TYPES OF REACTIONS
Chapter 6
Addition Reactions: In general, two molecules
“add” or combine to make one new substance.
“An Overview of Organic
Reactions”
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TYPES OF REACTIONS
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TYPES OF REACTIONS
Elimination Reactions: In general, one molecule
“splits” or breaks down to make two new
substances.
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TYPES OF REACTIONS
Substitution Reactions: In general, parts from one
molecule “exchange” with parts from a second
molecule.
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HOW ORGANIC REACTIONS OCCUR: MECHANISMS
Rearrangement Reactions: In general, a molecule
undergoes a “change” in connectivity to form a
second molecule.
Reaction Mechanisms: In an organic reaction, we
see the transformation that has occurred. The
mechanism describes the steps behind the changes
that we can observe. In a reaction, bond formation
or breakage can be symmetrical or unsymmetrical.
Homolytic cleavage; each
•  Symmetrical- homolytic
atom gets one electron.
•  Unsymmetrical- heterolytic
Heterolytic cleavage; one
atom gets both electrons.
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HOW ORGANIC REACTIONS OCCUR: MECHANISMS
When drawing mechanisms, curved arrows are
used to indicate breaking and forming of bonds.
Arrowheads with a “half” head (“fish-hook”)
indicate homolytic steps (called radical reactions),
and arrowheads with a complete head indicate
heterolytic steps (called polar processes).
A HOMOLYTIC, OR RADICAL REACTION MECHANISM
In a radical reaction there are at least three distinct
steps:
• 
Initiation – homolytic formation of two
reactive species with unpaired electrons.
• 
Propagation – reaction with another molecule
to generate a new radical.
• 
Termination – combination of two radicals to
form a stable product.
Homolytic arrows; the
transfer of one electron.
Heterolytic arrows; the
transfer of two electrons.
H
Cl
C
H
+ Cl2
H
C
H
H
H
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A HOMOLYTIC, OR RADICAL REACTION MECHANISM
In a radical reaction there are at least three distinct
steps:
• 
Initiation – homolytic formation of two
reactive species with unpaired electrons.
Cl
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A HOMOLYTIC, OR RADICAL REACTION MECHANISM
In a radical reaction there are at least three distinct
steps:
• 
Propagation – reaction with another molecule
to generate a new radical.
Cl
light
Cl
2 Cl
H
C
C
H
H
Cl
H
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A HOMOLYTIC, OR RADICAL REACTION MECHANISM
In a radical reaction there are at least three distinct
steps:
Termination – combination of two radicals to
form a stable product.
H
H
H
H
Cl
Cl
C
H
H
+ HCl
H
H
H
+ Cl
H
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A HOMOLYTIC, OR RADICAL REACTION MECHANISM
In a radical reaction there are at least three distinct
steps:
• 
Termination – combination of two radicals to
form a stable product.
Cl
Cl
C
H
H
C
• 
+ HCl
H
C
H
H
H
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A HOMOLYTIC, OR RADICAL REACTION MECHANISM
In a radical reaction there are at least three distinct
steps:
Cl
Cl
light
Initiation
2 Cl
Cl
H
C
C
H
H
Cl
H
H
H
+ Cl
H
C
H
H
H
H
Cl–
Termination
CH3Cl
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POLAR REACTION MECHANISMS
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SUBSTITUTION REACTIONS
As we have seen previously, molecules can contain
local unsymmetrical electron distributions due to
The carbon atom in
differences
in electronegativities. This causes a
chloromethane is electron Chloride anion has a high
partial
charge on
one atom
deficientnegative
and will function
electron
densityand
andawill
compensating
partial
on base
the
as a Lewis acid
(an positive
functioncharge
as a Lewis
electrophile).
(a nucleophile).
adjacent
atom. In a molecule,
the more
electronegative atom will have the greater electron
density.
Cl–
As we have seen previously, molecules can contain
local unsymmetrical electron distributions due to
differences in electronegativities. This causes a
partial negative charge on one atom and a
compensating partial positive charge on the
adjacent atom. In a molecule, the more
electronegative atom will have the greater electron
density.
Cl
Cl
C
+ HCl
Propagation
C
H
H
H
H
H
Cl
Cl
C
H
H
H
POLAR REACTION MECHANISMS
AsThis
weishave
molecules can contain
calledseen
an SNpreviously,
2 reaction
local
unsymmetrical electron distributions due to
(Substitution-NucleophilicBimolecular).
We will cover
differences
in electronegativities.
This causes a
in Chapter
10. on one atom and a
partialthese
negative
charge
compensating partial positive charge on the
adjacent atom. In a molecule, the more
electronegative atom will have the greater electron
density.
Cl–
CH3Cl
Cl–
CH3Cl
CH3Cl
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SUBSTITUTION REACTIONS
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REVISITING ADDITION REACTIONS
The actual process of bond-making and bond-breaking
is shown in the movie clip below:
Addition Reactions: In general, two molecules
“add” or combine to make one new substance.
H
H
CH3
H
+ HBr
Cl–
CH3Cl
CH3Cl
Cl–
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Br
CH3
H
H
H
H
2-Bromopropane is the only
significant product formed…
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ADDITION REACTIONS
ADDITION REACTIONS
The mechanism of the reaction be viewed as
involving a protonated π-cloud intermediate, which
forms by protonation of the electron rich π-cloud by
the strong acid.
H
The mechanism of the reaction be viewed as
involving a protonated π-cloud intermediate, which
forms by protonation of the electron rich π-cloud by
the strong acid.
H
H
H
H
H
CH3
H
CH3
H
A π-complex, and a
protonated π-cloud…
+ HBr
H
H
CH3
H
H in an H
Recall that
alkene, the
π-cloud is a rich source of
H
electron density.
CH3
H
CH3
H
H
CH3
H
H
+ HBr
H
CH3
CH3
H
H
H
CH3
H
H
H H
H
CH3
CH
3
H
CH3
H
H
H
CH3
H
C H3
+ HBr
H
CH3 CH3
H
CH3
CH3
H
H
Br
H
H
H3
CH
H
H
Br–
H
H
Br
H
H
H
Br
H
H
CH3
H
H
H
Br
H
H
H
H
CH
3
H
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Elimination Reactions: In general, one molecule
“splits” or breaks down to make two new
substances.
C H3
OH
C H 2 + H 2O
C H3
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ELIMINATION REACTIONS
Elimination Reactions: In general, one molecule
“splits” or breaks down to make two new
substances.
C H3
H H
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REVISITING ELIMINATION REACTIONS
OH
H
CH3
H
H H
H
CH3
CH
3
H
Br
H
The mechanism of the reaction be viewed as
H
H + HBr
H
involving a protonated
π-cloud intermediate,
which
H
forms by protonation of the electron rich π-cloud
H by
CH
3
CH3acid. H
the strong
+ HBr
Br
H
H
ADDITION REACTIONS
H H
H
H
H
CH3
H H
H
CH3
not observed
H
+ HBr
Br
H
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We will see that this
carbocation is formed
The mechanism of the reaction
viewed
as than
becausebe
it is
more stable
the other
option. We will
call
involving a protonated π-cloud
intermediate,
which
The π-complex can rearrange this Markovnikov’s Rule.
forms
by protonation
of the electron rich π-cloud by
to a σ-complex
to give two
the
strong
acid.
different
carbocations,
but
H
only the 2-propyl carbocation
is formed.
H
CH3
H
H
H
H
ADDITION REACTIONS
H
H
CH3
not observed
H
Br
H
+ HBr
C H3
C H3
C H3
C H3
+ H2S O4
H+
Base
H
H
C H3 O
H3C
CH
H
3C3
H
H3C
CC H2
C H3
H3C
+ H2S O4
H3C
H3C
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C C H2
C C H2 + H 2
ELIMINATION REACTIONS
REACTION COORDINATE DIAGRAMS
Elimination Reactions: In general, one molecule
“splits” or breaks down to make two new
substances.
The progress of any chemical reaction can also be
shown using a reaction coordinate diagram in
which the ground-state energy of reactants,
products and intermediates are plotted as a
function of the “progress” of the reaction (from
reactants to products).
Base
Base
:
H
O
H
H
H3C
C
H3C
H3C
C H3
H3C
C C H2 + H2O
H3C
C C H2
H3C
Cl–
Cl–
CH3Cl
CH3Cl
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REACTION COORDINATE DIAGRAMS
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SUBSTITUTION REACTIONS
In a reaction coordinate diagram, the ground-state
energy of reactants, products and intermediates are
plotted as a function of the “progress” of the
reaction.
Rate-limiting transition
The actual process of bond-making and bond-breaking
is shown in the movie clip below:
Br
Cl
Br–
H
H
CH3
H
CH3
-
+ CH
Cl– 3 O H
-
increasing energy
state; a transition state
is an energy maximum,
not an intermediate
H
CH3
CH
H 3
-
-
+
H
OCH
Cl 3
OCH3
+ CH3 O
H
+ Cl
Br–
Br
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REACTION COORDINATE DIAGRAMS
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REACTION COORDINATE DIAGRAMS
The addition of HBr to 2-propene can also be viewed
using a reaction coordinate diagram…
Higher energy
transition state;
the slow step.
H
CH3
H
H
+ HBr
CH3
Energy
Br
H
H
H
carbocation
intermediate
The addition of HBr to 2-propene can also be viewed
using a reaction coordinate diagram…
Br
H
H
Br
CH3
H
H
H
H
H
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CH3
H
H
+ HBr
CH3
Lower energy
transition state;
the fast step.
H
H
carbocation
intermediate
H
Br
CH3
H
H
H
H
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REACTION COORDINATE DIAGRAMS
IN-CLASS PROBLEM
Identify the compounds below that will each give a
single mono-chlorination product.
Enzymes accelerate reactions by lowering the
activation energy, as shown below…
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