Lecture 7 Organic Reaction Mechanisms.ppt

Lecture 5: Introduction to Organic Chemistry – A Comprehensive Guide for Undergraduate Students
This PowerPoint presentation, Lecture 5: Introduction to Organic Chemistry, is an essential resource designed for undergraduate students pursuing chemistry, biology, pharmacy, and other science-related disciplines. It provides a structured and detailed introduction to the fundamental concepts of organic chemistry, ensuring a solid foundation for further studies in the field.

Organic chemistry is a crucial branch of chemistry that focuses on the structure, properties, composition, reactions, and synthesis of carbon-containing compounds. It plays a pivotal role in various scientific fields, including medicine, pharmaceuticals, biochemistry, environmental science, and materials science. This lecture presentation covers the core principles necessary to understand organic molecules and their transformations, making it an invaluable tool for students and educators alike.

Key Topics Covered in this Lecture:
1. Introduction to Organic Chemistry
Definition and significance of organic chemistry in the scientific world.
Historical development of the field and major contributors.
The distinction between organic and inorganic compounds.
2. Structure and Bonding in Organic Compounds
Atomic structure and electronic configuration.
Covalent bonding and hybridization (sp, sp², sp³).
Molecular orbital theory and resonance structures.
The concept of electronegativity and polarity in organic molecules.
3. Functional Groups and Their Importance
Overview of key functional groups: alkanes, alkenes, alkynes, alcohols, ethers, aldehydes, ketones, carboxylic acids, esters, amines, amides, and aromatic compounds.
How functional groups determine the chemical properties and reactivity of organic molecules.
The role of functional groups in biological and industrial applications.
4. Nomenclature of Organic Compounds
Systematic IUPAC naming conventions for aliphatic and aromatic compounds.
Common names vs. IUPAC names.
Rules for naming branched-chain hydrocarbons, cyclic compounds, and complex organic molecules.
5. Isomerism: The Different Forms of Organic Molecules
Structural isomerism: Chain, positional, and functional group isomers.
Stereoisomerism: Geometrical (cis-trans) and optical isomerism.
Chirality and enantiomers in biological systems.
6. Reaction Mechanisms in Organic Chemistry
Introduction to reaction mechanisms and their importance in predicting product formation.
Classification of organic reactions:
Substitution reactions (SN1, SN2)
Addition reactions (electrophilic and nucleophilic addition)
Elimination reactions (E1, E2 mechanisms)
Rearrangement reactions
Oxidation and reduction reactions in organic synthesis
The role of catalysts and reaction conditions in determining reaction outcomes.
7. Organic Chemistry in Everyday Life
The impact of organic chemistry on medicine and drug development.
The significance of polymers and plastics in modern materials.

1. Introduction to Introduction to Organic Organic
Reaction MechanismsReaction Mechanisms
Lecturer:
Department of applied studies
February, 2017

2. Introduction
In general, organic chemical reactions are studied by
looking at:
◦what occurs (what kinds of reactions occur)
◦how it happens (how reactions occur)
An organic may be represented in general form as follows:
Organic reactions are written with reactants to the left of
the reaction arrow and products to the right like any
other reaction.
Substrate
Attacking
reagent
[Intermediate] Product(s) + By-product(s)

3. Introduction……..
There are usually two reactants in an organic reaction:
The substrate is the reactant that supplies carbon to the
newly formed bond.
The reagent is the chemical substance with which an organic
compound reacts
◦In an equation the reagent may be placed to the left
of the arrow or on the arrow itself. E.g.
+ HBrCH
3CHCH
2 CH
3CHCH
3
Br
CH
3CHCH
2 CH
3CHCH
3
Br
HBr

4. Introduction……Introduction……
In case, both reactants supply carbon to the new bond, the
molecule on which attention is focussed is arbitrarily chosen as the
substrate.
Other reaction conditions such as the solvent used and
temperature at which reaction is/was carried are often omitted but
at times they are placed above or below the arrow.
Some organic reactions are catalysed by light or supply of heat. In
such cases the symbols ‘h’ and ‘’ are used for light and heat,
respectively. E.g.
CH
4+ Cl
2
h
CH
3Cl + HCl
ethanol
H
2SO
4
170
o
C
ethene
CH
2CH
2CH
3CH
2OH

5. Types of Organic Reaction
Organic reactions are broadly classified into four:
◦Substitution
◦Addition
◦Elimination
◦Rearrangement reactions.

6. Substitution Reactions
Substitution reactions are those in which a group
attached to a carbon atom is substituted by another
group.
In substitution reactions, parts from two molecules
exchange E.g
In this example, Cl replaces H in CH
4
.
CH
3H+ Cl
2
h
CH
3Cl + HCl

7. Addition Reactions
Addition reactions occur in unsaturated systems such
as alkenes and alkynes.
In an addition reaction elements are added to the
starting material.
E.g. the addition of hydrogen halides e.g. hydrogen
bromide, HBr, to the carbon-carbon double bond of
an alkene, propene.
propene
+ HBr
Br
2-bromopropane

8. Elimination Reactions
Elimination reactions are the opposite of addition
reactions: a molecule is removed or lost or eliminated
or “subtracted” from the starting molecule.
In an elimination reaction, one molecule splits into
two
 An example: dehydrohalogenation of alkyl halides in
the presence of a base:
propene
Br
2-bromopropane
- HBr
OH

9. Rearrangement Reactions
In a rearrangement reactions under certain reaction certain reaction
conditions,conditions, a molecule undergoes changes in the way its
atoms are connected
A molecule may rearrange its structure to another form;
thermodynamically more stable (in having a lower energy
content).
E.g. Carbocation rearrangement forming from primary to tertiary
carbocation which is thermodynamically stable.
but-1-ene
acid catalyst
but-2-ene
2-methylpropan-1-ol
OHH
+
OH
2
protonation
-H
2O
protonated alcohol
Primary (1
o
)
carbocation
hydride
shift
Tertiary(3
o
)
carbocation
elimination
of water

10. Bond Breaking
In a reaction old bonds are broken and new ones formed.
Bond breaking (cleavage) in organic reactions requires energy
usually and occurs in two ways:
homolytic (symmetrical) fission:each atom leaves with one-half of
the shared electrons (one electron for a single bond, or two for
double bonds). E.g.
In homolytic cleavage, One bonding electron stays with each
product generating very reactive chemical species known as
radicals.
Such cleavage generally occurs in the gas phase or in non-polar
solvents and can be catalysed by light
A B A+B

11. Bond Breaking ….…Bond Breaking ….…
Heterolytic cleavage; One atom leaves with all of the
previously shared electrons and the other atom gets none
of them: two bonding electrons stay with one product
◦Results in the formation of an ion pair (charged intermediates).
◦Occurs in solution in polar solvents. Why?
(i) a polar solvent aids in the separation of charge (by favouring the bond
cleavage process)
(ii) the resultant ion pair so formed can be stabilised through solvation (by
the molecules of the polar solvent.)
A B A+B
A B A+B

12. Bond Making
Can be symmetrical (homolytic) or
unsymmetrical (heterolytic)
◦In homolytic bond formation one bonding electron
is donated by each reactant where as in heterolytic
bond making the two bonding electrons are supplied
by one reactant
A BA+B
(homolytic bond formation)
A BA+B (heterolytic bond formation)

13. Reaction ReagentsReaction Reagents
Recall there are usually two reactants in an
organic reaction: the substrate and the reagent.
Remember:
◦Substrate: the reactant that supplies carbon to the
newly formed bond
◦Reagent: chemical species that attacks the
substrate to form the product in a chemical
reaction.
Reagents are grouped into two depending on
the nature: electrophilic or nucleophilic

14. Electrophilic ReagentsElectrophilic Reagents
‘Electrophile’ literally means ‘electron- loving’.
They are chemical species that:
◦are “electron-loving”
◦are electron-deficient
◦can form a bond by accepting a pair of electrons
They react to increase their electron density: i.e.
electrophiles aim at gaining more electrons.
In a reaction, an electrophile will attract and accept
the pair of electrons from the electron rich centre in
a substrate eventually forming a covalent bond with it.

15. Electrophilic Reagents…….Electrophilic Reagents…….
All Lewis acids are electrophiles
Usually abbreviated a E, electrophiles may be
either neutral (E) or positively charged (E
+
)
◦Charged electrophiles tend to have a partial or full
positive charge,
◦Neutral electrophiles will not have a full electron
shell.
Examples of charged electrophiles:
H
+
, CH
3
+
, NO
2
+

Examples of neutral electrophiles include:
BH
3
, BF
3
, H
2
C=O.

16. Nucleophilic ReagentsNucleophilic Reagents
'nucleophile' literally means ‘nucleus loving’.
Nucleophiles are chemical species that:
◦are electron rich.
◦can form a bond by donating a pair of electrons
They are nucleus seeking reagents
Typically have a negative charge or a lone pair of
electrons
All Lewis bases are nucleophiles as they form a bond by donating a
pair of electrons
Nucleophiles tend to react to decrease their electron
density.
In an organic reaction nucleophilic reagents attack
electron deficient centre of the substrate and forms
covalent bond with it.

17. Nucleophilic Reagents…….Nucleophilic Reagents…….
Nucleophiles may be neutral or charged.
◦Charged nucleophiles tend to have partial of fully
negative charge and are abbreviated as Nu:
-
.
E.g. OH
-
,CN
-
◦Neutral nucleophiles have lone pairs of electrons and
are abbreviated as :Nu.
E.g. NH
3, H
2O, and alcohols (ROH).

18. What is a reaction Mechanism?
A reaction mechanism describes how a
reaction occurs
Illustration:
◦In a clock the hands move but the mechanism
behind the face is what causes the movement
◦Likewise 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

19. What is a reaction Mechanism…….What is a reaction Mechanism…….
Therefore the reaction mechanism shows:
◦which bonds are broken and which new ones are
formed
◦how bonds are broken and formed as starting
material is converted into a product the order and
relative rates of the various bond-breaking and
bond-forming steps
◦if in solution, the role of the solvent
◦if there is a catalyst, the role of a catalyst
In general reaction mechanism is a detailed description
of how bonds are broken and formed as starting
material is converted into a product

20. What is a reaction Mechanism…….What is a reaction Mechanism…….
An organic reaction can occur in more than one
step.
◦A one-step reaction is one;
The reactants are directly converted into products regardless
of how many bonds are broken or formed.
It is also known as a concerted reaction: several steps occur at
the same time
◦A multi-step or stepwise reaction;
The reactants are first converted into unstable intermediate
known as a reactive intermediate, which goes on to form the
product.
Each step involves either the formation or breaking of a
covalent bond

21. Indicating Steps in Reaction Mechanism
Both the homolytic and heterolytic processes
involve the movement of electrons.
When writing the reaction mechanisms, you
show the movement of the electrons using curly
(curved) arrows.
◦A half-headed curved arrow also known as a fishhook is
used to show the movement of a single electron i.e a
homolytic process:
◦A full headed arrow is used to show the movement of an
electron pair (two electrons) i.e. a heterolytic process
A B A B+
A B A+B
A B A+B

22. Reactive Intermediates
An organic reaction can occur in more than one
step.
In a multi-step reaction the reactants are first
converted into unstable intermediates known as
reactive intermediates which go on to form the
product.
Reactive intermediates are short lived, with
half-life ranging from milliseconds to minutes
and vary in their stability.
There are many types of reactive intermediates.
These include carbocations, free radicals and
carbanions

23. Reactive Intermediates:
Carbocations
A carbocation is an ion with a positively-charged
carbon atom.
The charged carbon atom in a carbocation has an
incomplete octet;
◦it has only six electrons in its outer valence shell instead of
the eight valence electrons that ensures maximum stability.
Therefore carbocations are often reactive as they seek
to fill the octet of valence electrons as well as regain a
neutral charge.
They are formed during heterolytic cleavage of a bond
between carbon and more electronegative element such
as oxygen (O), nitrogen (N) and a halogen (Cl, Br, I).

24. Reactive Intermediates:
Carbocations…..
Carbocations may also be formed when an organic
compound gets protonated as shown below:
CH
3 Cl CH
3+Cl
CH
3 OH CH
3+OH
CH
2
H
2
C
C
H
CH
2+ H
+
CH
2
H
2
C
C
H
CH
3

25. Reactive Intermediates:
Carbocations…..
Carbocations are classified as primary, secondary or
tertiary depending on how many other carbon atoms
are bonded to the charged carbon atom:
◦Primary, secondary and tertiary carbocations have one, two and
three carbon atoms, respectively bonded to the charged carbon
atom.
◦Tertiary carbocations are more stable than secondary
carbocations; primary carbocations are highly unstable.:
◦The stability of carbocations is influenced by three factors:
inductive effect, hyperconjugation and resonance effect
R'
C
H
R
R'
C
R"
R
H
C
H
R
Tertiary
carbocation
Secondary
carbocation
Primary
carbocation

26. Stability of Carbocations: Inductive
Effect
Inductive Effect: Electron redistribution due to
differences in electronegativities of substituents
Neighbouring alkyl groups contain electrons that are
polarizable, and these can shift towards the positive
charge. This is referred to as +I inductive effect.
◦The ‘I’ stands for "inductive" and the (+) sign indicates that the
group donates (or adds) electrons to the rest of the molecule.
◦All electron donating groups (EDG) such as alkyl groups are
referred to as +I groups.
◦Electron releasing such alkyl groups, stabilize the carbocation
making it easier to form.
◦Small hydrogen substituents cannot do this as well.
◦Opposed to electron donating groups are electron withdrawing
groups, such as -CF
3. These
destabilize the carbocation making it
harder to form.

27. Inductive Effect.......
H
3C
C
H
CH
3
H
3C
C
CH
3
CH
3
H
C
H
CH
3
H
C
H
H
> > >
+I Effect of electron donating methyl groups

28. Stability of Carbocations:
Hyperconjugation
Conjugation is a phenomenon where π electrons can be
shared over more than 3 or more atoms.
◦Conjugated molecules have π electrons that are not localized in
individual double or triple bonds. Rather their π electrons are
delocalized throughout an extended π system.
◦Conjugated unsaturated systems have a p orbital on a carbon
adjacent to a double bond
◦The p orbital may be empty (a carbocation), may hold one
electron (a radical) or may hold two electrons (carbanion)
◦The p orbital may be part of another multiple bond

29. Hyperconjugation…Hyperconjugation…
Hyperconjugation?
◦Hyper means above or beyond
◦Conjugation means getting together
Hyperconjugation involves the sigma () bonds
◦It involves the overlapping of a sigma bonding orbital with an
empty antibonding pi (π) orbital of an adjacent atom resulting
in the sharing of the electrons for example

30. Hyperconjugation…Hyperconjugation…
This overlapping and sharing of the electrons is known as
hyperconjugation. Hyperconjugation increases the stability of an
organic molecule
In carbocation, hyperconjugation spreads the positive charge onto
the adjacent alkyl group:This is due to orbital overlap between the
bond and the empty p orbital on the sp
σ
2
carbon.
Therefore increasing the number of alkyl substituent groups attached
to the charged carbon increases the stability of the carbocation.

31. Stability of Carbocations: Resonance
Resonance Effect: Conjugation with a multiple bond or
lone pairs of electrons increases the stability of a
carbocation.
Therefore allylic and benzylic systems are more stable
than their saturated counterparts.
◦The positive charge delocalized through resonance by utilizing
an adjacent pi system
CH
3
H
C
C
H
CH
2
CH
3
H
C
C
H
CH
2
Effect of Resonance in allylic carbocation

32. Stability of Carbocations:
Resonance....
Positive charge delocalized into the benzene ring.
Increases stability of carbocation.
CH
2 CH
2
CH
2 CH
2
Effect of Resonance in benzylic (aromatic) system

33. Reaction of Carbocations
Once a carbocations are formed as the reactive
intermediate, they undergo through some chemical
changes.
◦They react with negatively charged species (nucleophiles) to give
a neutral compound.
◦They may also lose a proton (deprotonation) to give a neutral
compound.
◦Carbocations can also undergo rearrangement to form another
carbocation.
CH
3
C
CH
3
CH
3
CH
3
C
CH
3
CH
3
+
-
OH
OH
Reaction of carbocation with a nucleophile

34. Reactions of Carbocations…..
Rearrangement of carbocations only occurs to give cations
of similar or increased stability. Rearrangement of a
carbocation frequently involves a 1,2 shift of a hydride. The
1,2 shift means the migration is from the carbon adjacent to
the positively charged carbon atom.
CH
3
C
C
H
2
CH
3
CH
3
C
CH
2
CH
3
Deprotonation of a carbocation
H
-H
+
CH
3
C
CH
2
CH
3
CH
3
C
CH
3
CH
3
Rearrangement of a carbocation
H

35. Carbanions
A carbanion is an organic species with a negative charge on the
carbon atom.
In a carbanion the negatively charged carbon has an unshared pair
of electrons, hence bearing the negative charge.
They are nucleophiles because they contain a carbon with a lone
pair (sp
3
carbon atoms).
A carbanion is formed due to heterolytic fission of a bond between
carbon and a less electronegative atom like H or any other metal
(M).
O
C
H
2
C
C
H
2
H
H
OH+ O
C
H
2
C
H
2C
H
+ H
2O

36. Carbanions……Carbanions……
The stability of carbanions is determined by three
factors: inductive effect, resonance and hybridisation
R'
C
R"
R
M
R'
C
R"
R
+ M
+

37. Stability of Carbanions:
Inductive Effect
Electronegative atoms adjacent to the negatively charged
carbon will stabilise the charge as the electron density shifts to
such atoms.
This is referred to as -I inductive effect. All electron withdrawing
groups (EWG) are referred to as -I groups.
Electron-withdrawing groups stabilize a carbanion while
electron-donating groups destabilize it.
Examples of electron withdrawing groups include -C=O, -NO
2, -CN, -SO
2.

38. Stability of Carbanions:
Resonance Effect
Resonance effects can stabilise the carbanion
especially in aromatic systems.
CH
2 CH
2
CH
2 CH
2
Effect of Resonance in aromatic system

39. Stability of Carbanions:
Hybridisation Effect
The fractional s-character of the C-H bonds has a major effect
on the stability of the carbanion.
The greater the s-character of the carbon bearing the negative
charge, the more stable the carbanion.
Only s-orbitals have electron density at the nucleus, and a lone
pair with high fractional s character has its electron density
closer to the nucleus, and is hence stabilized.
Carbanions are electron rich and therefore behave as
nucleophiles and therefore are involved in nucleophilic addition
or substitution reactions.
CH
3C C CH
3C
H
CH CH
3
H
2
C CH
2
sp
sp
2 sp
3
> >

40. Carbon Free Radicals
A carbon free radical is electrically neutral organic species in which
the carbon atom carries odd or unpaired electron.
Carbon free radical is produced during as a result of homolytic
fission of a bond between carbon and any other atom.
Carbon free radicals are formed when organic reactions are
carried out at high temperatures or in the presence of light,
peroxide or non-polar solvents.
The stability of carbon free radicals is influenced by resonance and
hyperconjugation and the order of stability is:
Benzyl > allyl> tertiary > secondary > primary > methyl free
radical

41. Carbon Free Radicals…..
Carbon free radicals are very reactive and react with each other or
themselves to form neutral compounds.
They are usually involved in setting up a chain reaction. For
example reactions of alkanes with halogens in the presence of
light.
Order the following carbon free radicals in the order of
increasing stability. Explain your answer.
H
C
H
CH
3
CH
3
C
H
CH
3
CH
3
C
CH
3
CH
3
CH
3
H
C
CH
2