1. Chapter 25 Organic Chemistry Part II
Dr. Al-Saadi

2. 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
CH3OCH3
CH3CH2COH
Propanone
Propanal
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3. Constitutional (structural) Isomers
25.4
Classes of Isomers
Isomers
Stereoisomers
Geometrical Isomers
Optical Isomers
Constitutional (structural) Isomers
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4. Constitutional Isomerism
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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5. Constitutional Isomerism
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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6. 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 Optical isomers
cis-platin
trans-platin
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7. 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
Opposite side
Same side
Geometrical isomers
Structural isomer
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8. 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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9. 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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10. Chiral molecules are optically active
25.4
Optical Isomers
To represent chirality of molecules when drawing them on a piece of paper, we use wedges and dashes.
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.
Chiral molecules are optically active
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11. Optical Activity of Enantiomers
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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12. Optical Activity of Enantiomers
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
plane-polarized
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13. Optical Activity of Enantiomers
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
Minimal transmission occurs when the plane of polarization of the light is perpendicular to that of the analyzer
Solution of optically active enantiomers
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14. Optical Activity of Enantiomers
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
Dr. Al-Saadi

15. 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:
Addition reactions.
Electrophilic addition.
Nucleophilic addition.
Substitution reactions.
Electrophilic substitution.
Nucleophilic substitution.
SN1 and SN2 reactions.
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16. Electrophiles and Nucleophiles
25.5
Electrophiles and Nucleophiles
An electrophile “loves electrons” is a species with a region of positive or partial positive charge, such as:
cations (H+ ion).
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:
anions (Cl‒ ion) , or
the negative region of polar molecules like HCl.
Nucleophiles are electron-rich.
The keys to understand organic reactions
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17. Electrophilic Addition Reactions
25.5
Electrophilic Addition Reactions
+ HCl
Where are the electron-rich and electron-poor areas?
Nucleophile
Electrophilic attack
Electrophile
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18. Electrophilic Addition Reactions
25.5
Electrophilic Addition Reactions
These curved pink arrows show the movement of electron pairs “Mechanism” 
intermediate  
Mechanism:
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.
A lone pair on the electron-rich Cl‒ ion forms a new σ bond with the electron-deficient carbocation.
The hybridization of the C atoms has changed from sp2 to sp3.
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19. Electrophilic Addition Reactions
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.
intermediate
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20. Electrophilic Addition Reactions
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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21. Nucleophilic Addition Reactions
25.5
Nucleophilic Addition Reactions O H H H O C O + O O C O H
Electrophile
Nucleophile
Where are the electron-rich and electron-poor areas?
Mechanism:
Nucleophilic attack.
Stabilized by resonance
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22. Nucleophilic Addition Reactions
25.5
Nucleophilic Addition Reactions
Breaking an O―H bond and forming H+ ion.
New O―H bond formed leading to the product.
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23. 25.5
Addition Reactions
Addition reactions are reactions that involve the addition of a molecule or an ion to another molecule.
Electrophilic addition. (species to be added is an electrophile)
Nucleophilic addition. (species to be added is a nucleophile)
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24. 25.5
Addition Reactions
Addition reactions are reactions that involve the addition of a molecule or an ion to another molecule.
Electrophilic addition. (species to be added is an electrophile)
Nucleophilic addition. (species to be added is a nucleophile)
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25. Addition Reactions Exercise:
25.5
Addition Reactions
Exercise:
Draw the mechanism for the nucleophilic addition of CN ion to CH3CHO.
Nucleophile
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26. Substitution Reactions
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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27. Electrophilic Substitution Reactions
25.5
Electrophilic Substitution Reactions
Mechanism:
Formation of the NO2+ electrophile.
Electrophilic attack which breaks a π bond.
Electrophile (E+)
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Carbocation

28. Electrophilic Substitution Reactions
25.5
Electrophilic Substitution Reactions
Carbocation is stabilized by resonance.
The π bond is restored and H+ ion is produced.
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29. Electrophilic Substitution Reactions
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
Dr. Al-Saadi

30. Nucleophilic Substitution Reactions
25.5
Nucleophilic Substitution Reactions
Nucleophilic substitution can be of two types:
SN2 reactions: Direct nucleophilic back-side attack .
In SN2 reactions the molecular configuration is inverted.
transition state
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31. Nucleophilic Substitution Reactions
10.5
Nucleophilic Substitution Reactions
The other type of nucleophilic substitution is:
SN1 reactions: First, a group is leaving the C atom. Then, a nucleophile attacks the carbocation intermediate.
Mechanism:
A group is leaving and a carbocation intermediate is formed.
Nucleophilic attack
carbocation
Dr. Al-Saadi

32. Nucleophilic Substitution Reactions
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.
The produced two enantiomers which are mirror images of each other “racemization”
Dr. Al-Saadi

33. Mechanisms of Substitution Reactions
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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34. Other Types of Organic Reactions
10.5
Other Types of Organic Reactions
Elimination reactions are reactions in which a double bond forms and a small molecule is removed.
Example is the following step in carbohydrate metabolism.
+ X‒
+ H―Nu
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35. Other Types of Organic Reactions
10.5
Other Types of Organic Reactions
Oxidation-reduction reactions are reactions that involve the loss and gain of electrons.
One example is the conversion (or oxidation) of ethanol to acetaldehyde which takes place in liver.
CH3CH2OH CH3CHO + H2
+ H2
Gaining O or losing H
oxidized
Losing O or gaining H
reduced
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36. Other Types of Organic Reactions
10.5
Other Types of Organic Reactions
Isomerization reactions are the reactions in which one isomer is converted to another.
An example is :
Aldose Ketose
C3O3H6 C3O3H6
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37. Organic reactions Organic Reactions Substitution reactions
10.5
Organic reactions
Organic Reactions
Substitution reactions
Electrophilic addition
Nucleophilic addition
SN1 reactions
SN2 reactions
Addition reaction
Other reactions
Elimination
Oxidation-reduction
Isomerization
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38. 25.6
Organic Polymers
Polymers are molecular compounds that are made up of many repeating units called monomers.
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39. 25.6
Organic Polymers
Polymers are molecular compounds that are made up of many repeating units called monomers.
Polymers:
can be natural or synthetic.
have physical properties different than the properties of monomers.
have a very high molar mass.
contain monomers attached together by chemical bonds.
Are very important in industrial and biological applications.
Dr. Al-Saadi

40. Types of Organic Polymers
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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41. Addition Polymers Initiation step Propagation step Termination step
25.6
Addition Polymers
Initiation step
Propagation step
Termination step
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42. Examples of Addition Polymers
25.6
Examples of Addition Polymers
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43. Condensation Polymers
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
Ester
Alcohol
Alcohol
ether
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44. Condensation Polymers
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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45. 25.6
Biological Polymers
These are naturally occurring polymers, and they include:
Proteins : polymers of amino acids.
Polysaccharides : polymers of sugars.
Nucleic acids : polymers of nucleotides.
DNA (deoxyribonucleic acid).
RNA (ribonucleic acid).
Dr. Al-Saadi

46. Proteins Formation Protein formation.
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.
Dr. Al-Saadi