The presentation gives an overall introduction about basic organic concepts

1. An Overview of Organic
Reaction Mechanisms
by
Dr.Y.Aparna
Assistant Professor of Chemistry
Dept.of Sciences and Humanities
Matrusri Engineering College
Saidabad

2. An Overview of Organic
Reaction Mechanisms
1. Introduction
2. Reacting species
3. Type of reactions
4. Reaction mechanisms

3. Types of organic reactions
 Substitution – an atom (group) of the molecule is replaced by
another atom (group)
 Addition – π-bond of a compound serves to create two new
covalent bonds that join the two reactants together
 Elimination – two atoms (groups) are removed from a molecule
which is thus cleft into two products
 Rearrangement – atoms and bonds are rearranged within the
molecule; thus, isomeric compound is formed

4. 4
Types of Organic Reactions
 Addition reactions
 Two molecules combine
 Elimination reactions – one molecule splits

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 Substitution
Species from two molecules exchange

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 Rearrangement reactions:
 A molecule undergoes changes in the way its
atoms are connected

7.  Elimination:
 In most cases, the two atoms/groups are removed from the
neighbouring carbon atoms and a double bond is formed (-
elimination)
– H2O

8. 8
Steps in Mechanisms
 A step involves either the formation or breaking of a
covalent bond
 Steps can occur in individually or in combination with
other steps
 When several steps occur at the same time they are
said to be concerted

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Types of Steps in Reaction
Mechanisms
 Bond formation or breakage can be symmetrical or
unsymetrical
 Symmetrical- homolytic
 Unsymmetrical- heterolytic

10. 10
Radical Reactions
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

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 Three types of steps
1.Initiation – homolytic formation of two reactive species with
unpaired electrons
 Example – formation of Cl atoms form Cl2 and light
2.Propagation – reaction with molecule to generate radical
 Example - reaction of chlorine atom with methane to
give HCl and CH3
.
3.Termination – combination of two radicals to form a stable
product: CH3
. + CH3
.  CH3CH3
Steps in Radical Substitution

12. 12
Polar Reactions
 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
 Elements such as O, F, N, Cl more electronegative than
carbon

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14. 14
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

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Generalized Polar Reactions
 An electrophile, an electron-poor species, combines
with a nucleophile, an electron-rich species
 An electrophile is a Lewis acid
 A nucleophile is a Lewis base
 The combination is indicated with a curved arrow
from nucleophile to electrophile

16. 16

17. 17
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

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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

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Rules for Using Curved Arrows
 The nucleophilic site can be neutral or negatively
charged

20. 20
 The electrophilic site can be neutral or
positively charged
 Don’t exceed the octet rule (or duet)

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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
products
 If Keq is 10, then the concentration of the product
is ten times that of the reactant
 A value of Keq less than one indicates that at
equilibrium most of the material is present as the
reactant
 If Keq is 0.10, then the concentration of the
reactant is ten times that of the product

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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‡)

23. 23

24. 24
First Step in Addition
 In the addition of HBr
the (conceptual)
transition-state
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

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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

26. Aromatic electrophilic substitution using
Lewis acids
 Halogenation:
 Very often, electrophilic substitution is used
to attach an alkyl to the benzene ring
(Friedel-Crafts alkylation):
benzene carbocation bromobenzene

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 Inductive Effect
 Permanent shift of -bond electrons in the molecule composed
of atoms with different electronegativity:
 – I effect is caused by atoms/groups with high
electronegativity that withdraw electrons from the
neighbouring atoms: – Cl, –C=O, –NO2:
 +I effect is caused by atoms/groups with low
electronegativity that increase electron density in their
vicinity: metals, alkyls:

28. Mesomeric effects
 Permanent shift of electron density along the -bonds (i.e. in
compounds with unsaturated bonds, most often in aromatic
hydrocarbons)
 Positive mesomeric effect (+M) is caused by atoms/groups
with lone electron pair(s) that donate π electrons to the system: –
NH2, –OH, halogens
 Negative mesomeric effect (–M) is caused by atoms/groups
that withdraw π electrons from the system: –NO2, –SO3H, –C=O

29. Activating/deactivating groups
 If inductive and mesomeric effects are contradictory, then the
stronger one predominates
 Consequently, the group bound to the aromatic ring is:
 activating – donates electrons to the aromatic ring, thus
facilitating the electrophilic substitution:
 a) +M > – I… –OH, –NH2
 b) only +I…alkyls
 deactivating – withdraws electrons from the aromatic ring,
thus making the electrophilic substitution slower:
 a) –M and –I… –C=O, –NO2
 b) – I > +M…halogens

30. Electrophilic substitution & M, I-effects
 Substituents exhibiting the +M or +I effect (activating groups,
halogens) attached to the benzene ring direct next substituent to the
ortho, para positions:
 Substituents exhibiting the –M and – I effect (–CHO, –NO2) direct
the next substituent to the meta position:

31. Nucleophilic substitution
 Electron-rich nucleophile introduces an electron pair into the
substrate; the leaving atom/group retains the originally bonding
electron pair:
|Nu– + R–Y  Nu–R + |Y–
 This reaction is typical of haloalkanes:
 Nucleophiles: HS–, HO–, Cl–
+
alcohol is produced

32. Radical addition
 Again: initiation (creation of radicals), propagation (radicals attack
neutral molecules, producing more and more radicals), termination
(radicals react with each other, forming a stable product; the chain
reaction is terminated)
 E.g.: polymerization of ethylene using dibenzoyl peroxide as an
initiator:

33. Electrophilic addition
 An electrophile forms a covalent bond by attacking an electron-
rich unsaturated C=C bond
 Typical of alkenes and alkynes
 Markovnikov´s rule: the more positive part of the agent (hydrogen
in the example below) becomes attached to the carbon atom (of
the double bond) with the greatest number of hydrogens:

34. Nucleophilic addition
 In compounds with polar unsaturated bonds, such as C=O:
 Nucleophiles – water, alcohols, carbanions – form a covalent bond
with the carbon atom of the carbonyl group:
used for synthesis
of alcohols
– carbon atom carries +
aldehyde/ketone

35. Rearrangement
Migration of a hydrogen atom, changing the position of the double
bond
 Keto-enol tautomerism of carbonyl compounds: equilibrium
between a keto form and an enol form:

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