Organic Reaction Mechanism : This topic is very-very important for CSIR-NET, GATE, IIT-JAM and other Competitive exams for Chemistry and Chemical Sciences.

1. Chemical Science
Organic Chemistry
Organic Reactions Mechanism
CSIR-NET (Chemical Science) & GATE (Chemistry)
Kendrika Academy
www.kendrika.com
Call @ 08726660111
Dr. Dhananjaya Sahoo

2. Outline of the Lecture
• Understanding of organic reaction
• Writing Equation for Organic Reaction
• Types of Organic Reaction & Brief Description
• Mechanism Behind the reaction
 Bond breaking and Bond formation
 Reaction Intermediates
 Curved arrow
• Describing a reaction in terms of Equilibria, Rates and
Energy Changes

3. 3
Understanding Organic Reactions
• Equations for organic reactions are usually drawn
with a single reaction arrow () between the
starting material and product.
• The reagent, the chemical substance with which
an organic compound reacts, is sometimes drawn
on the left side of the equation with the other
reactants. At other times, the reagent is drawn
above the arrow itself.
• Although the solvent is often omitted from the
equation, most organic reactions take place in
liquid solvent.
• The solvent and temperature of the reaction may
be added above or below the arrow.
• The symbols “h” and “” are used for reactions
that require light and heat respectively.
Writing Equations for Organic Reactions

4. Writing Equations for Organic Reactions
4
• When two sequential reactions are carried out without drawing any
intermediate compound, the steps are usually numbered above or
below the reaction arrow. This convention signifies that the first
step occurs before the second step, and the reagents are added in
sequence, not at the same time.

5. Types of Organic Reactions
The Organic Reactions Are Of Following Types
• Addition Reaction
• Elimination Reaction
• Substitution Reaction
• Rearrangement Reaction

6. • Addition is a reaction in
which elements are added
to the starting material.
• In an addition reaction,
new groups X and Y are
added to the starting
material. A  bond is
broken and two  bonds
are formed.
Addition Reactions

7. • Elimination is a reaction in
which elements of the starting
material are “lost” and a  bond
is formed.
• In an elimination reaction, two
groups X and Y are removed
from a starting material.
• Two  bonds are broken, and a
 bond is formed between
adjacent atoms.
• The most common examples of
elimination occur when X = H
and Y is a heteroatom more
electronegative than carbon.
Elimination Reaction
Examples

8. • Addition and elimination
reactions are exactly
opposite. A  bond is formed
in elimination reactions,
whereas a  bond is broken
in addition reactions.

9. Substitution Reaction
• A substitution is a reaction in
which an atom or a group of
atoms is replaced by another
atom or group of atoms.
• In a general substitution, Y
replaces Z on a carbon atom.
• Substitution reactions involve
 bonds: one  bond breaks
and another forms at the
same carbon atom.
http://chemistry.boisestate.edu/rbanks/organic/sn2.gif

10. • The most common examples of substitution occur when Z
is a hydrogen or a heteroatom that is more
electronegative than carbon.
Substitution Reaction

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

12. Classify each of the following as either substitution,
elimination or addition reactions.
OH
Br
substitution
b)
addition
c)
OH
elimination
a)

13. 13
• 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
 We classify the types of steps in a sequence
 A step involves either the formation or breaking of a covalent
bond
 Steps can occur in individually or in combination with other
steps
 When several steps occur at the same time they are said to be
concerted
How Organic Reactions Occur: Mechanisms

14. 14
Bond Making and Bond Breaking
• A reaction mechanism is a detailed description of how bonds
are broken and formed as starting material is converted into
product.
• A reaction can occur either in one step or a series of steps.
Mechanism

15. Regardless of how many steps there are in a reaction, there are
only two ways to break (cleave) a bond: the electrons in the
bond can be divided equally or unequally between the two
atoms of the bond.
• Breaking a bond by equally dividing the electrons between the two
atoms in the bond is called homolysis or homolytic cleavage
Homolysis generates uncharged reactive intermediates
with unpaired electrons

16. 16
• Breaking a bond by unequally dividing the electrons between
the two atoms in the bond is called heterolysis or hoterolytic
cleavage
• Heterolysis of a bond between A and B can give either A or B
with the two electrons. When A and B have different electro-
negativities, the electrons end up on the more
electronagetive atom
• Heterolysis generates charged intermediates.

17. 17
• Curved arrows indicate breaking and forming of bonds
• To illustrate the movement of a single electron, use a half-headed
curved arrow, sometimes called a fishhook.
• A full headed curved arrow shows the movement of an electron
pair.
Indicating Steps in Mechanisms

18. 18
Three reactive intermediates resulting from homolysis and heterolysis of
a C – Z bond
Reaction Intermediates
• Homolysis generates two uncharged species with unpaired electrons.
• A reactive intermediate with a single unpaired electron is called a radical.
• Radicals are highly unstable because they contain an atom that does not
have an octet of electrons.

19. 19
• Heterolysis generates a carbocation or a carbanion, which are
unstable intermediate.
• A carbocation contains a carbon surrounded by only six electrons, and
a carbanion has a negative charge on carbon.
Reaction Intermediates

20. 20
• Radicals and carbocations are electrophiles because they
contain an electron deficient carbon.
• Carbanions are nucleophiles because they contain a carbon
with a lone pair.
Reaction Intermediates

21. 21
Heterolytically cleave each of the carbon-hetratom bonds
and label the organic intermediate as a carbocation or
carbanion
a)
OH + OH
carbocation
b) H3CH2C Li H3C CH2
+ Li
carbanion

22. 22
• Bond formation occurs in two different ways and bond formation
always releases energy.
• Two radicals can each donate one electron to form a two-
electron bond.
How does bond formation occur?

23. 23
How does bond formation occur?
• Alternatively 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
• The combination is indicate with a curved arrow from
nucleophile to electrophile

24. 24
Types of arrows are used in describing
organic reactions

25. Using Curved Arrows in Reaction
Mechanisms
• Curved arrows are a way to keep track of changes in bonding
in a reaction
• The arrows track “electron movement”
• Electrons always move in pairs (expect radical reactions)
• 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

26. Rules for Using Curved Arrows
• The nucleophilic site can be neutral or negatively
charged

27. • The electrophilic site can be neutral or positively
charged
• Don’t exceed the octet rule (or duet)

28. 28
Use arrows to show the movement of electrons in
the following reactions.
a)
N N + N N
b)
HO OH OH2
c)
+ Br Br

29. Describing a Reaction: Equilibria, Rates,
and Energy Changes
• For a reaction to be practical, the equilibrium must favor products
and the reaction rate must be fast enough to form them in a
reasonable time. These two conditions depend on
thermodynamics and kinetics respectively.
• Thermodynamics describes how the energies of reactants and
products compare, and what the relative amounts of reactants
and products are at equilibrium.
• Kinetics describes reaction rates.
• The equilibrium constant, Keq, is a mathematical expression that
relates the amount of starting material and product at equilibrium.

30. • When Keq > 1, equilibrium favors the products (C and D)
and the equilibrium lies to the right as the equation is
written.
• When Keq < 1, equilibrium favors the starting materials (A
and B) and the equilibrium lies to the left as the equation is
written.
• For a reaction to be useful, the equilibrium must favor the
products, and Keq > 1.
Magnitudes of Equilibrium Constants

31. Free Energy and Equilibrium
• The position of the equilibrium is determined by the relative
energies of the reactants and products.
• G° is the overall energy difference between reactants and
products.

32. The relationship between free energy change and an
equilibrium constant is:
Gº = - 2.303 RT log Keq
Where:
• R = 1.987 cal/(K x mol)
• T = temperature in Kelvin
• When Keq > 1, log Keq is positive, making G° negative, and energy is
released. The reaction is exergonic. Thus, equilibrium favors the
products and the reaction is spontaneous
• When Keq < 1, log Keq is negative, making G° positive, and energy is
absorbed. The reaction is energonic. Thus, equilibrium favors the
reactants and the reaction is not spontaneous.
Relationship of Keq and Free Energy Change

33. G° is related to H° and S° by the following equation:

34. Energy Diagram and Transition state of an
Organic Reaction
• For the general reaction:
• The energy diagram would be shown as:
• The energy of activation Ea is the
minimum amount of energy needed to
break the bonds in the reactants.
• The larger the Ea, the greater the amount
of energy that is needed to break bonds,
and the slower the reaction rate

35. Examples

36. Examples

37. Examples

38. Examples

39. Comparing ∆H° and Ea in two energy diagrams

40. 40
• Consider the following two step reaction:
• An energy diagram must be drawn for each step.
• The two energy diagrams must then be combined to form an energy
diagram for the overall two-step reaction.
• Each step has its own energy barrier, with a transition state at the energy
maximum.

41. 41

42. 42

43. 43
Figure 6.6
Complete energy diagram for
the two-step conversion of

44. 44
Kinetics
• Kinetics is the study of reaction rates.
• Recall that Ea is the energy barrier that must be exceeded for
reactants to be converted to products.

45. • The higher the concentration, the faster the rate.
• The higher the temperature, the faster the rate.
• G°, H°, and Keq do not determine the rate of a reaction. These
quantities indicate the direction of the equilibrium and the relative
energy of reactants and products.
• A rate law or rate equation shows the relationship between the
reaction rate and the concentration of the reactants. It is
experimentally determined.
• Fast reactions have large rate constants.
• Slow reactions have small rate constants.
• The rate constant k and the energy of activation Ea are inversely
related. A high Ea corresponds to a small k.