Based on
McMurry’s Organic Chemistry, 7th edition, Chapter 5
AN OVERVIEW OF ORGANIC
REACTIONS
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
2
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
3
An Addition Reaction
4
An Elimination Reaction
5
A Substitution Reaction
6
A Rearrangement Reaction
7
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
8
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
9
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
10
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
11
Homogenic/Heterogenic Formation of a
Bond:
12
Homogenic Formation of a Bond:
Homogenic Breaking of a Bond:
13
Heterogenic Formation of a Bond:
Heterogenic Breaking of a Bond:
14
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
15
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
16
Radical Substitution
17
Chlorination of methane: a
radical substitution reaction
18
Steps in Radical Substitution
 Three types of steps
 Initiation – homolytic formation of two reactive
species with unpaired electrons
 Example – formation of Cl atoms form Cl2 and light
 Propagation – reaction with molecule to generate
radical
 Example - reaction of chlorine atom with methane
to give HCl and CH3.
 Termination – combination of two radicals to form
a stable product: CH3. + CH3.  CH3CH3
19
Intitiation
20
Propagation
21
Termination
22
Prostaglandin Biosynthesis:
23
Prostaglandin Biosynthesis:
24
Problem: Mechanism?
25
Radical Addition
26
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
27
Electronegativity of Some
Common Elements
 Higher numbers indicate greater electronegativity
 Carbon bonded to a more electronegative element has a
partial positive charge (+)
28
Polarity
29
Polarity is affected by structure changes:
30
31
And a few more:
32
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
33
Generalized Polar Reactions
 An electrophile, an electron-poor
species (Lewis acid), combines
with a nucleophile, an electronrich species (Lewis base)
 The combination is indicated with
a curved arrow from nucleophile
to electrophile
34
Generalized Polar Reactions
35
Electrophiles & Nuclephiles
36
Problem: BF3, electrophile or
nucleophile?
37
An Example of a Polar Reaction:
Addition of HBr to Ethylene
38
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
39
P-Bonds as Nucleophiles:
40
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
41
42
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
43
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
44
Rule 1: electrons move from Nu: to E
45
Rule 2: Nu: can be negative or neutral
46
Rule 3: E can be positive or
neutral
47
Rule 4: Octet rule!
48
Practice Prob.: Add curved
arrows
49
Prob.: Predict the products
50
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.
aA + bB
cC + dD
Keq = [Products]/[Reactants] = [C]c [D]d / [A]a[B]b
51
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)
52
For example:
53
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)
54
Free Energy and Equilibrium
55
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 = 1.987 cal/(K x mol) (gas constant)
 T = temperature in Kelvins
 ln = natural logarithm
 The exponential form: Keq = e-DG/RT
56
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
58
Ethylene + HBr
DGº = DHº - TDSº
59
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.
 Table 5.3 lists energies for many bond types
 Changes in bonds can be used to calculate net
changes in heat
60
61
More homolytic BDE’s:
62
Calculation of an Energy Change from
Bond Dissociation Energies
63
64
Describing a Reaction: Energy
Diagrams and Transition States
65
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‡)
66
67
First Step in the Addition
of HBr
 In the addition of
HBr the transitionstate 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
68
Energy Diagram for step 1
69
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
70
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
71
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
 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
73
Enzymes Change Mechanisms:
74
Enzyme Catalysis
75
76
Problem: Label the diagram:
77