Transcript Electrophilic Addition Reactions

Chapter 25
ORGANIC CHEMISTRY
Part II
Dr. Al-Saadi
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25.4
Isomerism
 Isomers are compounds with identical molecular formula but
different structural formulas.
For example, propanone (acetone) and propanal both have the
same molecular formula:
C3H6O
but they have different structural formulas.
Isomers
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CH3OCH3
CH3CH2COH
Propanone
Propanal
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25.4
Classes of Isomers
Isomers
Constitutional
(structural) Isomers
Stereoisomers
Geometrical
Isomers
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Optical
Isomers
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25.4
Constitutional Isomerism
 Constitutional (structural) isomerism occurs when the same
atoms can be connected in two or more different ways.
Constitutional isomers have different names and different
physical and chemical properties.
C5H12
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25.4
Constitutional Isomerism
 Exercise:
What are the constitutional (structural) isomers of C2H2Cl2
(with C=C bond)?
There are two constitutional (structural) isomers:
These two drawings show one constitutional
isomer because there is no breaking-reconnecting
of bonds between atoms. These are rather called
cis/trans isomers.
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25.4
Stereoisomerism
 Stereoisomers are compounds that contain identical bonds
but differ in the orientation of those bonds in space.
There are two kinds of stereoisomers:
Geometrical isomers
cis-platin
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Optical isomers
trans-platin
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25.4
Geometrical Isomers
 Geometrical isomers occur in compounds that have a
restricted rotation around a bond (a double or triple bond).
They usually have different chemical and physical properties.
C2H2Cl2
Same side
Opposite
side
Geometrical isomers
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Structural isomer
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25.4
Optical Isomers
 Optical isomers are compounds that are mirror images of
each other, but are not superimposable (not identical). They
usually have similar chemical and physical properties.
Your hands are
mirror images
of each other
mirror
But they are
NOT
superimposable
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25.4
Optical Isomers
 Such molecules are called chiral ; and a pair of mirror-image
molecules are called enantiomers. Enantiomers have most of
their chemical and physical properties to be identical.
These two
molecules are
mirror image
mirror
But you will never be
able to superimpose
them on top of each
other
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25.4
Optical Isomers
 To represent chirality of molecules
when drawing them on a piece of
paper, we use wedges and dashes.
H
COOH
COOH
C
C
R
R
NH2
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Chiral molecules are
optically active
H
NH2
Chiral molecules exist as
mirror images of each other,
just as the right hand is a
mirror image of the left hand
(the word chiral comes from
the Greek word for ‘hand’).
Chiral molecules have at least
one central carbon atom
connected to four different
groups/atoms.
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25.4
Optical Activity of Enantiomers
 A pair of enantiomers rotate the plane of a plane-polarized
light in opposite directions.
 What is plane-polarize light?
Plane-polarized light oscillates only in a single plane, while
ordinary light (that is not plane-polarized) oscillates in all
directions.
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25.4
Optical Activity of Enantiomers
 A pair of enantiomers rotate the plane of a plane-polarized
light in opposite directions.
 What is plane-polarize light?
Plane-polarized light oscillates only in a single plane, while
ordinary light (that is not plane-polarized) oscillates in all
directions.
Non plane-polarized
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plane-polarized
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25.4
Optical Activity of Enantiomers
 Measuring the rotation of polarized light by optical isomers.
Plane of polarization
gets rotated by 90
Plane-polarized light
Solution of optically
active enantiomers
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Minimal transmission
occurs when the plane
of polarization of the
light is perpendicular to
that of the analyzer
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25.4
Optical Activity of Enantiomers
 When the two enantiomers of CHFClBr molecule have a net
rotation of zero, it means that the mixture of the two
enantiomers is equimolar (same number of moles). Such a
mixture is called a racemic mixture.
Non plane-polarized
plane-polarized
Net rotation
is zero
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25.5
Organic Reactions
 An organic reaction is the chemical process
that involves the addition of an organic
molecule or an ion to another organic
molecule.
 Some examples of organic reactions:
o Addition reactions.
• Electrophilic addition.
• Nucleophilic addition.
o Substitution reactions.
• Electrophilic substitution.
• Nucleophilic substitution.
– SN1 and SN2 reactions.
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25.5
Electrophiles and Nucleophiles
 An electrophile “loves electrons” is a species with a region of
positive or partial positive charge, such as:
+
o cations (H ion).
o the positive region of polar molecules (HCl).
Electrophiles are electron-poor.
 A nucleophile “loves nucleus” is a species with a region of
negative or partial negative charge, such as:
o anions (Cl‒ ion) , or
o the negative region of polar molecules like HCl.
Nucleophiles are electron-rich.
The keys to
understand
organic reactions
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25.5
Electrophilic Addition Reactions
H
Cl
C
C
H
H
H
H
C
C
H
+ HCl
H
H
Where are the electron-rich
and electron-poor areas?
H
Nucleophile
Electrophilic
attack
Electrophile
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25.5
Electrophilic Addition Reactions
These curved
pink arrows
show the
movement of
electron pairs
“Mechanism”

intermediate


 Mechanism:
1. The π bond breaks and a new C―H bond is produced. As a result,
the H―Cl bond also breaks and a Cl‒ ion is produced.
2. A lone pair on the electron-rich Cl‒ ion forms a new σ bond with
the electron-deficient carbocation.
3. The hybridization of the C atoms has changed from sp2 to sp3.
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25.5
Electrophilic Addition Reactions
 Viewing the atomic orbitals in the previous addition reaction
and how the hybridization of C atoms has changed.
carbocation
A carbocation is
showing an sp2
hybridization which
implies a planar
structure.
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25.5
Electrophilic Addition Reactions
 Viewing the atomic orbitals in the previous addition reaction
and how the hybridization of C atoms has changed.
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25.5
Nucleophilic Addition Reactions
O
C
O
Electrophile
+
H
H
O
O
O
H
O
H
C
Nucleophile
Where are the electron-rich
and electron-poor areas?
 Mechanism:
o
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Nucleophilic attack.
Stabilized by resonance
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25.5
Nucleophilic Addition Reactions
o
Breaking an O―H bond and forming H+ ion.
o
New O―H bond formed leading to the product.
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25.5
Addition Reactions
 Addition reactions are reactions that involve the addition of
a molecule or an ion to another molecule.
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o
Electrophilic addition. (species to be added is an electrophile)
o
Nucleophilic addition. (species to be added is a nucleophile)
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25.5
Addition Reactions
 Addition reactions are reactions that involve the addition of
a molecule or an ion to another molecule.
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o
Electrophilic addition. (species to be added is an electrophile)
o
Nucleophilic addition. (species to be added is a nucleophile)
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25.5
Addition Reactions
 Exercise:
Draw the mechanism for the nucleophilic addition of CN ion
to CH3CHO.
O
H3C C H
O
+
C N
H3C C H
C
Nucleophile
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N
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25.5
Substitution Reactions
 A substitution reaction is the reaction during which one
group is replaced by another group.
Electrophilic substitution
An electrophile (NO2+)
attacks an aromatic
molecule and replaces a
hydrogen atom.
Nucleophilic substitution
A nucleophile (OH‒) attacks
an electron-deficient C
atom and replaces another
group on that C atom.
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25.5
Electrophilic Substitution Reactions
 Mechanism:
o
Formation of the NO2+ electrophile.
Electrophile (E+)
o
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Electrophilic attack which breaks a π bond.
Carbocation
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25.5
Electrophilic Substitution Reactions
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o
Carbocation is stabilized by resonance.
o
The π bond is restored and H+ ion is produced.
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25.5
Electrophilic Substitution Reactions
 Exercise:
Predict the product and give the mechanism for the following
electrophilic substitution reaction:
H
+
tert-butyl cation
tert-butyl benzene
H
H
H
H
+
C(CH3)3
C(CH3)3
H
H
First step:
+
H
H
H
H
H
H
H
C(CH3)3
H
C(CH3)3
H
H
Second step:
+
+
H
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H
H
H
H
H+
H
H
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25.5
Nucleophilic Substitution Reactions
 Nucleophilic substitution can be of two types:
o SN2 reactions: Direct nucleophilic back-side attack .
 In SN2 reactions the molecular configuration is inverted.
transition state
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10.5
Nucleophilic Substitution Reactions
 The other type of nucleophilic substitution is:
o SN1 reactions: First, a group is leaving the C atom. Then, a
nucleophile attacks the carbocation intermediate.
 Mechanism:
o
A group is leaving and a carbocation intermediate is formed.
carbocation
o
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Nucleophilic attack
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10.5
Nucleophilic Substitution Reactions
 In SN1 reactions for a molecule with a chiral C atom (bonded
to four different atoms/groups), a pair of enantiomers is
formed. This is known as racemization.
The carbocation intermediate is planar. Thus,
nucleophilic attack could also take place from
the opposite side of the carbocation.
Dr. Al-Saadi
The produced two
enantiomers which are
mirror images of each
other “racemization”
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10.5
Mechanisms of Substitution Reactions
Electrophilic substitution
Nucleophilic substitution (SN1)
Nucleophilic substitution (SN2)
Formation of carbocation intermediate
Nucleophile attacking from back
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10.5
Other Types of Organic Reactions
 Elimination reactions are reactions in which a double bond
forms and a small molecule is removed.
+ X‒
o
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+ H―Nu
Example is the following step in carbohydrate metabolism.
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10.5
Other Types of Organic Reactions
 Oxidation-reduction reactions are reactions that involve the
loss and gain of electrons.
o One example is the conversion (or oxidation) of ethanol to
acetaldehyde which takes place in liver.
CH3CH2OH
CH3CHO + H2
+
Gaining O or losing H
oxidized
Losing O or gaining H
reduced
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H2
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10.5
Other Types of Organic Reactions
 Isomerization reactions are the reactions in which one isomer
is converted to another.
o An example is :
Aldose
Ketose
C3O3H6
C3O3H6
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10.5
Organic reactions
Organic Reactions
Substitution reactions
Electrophilic
addition
Nucleophilic
addition
SN1 reactions
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Addition reaction
Electrophilic
addition
Nucleophilic
addition
Other reactions
Elimination
Oxidationreduction
Isomerization
SN2 reactions
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25.6
Organic Polymers
 Polymers are molecular compounds that
are made up of many repeating units called
monomers.
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25.6
Organic Polymers
 Polymers are molecular compounds that
are made up of many repeating units called
monomers.
 Polymers:
o can be natural or synthetic.
o have physical properties different than
the properties of monomers.
o have a very high molar mass.
o contain monomers attached together by
chemical bonds.
o Are very important in industrial and
biological applications.
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25.6
Types of Organic Polymers
 Addition polymers form when
monomers join end to end.
Polyethylene is an example of
addition polymer.
 Condensation polymers form
when two different functional
groups combine in an
elimination reaction.
 Biological polymers are
naturally occurring polymers.
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25.6
Addition Polymers
Initiation step
Propagation step
Termination step
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25.6
Examples of Addition Polymers
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25.6
Condensation Polymers
 Condensation reactions are the basis of condensation
polymers. In a condensation reaction, two or more molecules
each with different functional group become connected with
the elimination of a small molecule (often water).
Carboxylic acid
Alcohol
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Alcohol
Ester
Alcohol
ether
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25.6
Condensation Polymers
 An example of condensation polymers is Nylon 66.
Diamine
Dicarboxylic acid
New bond formed
Monomer
New bonds formed
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25.6
Biological Polymers
 These are naturally occurring polymers, and they include:
o Proteins : polymers of amino acids.
o Polysaccharides : polymers of sugars.
o Nucleic acids : polymers of nucleotides.
• DNA (deoxyribonucleic acid).
• RNA (ribonucleic acid).
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20.6
Proteins Formation
 Protein formation.
Peptide bonds are also called “amide linkages” since they
contain an amide functional group.
 Very long chains are called proteins, while shorter chains are
called polypeptides.
o
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