1. An Overview of Organic Reactions
Why and how chemical reactions take place?
How a reaction can be described?

2. Kinds of Organic Reactions
Addition reactions – two molecules combine

3. Elimination reactions – one molecule splits into two

4. Substitution – parts from two molecules exchange

5. Rearrangement reactions – a molecule undergoes changes in the way its atoms are connected

6. How Organic Reactions Occur: Mechanisms
In a clock the hands move but the mechanism behind the face is what causes the movement
In an organic reaction, we see the transformation that has occurred. The mechanism describes the steps behind the changes that we can observe
Reactions occur in defined steps that lead from reactant to product

7. Steps in Mechanisms We classify the types of steps in a sequence
A step involves either the formation or breaking of a covalent bond
Steps can occur individually or in combination with other steps
When several steps occur at the same time they are said to be concerted

8. Types of Steps in Reaction Mechanisms
Bond formation or breaking can be symmetrical or unsymetrical
Symmetrical - homolytic
Unsymmetrical - heterolytic

9. Indicating Steps in Mechanisms
Curved arrows indicate breaking and forming of bonds
Arrowheads with a “half” head (“fish-hook”) indicate homolytic and homogenic steps (called ‘radical processes’)
Arrowheads with a complete head indicate heterolytic and heterogenic steps (called ‘polar processes’)

10. Radical Reactions
A radical can break a bond in another molecule and abstract a partner with an electron, giving substitution in the original molecule

11. Steps in Radical Substitution
Initiation – homolytic formation of two reactive species with unpaired electrons
Propagation – reaction with molecule to generate radical
Termination – combination of two radicals to form a stable product:

12. Polar Reactions
Molecules can contain local unsymmetrical electron distributions due to differences in electronegativities
This causes a partial negative charge on an atom and a compensating partial positive charge on an adjacent atom
The more electronegative atom has the greater electron density
Elements such as O, F, N, Cl are more electronegative than carbon

14. Polarity patterns in some functional groups

15. Polarizability is the tendency to undergo polarization
Polar reactions occur between regions of high electron
density and regions of low electron density
Polarizability is the tendency to undergo polarization

16. Polar Reactions
An electrophile, an electron-poor species, combines with a nucleophile, an electron-rich species
An electrophile is a Lewis acid
A nucleophile is a Lewis base

17. Some nucleophiles and electrophiles

18. An Example of a Polar Reaction: Addition of HBr to Ethylene
HBr adds to the  part of C-C double bond
The  bond is electron-rich, allowing it to function as a nucleophile
H-Br is electron deficient at the H since Br is much more electronegative, making HBr an electrophile

19. Addition of HBr to Ethylene - Mechanism

20. Using Curved Arrows in Polar Reaction Mechanisms
Curved arrows are a way to keep track of changes in bonding in polar reaction
The arrows track “electron movement”
Electrons always move in pairs
Charges change during the reaction
One curved arrow corresponds to one step in a reaction mechanism
The arrow goes from the nucleophilic reaction site to the electrophilic reaction site

21. The nucleophilic site can be neutral or negatively charged

22. The electrophilic site can be neutral or positively charged

23. Describing a Reaction: Equilibria
Reactions can go either forward or backward to reach equilibrium
The multiplied concentrations of the products divided by the multiplied concentrations of the reactant is the equilibrium constant, Keq
Each concentration is raised to the power of its coefficient in the balanced equation.

24. If the value of Keq > 1, this indicates that at equilibrium
most of the material is present as products
If the value of Keq < 1, this indicates that at equilibrium
most of the material is present as substrates
When Keq > 1000, reaction is considered „complete” –
amount of reactant left less than 0.1%

25. Equilibrium and Free Energy
The ratio of products to reactants is controlled by their relative Gibbs free energy
This energy is released on the favored side of an equilibrium reaction
The change in Gibbs free energy between products and reactants is written as “ΔG”
If Keq > 1, energy is released to the surroundings (exergonic reaction)
If Keq < 1, energy is absorbed from the surroundings (endergonic reaction)

26. Relationship of Keq and Free Energy Change
The standard free energy change at 1 atm pressure and 298 K is ΔGº
The relationship between free energy change and an equilibrium constant is:
ΔGº = - RT ln Keq
where
R = cal/(K x mol) gas constant
T = temperature in Kelvin
ln Keq = natural logarithm of Keq

27. Describing a Reaction: Thermodynamic parameters
The Gibbs free energy change is attributable to a combination of two factors – an enthalpy factor ΔHo and an entropy factor ΔSo
ΔGº = ΔHº - TΔSo
ΔHº - enthalpy of reaction (change in total bonding energy during a reaction)
ΔSo - entropy of reaction (change in in the amount of molecular disorder caused by reaction)

29. Bond Dissociation Energies
Bond dissociation energy (D): amount of energy required to break a given bond to produce two radical fragments when the molecule is in the gas phase at 25˚ C
Changes in bonds can be used to calculate
net changes in heat (Enthalpy = ΔH)

30. Bond Dissociation Energies

31. Bond Dissociation Energies

32. Addition of HBr to Ethylene - Mechanism32

33. Energy Diagrams for Single-step Reaction
The highest energy point in a reaction step is called the transition state
The energy needed to go from reactant to transition state is the activation energy (ΔG‡)

34. First Step of HBr Addition to Ethylene
In the addition of HBr the (conceptual) transition-state structure for the first step
The  bond between carbons begins to break
The C–H bond begins to form
The H–Br bond begins to break

35. Reaction Intermediates
If a reaction occurs in more than one step, it must involve species that are neither the reactant nor the final product
These are called reaction intermediates or simply “intermediates”
Each step has its own free energy of activation
The complete diagram for the reaction shows the free energy changes associated with an intermediate

36. Reaction is thermodynamically favourable when the free energy of products is lower than free energy of reactants

37. Energy Diagrams for Single-step Reaction

38. ALKANES 38

39. Petroleum – natural source of alkanes Straight-run gasoline
Natural gas
C1 – C4
Asphalt
PETROLEUM
Straight-run gasoline
C5 – C11
b.p C
Lubricating oil
Waxes
Catalytic reforming to aromatics:
benzene, toluene
Kerosene
C11 – C14
b.p C
Gas oil
C14 – C25
b.p C
Catalytic cracking to C3- C5
Catalytic recombination to C7- C10 39

40. Alkane isomerism C6H14 C10H22 C7H16 C15H32 C8H18 C20H42 C9H20 C30H62 5
Formula
Number of isomers
C6H14 5
C10H22 75
C7H16 9
C15H32
4 347
C8H18 18
C20H42
C9H20 35
C30H62 40

41. Types of Alkyl Groups Classified by the connection site
a carbon at the end of a chain (primary alkyl group)
a carbon in the middle of a chain (secondary alkyl group)
a carbon with three carbons attached to it (tertiary alkyl group) 41

42. Types of Hydrogen Atoms

43. Alkane intermolecular forces+ - + - + - + - + - + - + - + - + - + - + - + -
Attractive van der Waals forces caused by temporary dipoles 43

44. Alkane intermolecular forces
Neopentane Pentane
(2,2-dimethylpropane)
Straigt-chain versus branched alkanes 44

45. Physical properties of alkanes
C1 – C4 gases at standard conditions
C5 – C19 liquids at standard conditions
> C solids at standard conditions 45

46. Physical properties of alkanes

47. Physical properties of alkanes
Melting point (C)
Boiling point (C)
Density
(g/mL)
Pentane
-129.7
36.1
0.6262
Isopentane
-159.9
27.9
0.6201
Neopentane
-16.5
9.5
0.6135
Straigt-chain versus branched alkanes 47

48. Chemical behaviour of alkanes
1. Combustion of alkanes (oxidation reaction)
2. Chlorination (bromination) of alkanes 48

49. Chlorination of alkanes – radical chain reaction mechanism
1st step - initiation 49

50. Chlorination of alkanes – radical chain reaction mechanism
2nd step - propagation 50

51. Chlorination of alkanes – radical chain reaction mechanism
3rd step - termination 51