1. Iran University of Science & Technology
Organic Chemistry
M. R. Naimi-Jamal
Faculty of Chemistry
Iran University of Science & Technology

2. Chapter 3. An Overview of Organic Reactions
Based on:
McMurry’s Organic Chemistry, 6th edition, Chapter 3

3. 3.1 Kinds of Organic Reactions
In general, we look at:
what occurs,
and try to learn how it happens
What includes reactivity patterns and types of reaction
How refers to reaction mechanisms

4. 3.1 Kinds of Organic Reactions
Addition reactions – two molecules combine
Elimination reactions – one molecule splits into two
Substitution – parts from two molecules exchange
Rearrangement reactions – a molecule undergoes changes in the way its atoms are connected

5. An Addition Reaction

6. An Elimination Reaction

7. A Substitution Reaction

8. A Rearrangement Reaction

9. 3.2 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, by isolating and identifying the products, 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

10. Steps in Mechanisms
A step usually 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

11. Types of Steps in Reaction Mechanisms
Formation of a covalent bond
Homogenic or heterogenic
Breaking of a covalent bond
Homolytic or heterolytic
Oxidation of a functional group
Reduction of a functional group

12. Homogenic Formation of a Bond
One electron comes from each fragment
No electronic charges are involved

13. Heterogenic Formation of a Bond
One fragment supplies two electrons (Lewis Base)
One fragment supplies no electrons (Lewis Acid)
Combination can involve electronic charges

14. 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’)—the motion of one electron
Arrowheads with a complete head indicate heterolytic and heterogenic steps (called ‘polar processes’)—the motion of an electron pair

15. Bond Making

16. Homolytic Breaking of Covalent Bonds
Each product gets one electron from the bond

17. Heterolytic Breaking of Covalent Bonds
Both electrons from the bond that is broken become associated with one resulting fragment
A common pattern in reaction mechanisms

18. Bond Breaking

19. Radicals Alkyl groups are abbreviate “R” for radical
Example: Methyl iodide = CH3I, Ethyl iodide = CH3CH2I, Alkyl iodides (in general) = RI
A “free radical” is an “R” group on its own:
CH3 is a “free radical” or simply “radical”
Has a single unpaired electron, shown as: CH3.
Its valence shell is one electron short of being complete

20. 3.3 Radical Reactions and How They Occur
Radicals react to complete electron octet of valence shell
A radical can break a bond in another molecule and abstract a partner with an electron, giving substitution in the original molecule
A radical can add to an alkene to give a new radical, causing an addition reaction

21. Radical Substitution

22. Radical Addition

23. Chlorination of methane: a radical substitution reaction

24. 3.4 Polar Reactions and How They Occur
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

25. Electronegativity of Some Common Elements
Higher numbers indicate greater electronegativity
Carbon bonded to a more electronegative element has a partial positive charge (+)

26. Polarity

27. Polarity is affected by structure changes:

29. And a few more:

30. Polarizability
Polarization is a change in electron distribution as a response to change in electronic nature of the surroundings
Polarizability is the tendency to undergo polarization
Polar reactions occur between regions of high electron density and regions of low electron density

31. Generalized Polar Reactions
An electrophile, an electron-poor species (Lewis acid), combines with a nucleophile, an electron-rich species (Lewis base)
The combination is indicated with a curved arrow from nucleophile to electrophile

32. Electrophiles & Nuclephiles

33. Problem 3.5: BF3, electrophile or nucleophile?

34. 3.5 An Example of a Polar Reaction: Addition of HBr to Ethylene

35. 5.5 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

36. Addition of HBr to Ethylene

37. P-Bonds as Nucleophiles:
P-bonding electron pairs can function as Lewis bases:

38. Mechanism of Addition of HBr to Ethylene
HBr electrophile is attacked by  electrons of ethylene (nucleophile) to form a carbocation intermediate and bromide ion
Bromide adds to the positive center of the carbocation, which is an electrophile, forming a C-Br  bond
The result is that ethylene and HBr combine to form bromoethane
All polar reactions occur by combination of an electron-rich site of a nucleophile and an electron-deficient site of an electrophile

40. 3.6 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 in polar reactions
Charges change during the reaction
One curved arrow corresponds to one step in a reaction mechanism

41. Rules for Using Curved Arrows
The arrow goes from the nucleophilic reaction site to the electrophilic reaction site
The nucleophilic site can be neutral or negatively charged
The electrophilic site can be neutral or positively charged

42. Rule 1: electrons move from Nu: to E

43. Rule 2: Nu: can be negative or neutral

44. Rule 3: E can be positive or neutral

45. Rule 4: Octet rule!

46. Rule 4: Octet rule!

47. Practice Prob. 3.2: Add curved arrows

48. Solution:

49. 3.7 Describing a Reaction: Equilibria, Rates, and Energy Changes
Reactions can go in either direction 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.
Keq = [Products]/[Reactants] = [C]c [D]d / [A]a[B]b

50. Magnitudes of Equilibrium Constants
If the value of Keq is greater than 1, this indicates that at equilibrium most of the material is present as product(s)
A value of Keq less than one indicates that at equilibrium most of the material is present as the reactant(s)

51. For example:

52. Free Energy and Equilibrium
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 reacts is written as “DG”
If Keq > 1, energy is released to the surrounding (exergonic reaction)
If Keq < 1, energy is absorbed from the surroundings (endergonic reaction)

53. Free Energy and Equilibrium

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

55. Changes in Energy at Equilibrium
Free energy changes (DGº) can be divided into
a temperature-independent part called entropy (DSº) that measures the change in the amount of disorder in the system
a temperature-dependent part called enthalpy (DHº) that is associated with heat given off (exothermic) or absorbed (endothermic)
Overall relationship: DGº = DHº - TDSº

57. Ethylene + HBr
DGº = DHº - TDSº

58. 5.8 Describing a Reaction: Bond Dissociation Energies
Bond dissociation energy (D): Heat change that occurs when a bond is broken by homolysis
The energy is mostly determined by the type of bond, independent of the molecule
The C-H bond in methane requires a net heat input of 105 kcal/mol to be broken at 25 ºC.
Changes in bonds can be used to calculate net changes in heat

59. Calculation of an Energy Change from Bond Dissociation Energies

60. 5.9 Describing a Reaction: Energy Diagrams and Transition States

61. 5.9 Describing a Reaction: Energy Diagrams and Transition States
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 (DG‡)

62. Energy Diagram for step 1

64. First Step in the Addition of HBr
In the addition of HBr the 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

65. Energy Diagram for step 1

66. First Step in the Addition of HBr

67. 5.10 Describing a 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

68. Formation of a Carbocation Intermediate
HBr, a Lewis acid, adds to the  bond
This produces an intermediate with a positive charge on carbon - a carbocation
This is ready to react with bromide

69. Carbocation Intermediate Reactions with Anion
Bromide ion adds an electron pair to the carbocation
An alkyl halide produced
The carbocation is a reactive intermediate

70. Reaction Diagram for Addition of HBr to Ethylene
Two separate steps, each with a own transition state
Energy minimum between the steps belongs to the carbocation reaction intermediate.

72. Biological Reactions
Reactions in living organisms follow mechanisms (with reaction diagrams) too
They take place under very specific conditions:
Aqueous environment with a pH close to 7
Temperature of 37oC

73. Biological Reactions
They are promoted by catalysts that lower the activation energy
The catalysts are usually proteins, called enzymes
Enzymes provide an alternative mechanism that is compatible with the conditions of life

74. Enzymes Change Mechanisms:

75. Explosives: Nitroglycerine

76. Explosives: Research Department composition X
(1,3,5-Trinitro-1,3,5-triazacyclohexane)

77. Prob. 5.21: Label the diagram: