

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PROPAGATION

The only way that free radical can become stable is for it to lose

an electron to gain an empty orbital or gain a further electron to

pair up with its existing unpaired electron. This process generates

a new free-radicals. It propagates free-radical formation.

Propagation is the continued generation of free-radicals in a

sequential manner. It may lead to the setting up of a chain

reaction.

106 107

TERMINATION

Free-radical reactions do not continue

indefinitely. Termination of a free-radical reaction

occurs when two free radicals collide, and

combine to form a stable molecule.

The termination is the ending of a chain

reaction.

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112

Destruction of the ozone layer by CFC radicalsFREE RADICALS IN BIOLOGICAL SYSTEMS

Free radicals react with water molecule in the following

stages:

water + free radical → hydroxyl ( radical)

2 Hydroxyl → hydrogen peroxide

Hydrogen peroxide →superoxide ( radical)

Superoxide →O2

When hydroxyl radical reacts with a protein, lipid, or DNA molecule it snatches an electron to itself and then sinks back into the sublime chemical stability of water. But this process leads to formation of reactantmolecules short of the electron (molecules become freeradical).

Free radicals damage of DNA causes genetic mutationand leads to an increased disposition to cancer.

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O2

-*

*OH

The free radical reaction are an inevitable by-

product of respiration.

The propagation of free-radical reactions can be

inhibited by a group of molecules called

antioxidant (e.g. Vitamins C and E). They prevent

the oxidation associated with the activity of free

radicals.

114

QUIZ

115

Imagine the reaction below is part of the free radical

substitution mechanism that occurs when chlorine and

methane react at room temperature and pressure and

classify the reaction accordingly.

a) initiation; b) propagation; c) termination; d) none of above

1

2

3

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