Information about different organic reactions and their mechanism of action. It is basically a part of medicinal chemistry.

1. ORGANIC REACTIONS AND THEIR MECHANISMS
Reactants
Conditions
Products
Classical Structural Theory of Organic Reactions
Classical structural theory is that the chemical behavior of an organic compound is
determined by the functional groups in the molecules. Thus, the course of an organic
reaction could be easily interpreted by indicating the interaction between functional
groups of reacting molecules.
Incompetence of the classical structural theory
H2C CH2
C2H5Br Br easily replaced by OH
C6H5Br
C O in aldehhyde readily formed addition product with
in C
O
OH did notC OHCN but
addition products easily did under forced condition
Br can not be replaced by OH

2. Reaction Mechanism -- Fundamental Aspects
Substrate: A substrate may be defined as the reactant that contain carbon atoms some of whose
bonds with other atoms are broken and some new bonds are formed as a result of reaction with an
attacking agent.
The carbon bonds in the substrate molecule are broken or cleaved to give fragments which are very
reactive and constitute transitory intermediate. At once they may react with other similar species or
with molecules in their environment, thus establishing the new bonds to give product.
Attacking agents: For a mechanistic approach, an organic reaction is believed to take place by the
attack of a reagent on a compound containing carbon (substrate) and the reagent is known as
attacking agents.
Attacking reagent bear either +ve or -ve charge. Therefore, attacking agent will not attack the
substrate successfully unless the latter somehow possessed oppositely charged center in the
mole.
Substrate molecule must develop polarity on some of its carbon atoms and constituents linked
together. Polarity is developed by the displacement of bonded electrons.
Attacking Reagent + Substrate Product
Substrate Product
(Transitory)
Intermediate
Attacking Reagent bear either +ve or -ve charge
Substrate ------- electrically neutral ------- develop polarity on some
of its carbon atom

3. Bond Fission: Bond fission means separation of two atoms from each other in a
covalently bonded molecule.
Bond Fission
Homolytic fission Heterolytic fission
Each of the atoms acquire one of the bonded electrons
One of the atoms acquire both of the bonded electrons because A is more
electronegative than B which thereby acquires both the bonding electrons
and becomes negatively charged.
A B A + Heterolytic fissionB
- +
A B A + Homolytic fissionB
Light
Energy

4. Carbonium Ions: An organic ion containing a positively charged carbon
atom is called a carbonium ion
C
H
H
H
C
H
H
H
X
Heterolytic
fission
+ X
-
+
C
H
H
H
C
H
H3C
H
C
CH3
H3C
H
C
CH3
CH3
H3C
10
20
30
+ + + +
C
R
R
R + C
H
H
R +C
R
H
R +> >
30
20
10

5. THE RELATIVE STABILITIES OF CARBOCATIONS
Carbanium ion being deficient in electrons is very unstable. Electron realising group
such as alkyl group adjacent to the carbon atom makes the carbanium ion stable by
partial neutraliztion of the positive charge on carbon
Alkyl groups, when compared to hydrogen atoms, are electron releasing thus the
order of stability of carbocations parallel the number of attached alkyl / methyl
groups.
C
R
R
R + C
H
H
R +C
R
H
R +> >
30
20
10
C
H
H
H
+
>
The relative stabilities of carbocations is 3° > 2° > 1° > methyl

6. Molecular rearrangement /Carbocation Rearrangements
When carbocations are intermediates, a less stable carbocation can rearrange to a
more stable carbocation by a shift of a hydrogen or an alkyl group. This is called a
carbocation rearrangements.
The migrating group in a 1,2-shift moves with two bonding electrons, giving
carbon a net positive (+) charge.
A 1,2-shift converts a less stable carbocation to a more stable carbocation.
C C
H
C C CC
HH
+
C C
R
C C CC
RR
+
1,2 Hydride shift: Movement of a hydrogen atom is called 1,2 Hydride shift
1,2 alkyl shift: Movement of an alkyl group is called 1,2 alkyl shift

7. CH3 C C
CH3
CH3
CH3
H
CH3 C C
CH3
CH3
H
CH3
2° carbocation 3° carbocation
1,2 methyl shift
H
CH3 C
H
C H
H
CH3 C
H
C H
H
H
1,2 Hydride shift
1° carbocation 2° carbocation

8. Reactions of Carbonium ion
Proton loss CH3 CH CH2
H
- H+
CH3 CH CH2
Propylene
+
Combination
with nucleophile
Addition to
an alkene
Abstraction of
Hydride ion
CH3 C +CH2
CH3
CH3
C
CH3
CH3 CH3 C CH2
CH3 CH3
C
CH3
CH3
CH3
CCH2
CH3CH3
C
CH3
H3C
CH3
C
CH3
CH3H+
CH3
CHCH2
CH3CH3
C
CH3
H3C
CH3
C
CH3
CH3+
CH3 CH2 CH3 CH2 Br+ Br

9. Carbanions: An organic ion with a pair of electrons and a negative charge on the
central carbon atom is called a carbanion (carb, from carbon + anion, negative charge)
C
H
H
H
C
H
H
H
Y
Heterolytic
fission
+
Y
-
+
H C CH2
O
- C O+ H C CH2
O
C O
-
Reactions of Carbanion
Addition reaction
Substitution reaction
Na+
HC
OOCC2H5
OOCC2H5
CH3 I
-
+ HC
OOCC2H5
OOCC2H5
NaI+CH3

10. Stability of Carbanions: Carbanions being rich in electrons is very unstable,
so electron attracting/withdrawing group such as CN, NO2 group makes the
carbanion more stable
+- C
CN
CH
>
- -
More stable
On the other hand, electron realising group such as alkyl group adjacent to the
carbon atom makes the carbanion ion less stable by increasing the electron
density on the negatively charged carbon.
The relative stabilities of carbanion is Methyl > 1° > 2° > 3°
C
H
H
H
C
H
H3C
H
C
CH3
H3C
H
C
CH3
CH3
H3C
10
20
30
> > >

11. Homolytic bond fission means separation of two atoms in which each of the atoms
acquire one of the bonded electrons
A B A + Homolytic fissionB
Light
Energy
Free radicals: An atom or group of atoms possessing an odd (unpaired) electrons is known
as free radical. A dot generally represents the odd electrons, the symbol of free radical.
They are produced by homolytic fission of the molecule
A : B A + B. .Light/heat

12. Alkyl radicals are classified as being 1°, 2°, or 3° on the basis of the carbon atom that
has the unpaired electron.
Propyl radical (a 1° radical)
Isopropyl radical (a 2° radical)
The relative stabilities of alkyl radicals: The order of stability of alkyl radicals is the
same as for carbocations. Alkyl radicals are uncharged, the carbon that bears the odd
electrons is electron deficient. Therefore, electron-releasing alkyl groups attached to
this carbon provide a stabilizing effect, and more alkyl groups that are attached to this
carbon, the more stable the radical is.

13. Electrophiles: An agent which can accept an electron pair in a reaction is called electrophile.
Electrophiles are electron-deficient. The name electrophile means (electro = electrons and
philic = loving) “electron-loving” and indicates that it attacts regions of high electron density
(negative centre) of the substrate molecules.
Electrophiles
Positive electrophiles Neutral electrophiles
They are deficient
of two electrons and
carry a positive charge
Neutral molecules with
electron deficient centre
C+
H
H
H
Br ::
:
+
B
F
F
F Al
Cl
Cl
Cl
Nucleophile: An agent which can donate an electron pair in a reaction is called nucleophile.
Nucleophiles are electron rich. The name nucleophile means (nucleo = nucleus and
philic = loving) “nucleus-loving”. Since the nucleus is electrically positive, the nucleophile
will attacts regions of low electron density (positive centre) of the substrate molecules
Nucleophiles
Negative Nucleophiles Neutral Nucleophiles
C:
H
H
H - An excess electron pair
and carry a negative charge
N:
H
H
H
Br ::
::
Posses spare electron
pair and no charge

14. Electron Displacement Effects
Inductive effect: The permanent effect whereby polarity is induced on the carbon atom
and the substituent attached to it due to minor displacement of bonding electron pair caused
by their different electronegativities, is known as inductive effect or simply as I-effect.
Inductive effect (I effect) refers to polarity produced in a molecule as a result of
higher electronegativity of one atom compared to other
Inductive effect
Positive Inductive
Effect (+ I effect)
Negative Inductive
Effect (- I effect)
C X
 
C : X C X
 
C Y


C : Y C Y

The substituent bonded to carbon atom is electron
attracting /withdrawing group, it develops negative
charge and the inductive effect is called negative
inductive effect (-I effect)
The substituent bonded to carbon atom which loses
electrons toward carbon atom (electron releasing
group), it develops positive charge and the inductive
effect is called positive inductive effect (+I effect)

15. C : H C H
H C
H
H
H C X
 
C : X C X
 
C Y


C : Y C Y

C X C Y
Standard -I Effect +I Effect
C C C X
   
C C C X
   
-I Effect group (electron
attracting/withdrawing group):
NO2 > F > COOH > Cl >
Br > I > OH > C6H5
+I Effect group (electron
resealing group):
(CH3)3C- > (CH3)2CH- >
CH3CH2
- > CH3-
C--C---C---X
g---b---a
-I Effect +I Effect

16. Electromeric effects: The effect which causes a temporary polarization in the substrate at
the seat of a multiple bond by shift of an electron pair from one atom to other under the
influence of an eletrophillic agent is called electrometric effect.
When a double or triple bond is exposed to attack by an electrophillic agent, a pair of
bonding electron is transferred completely from anoe atom to another. The atom that takes
the electron pair becomes negatively charged and the other develops positive charge.
or bond Temporary effect
A B
Electrophilic reagent
added
removed
A B
+ -
C C C C
reagent
E+
+ electron

17. Mesomeric effect: When an electron withdrawing or pumping group is conjugated
with a  bond or a set of alternately arranged s and  bonds, the electron displacement
is transmitted trough the  electrons in the chain. The  electrons will be displaced to
more electronegative atoms gives resonance in a molecule and thus develops polarity in
a molecule.
CH CH CH CH O
CH CH CH CH OH2C
+ _
-M effect
H2C
CH CH CH NH2
CH CH CH NH2H2C
+
+M effect
H2C
-

18. Hyperconjugative effect: Hyperconjugation effect takes place through the
interaction of s electrons of C-H bond with  electrons of the double bond.
CH CH2C
H
H
H
CH CH2C
H
H
H
+
-
*
Greater the number of C-H bonds at a position to the unsaturated system,
greater would be the electron release towards the terminal C (* mark) creating
high electron density.
C
H
H
H
C
H
H3C
H
C
CH3
H3C
H
C> > >
CH3
CH3
H3C
10
20
30

19. Types of Reactions
• Addition reaction
• Substitution reaction
• Elimination reaction
• Rearrangement reaction

24. Electrophilic Addition reactions in Conjugated Dienes
Conjugated dienes are compounds having two double bonds joined by one s bond.
Conjugated dienes are also called 1,3-dienes.
1,3-Butadiene (CH2=CH-CH=CH2) is the simplest conjugated diene.
•A conjugated diene is more stable than an isolated diene because a conjugated diene
has overlapping p orbitals on four adjacent atoms. Thus, its  electrons are delocalized
over four atoms.

25. Electrophilic Addition: 1,2- Versus 1,4-Addition
The  bonds in conjugated dienes undergo addition reactions that differ in two ways from
the addition reactions of isolated double bonds.
Electrophilic addition in conjugated dienes gives a mixture of products.
Conjugated dienes undergo a unique addition reaction not seen in alkenes or isolated
dienes.
Electrophilic addition of one equivalent of HBr to an isolated diene yields one product
and Markovnikov’s rule is followed.
Electrophilic addition in conjugated dienes gives a mixture of products
1,2-addition product - addition at the 1- and 2-positions - direct addition
1,4-addition product - addition at the 1- and 4-positions - conjugate addition

26. Electrophilic addition of one equivalent of HBr in a conjugated diene affords two products.
• The 1,2-addition product results from Markovnikov addition of HBr across two adjacent carbon
atoms (C1 and C2) of the diene.
• The 1,4-addition product results from addition of HBr to the two end carbons (C1 and C4) of the
diene. 1,4-Addition is also called conjugate addition.
• Addition of HX to a conjugated diene forms 1,2- and 1,4-products because of the resonance-
stabilized allylic carbocation intermediate.
• The ends of the 1,3-diene are called C1 and C4 arbitrarily, without regard to IUPAC numbering.

28. Kinetic Versus Thermodynamic Products
The amount of 1,2- and 1,4-addition products formed in electrophilic addition reactions of
conjugated dienes depends greatly on the reaction conditions
When a mixture containing predominantly the 1,2-product is heated, the 1,4-addition
product becomes the major product at equilibrium

29. Nucleophilic substitution is a fundamental class of substitution reaction in which an
"electron rich" nucleophile selectively bonds with or attacks the positive or partially
positive charge of an atom attached to a group or atom called the leaving group.
In nucleophilic substitution reactions, the C–X bond of the substrate undergoes
heterolysis, and the lone-pair electrons of the nucleophile is used to form a new
bond to the carbon atom
A nucleophile is an the electron rich species that will react with an electron poor
species. A leaving group , LG, is an atom (or a group of atoms) that is displaced
as stable species taking with it the bonding electrons.

30. SN2 reaction: When the rate of a nucleophilic substitution reaction depends on both
of the concentration of substrate and nucleophile is known as SN2 reaction. In SN2
reaction, a lone pair of electron from a nucleophile attacks an electron deficient
electrophilic center and bonds to it, expelling another group called a leaving group.
Thus the incoming group replaces the leaving group in one step. Since two reacting
species are involved, rate-determining step of the reaction, this leads to the name
bimolecular nucleophilic substitution, or SN2.
Kinetics: The rate of the reaction depends on the concentration of substrate and the
concentration of nucleophile. Rate of reaction ∝ [Substrate] [Nucleophile]. Thus,
the reaction is bimolecular i.e. two species are involved in the rate-determining step.
Reaction order: The reaction is second order overall
The SN2 reaction: Substitution, Nucleophilic, bimolecular

31. MECHANISM OF SN2 REACTION

32. MECHANISM OF SN2 REACTION
• The nucleophile attacks the carbon bearing the leaving group from the backside.
The nucleophile attacks the carbon at 180° to the leaving group and the leaving
group is then pushed off the opposite side and the product is formed.
• The bond between the nucleophile and the carbon atom is forming, and the bond
between the carbon atom and the leaving group is breaking. The formation of the
bond between the nucleophile and the carbon atom provides most of the energy
necessary to break the bond between the carbon atom and the leaving group.
• The transition state involves both the nucleophile and the substrate, in which the
nucleophile and the leaving group are both bonded to the carbon atom undergoing
attack so the carbon under nucleophilic attack is pentacoordinate.
• Bond formation and bond breaking occur simultaneously in a single transition state,
the SN2 reaction is a concerted reaction.
• The configuration of the carbon atom becomes inverted during SN2 reaction.

33. STEREOCHEMISTRY OF SN2 REACTIONS
In an SN2 reaction, the nucleophile attacks from the back side, that is, from the
side directly opposite the leaving group and this attack causes a change in the
configuration of the carbon atom so the configuration of the carbon atom becomes
inverted which is known as Walden inversion because the first observation of such
an inversion was made by the Latvian chemist Paul Walden in 1896.

34. Inversion of configuration can be observed when hydroxide ion reacts with
cis-1-chloro-3-methylcyclopentane in an SN2 reaction
Inversion of configuration can be also observed when the SN2 reaction takes place at
a stereocenter (with complete inversion of stereochemistry at the chiral carbon center)

35. SN1 reaction: When the rate of a nucleophilic substitution reaction depends only the
concentration of substrate molecule is known as SN1 reaction. In an SN1 reaction,
there is loss of the leaving group generates an intermediate carbocation which is then
undergoes a rapid reaction with the nucleophile.
H3C
C Br
H3C
H3C
OH-
OH-
H3C
C
H3C
H3C
Kinetics: The rate of the reaction depends on the concentration of substrate but not
the concentration of nucleophile.
Rate of reaction ∝ [Substrate]. Thus, the reaction is uniimolecular i.e. one species
is involved in the rate-determining step.
Reaction order: The reaction is first order overall
The SN1 reaction: Substitution, Nucleophilic, unimolecular

36. MECHANISM OF SN1 REACTION
Step 1: Rate determining (slow) loss of the leaving group, LG, to generate a carbocation
intermediate
H3C
C
CH3
CH3
+
OH-
CH3
CHO
CH3
CH3
H3C
C OH
H3C
H3C
(a)
(b)(a) (b)
Inversion
(Predominates)
Retension
Step 2: Rapid attack of a nucleophile on the electrophilic carbocation to form a new σ bond
H3C
C Br
H3C
H3C
H3C
C
CH3
CH3
+ Br
-
+
Carbonium ion

37. STEREOCHEMISTRY OF SN1 REACTIONS
The carbocation has a trigonal planar structure and it may react with a nucleophile
from either the front side or the back side:
H3C
C Br
H3C
H3C
H3C
C
CH3
CH3
+ Br
-
+
OH OH
With the tert-butyl cation it makes no difference but with some cations
(optically active compounds), different products arise from the two
reaction possibilities.

38. SN1 reaction proceeds with racemization
In SN1 reaction, racemization (a reaction that transforms an optically active compound
into a racemic form) takes place whenever the reaction causes chiral molecules to
be converted to an achiral intermediate. The SN1 reaction proceeds with racemization
because the intermediate carbocation is achiral and attacked by the nucleophile can occur
from either side.
The carbocation has
a trigonal planar
structure and is achiral.
(S)-3-bromo-3-methylhexane
(optically active)
(S)-3-methyl-3-hexanol (R)-3-methyl-3-hexanol
(optically inactive, a racemic form)

39. OH-
This side open
to attack
This side shielded
from attack
(retention)(inversion)

40. OH-
This side open
to attack
This side shielded
from attack
(retention)(inversion)

41. FACTORS AFFECTING THE RATES OF SN1 AND SN2 REACTIONS
• The structure of the substrate
• The concentration and reactivity of the nucleophile
• The effect of the solvent
• The nature of the leaving group.
The effect of the structure of the substrate on SN2 reactions:
In an SN2 reaction, the nucleophile attacks from the back side, that is, from the
side directly opposite the leaving group. Substituents on or near the reacting carbon
have a dramatic inhibiting effect.
Simple alkyl halides show the following general order of reactivity in SN2 reactions:
methyl > 1° > 2° >> 3° (unreactive) because the central carbon atom surrounded
by bulky groups will be sterically hindered for SN2 reactions.
A steric effect is an effect on relative rates caused by the space-filling properties of
those parts of a molecule attached at or near the reacting site.
Steric hindrance: the spatial arrangement of the atoms or groups at or near the
reacting site of a molecule hinders or retards a reaction.
Although most molecules are reasonably flexible, very large and bulky groups

42. H
C X
H
H
OH-
H3C
C X
H3C
H3C
OH-
Easy attack
Difficult attack
Methyl 1°
2° 3° Neopentyl

43. SN1 Reactions: In an SN1 reaction, there is loss of the leaving group generates an
intermediate carbocation. The primary factor that determines the reactivity of organic
substrates in an SN1 reaction is the relative stability of the carbocation that is formed.
Organic compounds that are capable of forming relatively stable carbocation
can undergo SN1 reaction at a reasonable rate.
The stability order of carbocations is exactly the order of SN1 reactivity for alkyl
halides. The relative stabilities of carbocations is 3° > 2° > 1° > methyl
C
R
R
R + C
H
H
R +C
R
H
R +> >
30
20
10
C
H
H
H
+
Thus the order of reactivity in SN1 reactions: 3° > 2° > 1° > methyl (unreactive)

44. Thus, the order of reactivity in SN2 reactions: methyl > 1° > 2° >> 3°
(unreactive) and in SN1 reactions: 3° > 2° > 1° > methyl (unreactive)

45. Effect of the nature and the concentration of the nucleophile
SN1 reactions: Neither the concentration nor the structure of the nucleophile affects
the rates of SN1 reactions since the nucleophile does not participate in the rate-
determining step.
SN2 reactions: The rates of SN2 reactions depend on both the concentration and the
structure of the nucleophile. A stronger nucleophile attacks the substrate faster.
A strong nucleophile as well as high concentration of the nucleophile always
favor the SN2 reactions
A negatively charged nucleophile is always a more reactive nucleophile than its
conjugate acid. HO– is a better nucleophile than H2O; RO– is a better nucleophile
than ROH.
“Nucleophilicity” measures the affinity (or how rapidly) of a Lewis base for a
carbon atom in the SN2 reaction (relative rates of the reaction).
“Basicity”, as expressed by pKa, measures the affinity of a base for a proton (or
the position of an acid-base equilibrium).

46. SOLVENT EFFECTS ON SN2 REACTIONS
Protic Solvents: The solvent molecule has a hydrogen atom attached to an atom of a
strongly electronegative element e.g. hydroxylic solvents such as alcohols and water
Molecules of protic solvents form hydrogen bonds with the nucleophiles and slows
down the rate of SN2 reactions.
Polar Aprotic Solvent: Aprotic solvents are those solvents whose molecules do not
have a hydrogen atom attached to an atom of a strongly electronegative element.
Polar aprotic solvents are especially useful in SN2 reactions because polar aprotic
solvents do not solvate anions to any appreciable extent because they cannot form
hydrogen bonds. The rates of SN2 reactions generally are vastly increased when
they are carried out in polar aprotic solvents.

47. (DMF) (DMSO) (DMA)
Polar aprotic solvents dissolve ionic compounds, and they solvate cations very well.
A sodium ion solvated by
molecules of the protic
solvent water

48. SOLVENT EFFECTS ON SN1 REACTIONS
Since SN1 reaction involves the the formation of carbocation intermediate in the
rate determining step, anything that can facilitate will speed up the reaction.
Polar protic solvent will greatly increase the rate of ionization of an alkyl halide
in any SN1 reaction.
Polar protic solvents solvate cations and anions effecttively. Cations are solvated
chiefly through unshared pares of elctrons: anions are solvated chiefly through hydrogen
bonding.
Solvation stabilizes the transition state leading to the intermediate carbocation and
halide ion more it does the reactants.
Thus SN1 reactions proceed more rapidly in water, alcohols, and mixture of
water and alcohols than in aprotic solvents like DMF and DMSO

49. Elimination reactions: Two or four atoms or groups attached to the adjacent carbon
atoms in the substrate molecule are eliminated to form a multiple bond. Elimination
reactions are important as a method for the preparation of alkenes. The two most
elimination reactions are:
• Dehydration (-H2O) of alcohols, and
• Dehydrohalogenation (-HX) of alkyl halides.
Dehydrohalogenation involves the elimination of the halogen atom and of a hydrogen
atom from a carbon adjacent to the one losing the halogen. The reagent required is a
base, whose function is to abstract the hydrogen as a proton.
Dehydrohalogenation is a b elimination or1,2-elimination reaction because the carbon
holding the halogen is designated as a or 1-carbon and elimination involves the loss of b
or 2-hydrogen from b or 2-carbon adjacent carbon holding the halogen.
There are three fundamental events in these elimination reactions:
• Removal of a proton
• Formation of the CC  bond
• Breaking of the bond to the leaving group
R CH CH2 X
HOH-
R CH CH2
H2O + X-
+
12

50. How does such an elimination reaction generate a double bond?
Regardless of the exact mechanisms,
• Halogen leaves the molecule as halide ion and hence must take its electron pair
along.
• Hydrogen is abstracted by the base as a proton and hence leave its electron pair
behind.
• It is this electron pair that is available to form the second bond, the  bond
between the carbon atoms.
R CH CH2 X
HOH-
R CH CH2
H2O + X-+
Types of elimination reactions:
• Bimolecular Elimination Reaction (E2)
• Unimolecular Elimination Reaction (E1)

51. Bimolecular Elimination Reaction (E2): E2 reactions proceed by second order kinetics
and the reaction involves in a single step:
Base pulls a proton away from the carbon; simultaneously a halide ion departs and the
double bond forms between the carbons.
Halogen takes its electron pair along with it and hydrogen leaves its electron pair behind
to form the double bond.
R CH CH2 X R CH CH2
H
R C
H
CH2 X
H OH
Transition state
OH-
The base begins to remove a proton
from the β-carbon using its
electron pair to form a bond to it.
At the same time, the electron pair
of the β C−H bond begins to move
in to become the π bond of a double
bond, and the bromide begins to
depart with the electrons that bonded
it to the α carbon.
Partial bonds now exist
between the oxygen and
the β hydrogen and
between the α carbon
and the bromine.
The carbon-carbon bond is
developing double bond
character.
Now the double bond
of the alkene is fully
formed

52. Orientation of elimination reactions
CH3 CH2 CH2 Br
KOH (alc)
CH3 CH CH2
Dehydrohalogenation yields a single alkene
C
H
b Dehydrohalogenation yields a mixture of alkenes
corresponding to the loss of any one of the b-hydrogen
CH3 CH2 CH CH3
KOH (alc) CH3 CH CH
Br
CH3
CH3 CH2 CH CH2
+
CH3 CH2 C CH3
KOH (alc)
CH3 CH C
Br
CH3
CH3 CH2 C CH2
+
CH3
CH3
CH3
In a mixture of alkenes which one is the preferred product and why?

53. Zaitsev's rule, Saytzeff's rule or Saytsev's rule named after Alexander Mikhailovich
Zaitsev is a rule that states that if more than one alkene can be formed by an
elimination reaction, the more stable alkene is the major product.
When an alkylhalide has two or three b carbons, a mixture of isomers is possible and
the more substituted alkene is the major product because the compound that has a
more highly substituted C=C double bond is more stable due to the electron donating
properties of the alkyl group.
• The major product will be the most highly substituted alkene, the product
with the fewest H substituents on the double bonded carbons

54. Unimolecular Elimination Reaction (E1): E1 reaction proceeds by first order
kinetics and involves of two steps:
Step 1: Substrate undergoes heterolysis to form halide ion and carbocation
Step 2: The carbocation rapidly loses the proton to the base and forms alkenes
CH3 C X
CH3
CH3
C X-
CH2
H3C CH3
+
H
Slow
C X-
CH2
H3C CH3
+
H
OH-
Slow
C
CH2
H3C CH3
H2O + X-+

55. E1 reactions accompained by rearrangement, where structure permits
CH3 C C
CH3
CH3
CH3
H
CH3 C CH
CH3
CH3
CH3
Br
CH3 C C
CH3
CH3
CH3
H
CH3 C C
CH3
CH3
H
CH3
CH3 C C
CH2
CH3
H
CH3
CH3 C C
CH3
CH3
CH3
2° carbocation
Methyl shift
3° carbocation
Major productMinor product

56. SN2 VERSUS E2
Since eliminations occur best by an E2 path when carried out with a high
concentration of a strong base (and thus a high concentration of a strong
nucleophile), substitution reactions by an SN2 path often compete with the
elimination reaction.
(a) When the nucleophile (base) attacks a β carbon atom, elimination occurs.
(b) When the nucleophile (base) attacks the carbon atom bearing the leaving
group, substitution results.
Primary halides: substitution is favored, Secondary halides: elimination is favored
and Tertiary halides: no SN2 reaction, elimination reaction is highly favored

57. THE NATURE OF THE LEAVING GROUP
The best leaving groups are those that become the most stable ions after they
depart. Most leaving groups leave as a negative ion and the best leaving groups are
those ions that stabilize a negative charge most effectively.
The leaving group begins to acquire a negative charge as the transition state is
reached in either an SN1 or SN2 reaction
SN1 Reaction
(rate-limiting step)
SN2 Reaction
Stabilization of the developing negative charge at the leaving group stabilizes the
transition state (lowers its free energy) lowers the free energy of activation and
increases the rate of the reaction.
The effect of the leaving group is the same in both SN1 and SN2:
R–I > R–Br > R–Cl

58. Rearrangement Reaction
A rearrangement reaction is a broad class of organic reactions where the carbon
skeleton of a molecule is rearranged to give a structural isomer of the original
molecule.
The reactions which proceed by a rearrangement or reshufling or shifting of
atoms/groups from one position to another in the molecule to produce a
structural isomer of the original substance are called rearrangement reaction.
CH3 CH2 CH2 CH3 CH3 CH CH3
CH3
n-Butane iso-Butane
• Intramolecular rearrangement
• Intermolecular rarrangement

59. Hofmann rearrangement
• Hofmann rearrangement is the organic reaction of a primary amide to a primary amine
with one fewer carbon atom.
• Primary amide is transferred into an intermediate isocyanat in the presence of bromine
with sodium hydroxide
• The intermediate isocyanate is hydrolyzed to a primary amine giving off carbon dioxide.

61. Beckmann Rearrangement
• When ketoxime is treated with an acidic catalyst such as H2SO4, H3PO4, SOCl2,
PCl5 etc., it is converted into a substituted amide.
• The reaction mechanism of the Beckmann rearrangement is generally believed to
consist of an alkyl migration with expulsion of the hydroxyl group to form a
nitrilium ion followed by hydrolysis
CH3 C CH2 CH3
N OH
C CH2 CH3
HN CH3
O
PCl5
R C R'
N OH
R C R'
N Cl
C R'
NR
+
H2O:
C R'
NR
OH
H
+C R'
NR
OHC R'
NHR
O
PCl5

62. Fries rearrangement is a rearrangement reaction of a phenyl ester to a hydroxy
aryl ketone by catalysis of lewis acids
OH OCOR OH OH
RCOCl AlCl3
COR
COR
+
O
C
R
O
AlCl3
O
C
R
O
O
C
R
O
AlCl3
AlCl3
O
C
R
O
AlCl3
O
C
R
O
AlCl3
O
C R
O
AlCl3
H
O
C R
O
AlCl3
O
C R
O
H2O
H
+
_
_
+
_
+
_
++
_
+
- H+
_
Mechanism

63. Mechanism of Fries rearrangement
• Lewis acid, AlCl3 co-ordinates to the carbonyl oxygen atom of the acyl group
because this oxygen atom is more electron rich than the phenolic oxygen atom
• Polarization of the bond between the acyl residue and the phenolic
oxygen atom and AlCl3 group rearranges to the phenolic oxygen atom.
• Generation of a free acylium carbocation which reacts in a classical electrophilic
aromatic substitution with the aromat.
• The abstracted proton is released as hydrochloric acid where the chlorine is
derived from aluminium chloride.
• The orientation of the substitution reaction is temperature dependent. A low
reaction temperature favors para substitution and with high temperatures the
ortho product