Sem I A) Electronic displacements by Dr Pramod R Padole

1
B.Sc. First year
Students
Semester – I
Electronic Displacements
by
Dr Pramod R Padole

Organic Chemisrtry
Unit-III
A) Electronic Displacements:
B) Reactive Intermediates:
C) Aliphatic Hydrocarbons:

III A) Electronic Displacements:
Electromeric
effect
Inductive
effect
1 2 3 4
Hyperconjugation
Resonance
(Definition, and applications of these effects))

III B) Reactive Intermediates:
First Carbocations
:their generation stability and reactions
Second Carbanions
Third Free Radicals

3. Alkynes: Preparation from vicinal and germinal dihalides,
Reaction- Hydrogenation
2. Alkenes: Methods of formation (With mechanism):
i) Dehydrohalogenation of alkyl halides (E1 & E2)
ii) Dehydration of alcohols, Reactions: Electrophilic
& free radical addition of HX and X2 (with mechanism)
1. Alkanes:
Methods of formation: i) Wurtz reaction and ii) Corey-House reaction,
Reactoins: i) Halogenatiom (With mechanism), ii) Aromatisation.
4. Alkadienes: Classification,1,3-Butadiene-
Preparation from cyclohexene, Reactions- Addition of H2, Br2 and HBr.
C) Aliphatic Hydrocarbons:

LOGO
Unit-III

COMPANY LOGO
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‘The distortion of the symmetrical electron charge cloud
of an anion by the cation is termed as polarisation’.
The power or ability of a cation to polarize
(distort) a nearby anion is known as polarising
power and the tendency of the anion to get
distort or polarise by cation is known as
polarizability.

Q.1) What if two atoms of equal electronegativity
bond together?
 Consider a bond between two atoms, A and B.
 If the atoms are equally electronegative, both have the same
tendency to attract the bonding pair of electrons, and so it will
be found on average half way between the two atoms:
 To get a bond like this, A and B would usually have to be the
same atom.
 For example, H2 or Cl2 molecules.
 Note: The electrons are actually in a molecular orbital, and are
moving around all the time within that orbital. This sort of bond
could be thought of as being a "pure" covalent bond -
where the electrons are shared evenly between the two atoms.

Q.2) What if B is slightly more electronegative
than A?
 B will attract the electron pair rather more than A
does.
 That means that the B end of the bond has more than its fair
share of electron density and so becomes (δ-) slightly negative.
 At the same time, the A end (rather short of electrons) becomes
(δ+) slightly positive.
 A polar bond is a covalent bond in which there is a separation of
charge between one end is slightly positive and the other slightly
negative.
 Examples include most covalent bonds. The hydrogen-chlorine
bond in HCl or the hydrogen-oxygen bonds in water are typical.

Q.3) What if B is a lot more electronegative
than A?
 If B is a lot more electronegative than A, then
the electron pair is dragged right over to B's
end of the bond.
 To all intents and purposes, A has lost control of its
electron, and B has complete control over both
electrons. Ions have been formed.
 The bond is then an ionic bond rather than a covalent
bond.

Or Mobility of electrons:
 Basically covalent bond is non-polar
(e.g. Cl-Cl), but due to some factors it
becomes polarised.
 Mobility (or migration or transmission or
displacement) of electrons in the
covalent bonds, can be studied with the
help of two effects, which are
(i) Inductive effect &
(ii) Electromeric effect.

Inductive Effect:
Or Transmission Effect:
Mobility or Migration or Transmission of electrons in the single
covalent bond

Inductive Effect:
Q.1) Define Inductive effect with suitable examples.
(S-11, S-12 & S-17, 2 Mark)
Q.2) What is Inductive effect? Explain –I & +I effects with
suitable examples. (W-12, 4 Mark)
Q.3) Explain Inductive effect with suitable examples.
(W-13 & S-15, 4 Mark)
Q.4) Explain the term: Inductive effect with suitable examples.
(W-14 & S-15, 2-4 Mark)
Q.5) Discuss the application of Inductive effect.
(S-18, 4 Mark)
Q.6) Define Inductive effect and discuss the applications of
Inductive effect. (S-19, 4 Mark)

pramodpadole@gmail.com By Dr. Pramod R. Padole
Inductive Effect:
 Permanent polarization (polarity produced, i.e., of
single covalent bond due to difference in
electronegativity between two bonded atoms is
known as Inductive effect.
Or
Effect of electric dipole on single covalent bond
is known as Inductive effect.
Defination:
CH3 CH2 CH2 CH2 Cl




In this, the electronegativity of Chlorine is greater than Carbon.

a)

b) CH3 CH2 CH2 CH2 Li


 
In this, the electronegativity of Carbon is greater than Lithium.

Inductive Effect:
Salient features:
i) It occurs in saturated compounds.
ii) It is a permanent effect.
iii) It involves the displacement or migration of the
sigma electrons (bond).
iv) It is observed only when the atoms are joined
together by single covalent bond, with different
electronegatvity.
v) In this effect, displacement of electron shared pair
within the shell (i.e, without leaving the octet
of the bonded atoms take place).
vi) Attacking reagent is not required.

Inductive Effect:
vii) Due to this effect, partial opposite but equal
charges (developed on the two atoms.
viii) Effect of inductive effect decreases, as the length
of carbon chain in atoms, this effect falls off.
ix) Inductive effect can be relayed from one atom
to another in a chain of carbon atoms.
x) It is denoted by an arrow ( ) which shows the
direction of displacement of electron shared pair.
xi) It is represented / designated by ‘I’.
xii) Examples or Representation:
CH3 CH2 CH2 CH2 Cl





In this, the electronegativity of Chlorine is greater than Carbon.
b) CH3 CH2 CH2 CH2 Li





In this, the electronegativity of Carbon is greater than Lithium.
a)

pramodpadole@gmail.com
Types of Inductive Effect:
Positive
inductive
effect
(+I effect):
I-effect
Negative
inductive
effect
(-I effect):
By Dr. Pramod R. Padole

Positive inductive effect
(+I effect):
Defination:
 The atoms or group of atoms which donates ( or
release or repel or pushes) shared pair of electron
(bonded pair) from itself and thus acquires partial
positive charge (+δ), called positive inductive effect
(+I effect).
 Note: i) +I group: Electron donating or releasing
or repelling group show +I effect.
ii) Representation:
C CH3
C D
Donar atom / group



Positive inductive effect
(+I effect):
iii) The + I effect of atoms or group of atoms
decreases as the number of α-hydrogen decreases.
iv) Examples:
Alkyl groups show +I effect:
(CH3)3 C- > (CH3)2 CH- > CH3CH2- > CH3-

ii) Negative inductive effect (-I effect):
 The atoms or group of atoms which withdraw
or attract shared pair of electron towards itself and
thus acquires partial negative charge (-δ), called
negative inductive effect (-I effect).
 Note:
i) -I group: Electron attracting or withdrawing or
accepting group show -I effect.
ii) Representation:

C Cl
C W
Withdrawing atom / group


ii) Negative inductive effect (-I effect):
iii) The - I effect of atoms or group of atoms decreases
as the electronegativity of the atom or group of
atoms decreases.
iv) Examples:
-OH, -OCH3 , -Cl, -Br, -I, –COOH, -CN, -NO2, etc.
[Note that:
i)The terminology +I and –I is due to Ingold, while
Robinson has suggested opposite sign.
ii) To avoid confusion, we shall use Ingold
terminology throughout.]

LOGO
Applications of inductive effect:
Q.1) Discuss the application of Inductive effect. (S-18, 4 Mark)
Q.2) Define Inductive effect and discuss the applications of
Inductive effect. (S-19, 4 Mark)

(i) To determing the stability of free
radicals, cation and anions following
relationships are considered:
a) Stability of carbocation or free
radical is directly proportional to +I
power of the group.
Thus, the stability order for carbonium
ions and free radicals is 30 > 20 > 10
b) Stability of carbanions directly
proportional to -I power of the group.
Thus, the stability order for carbanion
is 10 > 20 > 30

(ii) To determining the strength of acids and bases
anions, following relations are considered:
(a) Strength of acid is directly
proportional to -I power of the group
present in acid.
For example:
(i) Relative strength of chloroacetic acid
and acetic acid.
 Strength of an acid depends on its tendency to release H+ ion
in aqueous solution in the form of H3O+ ions. The greater is
the tendency to release H+ ions in the aqueous solution in the
form of H3O+ ions greater is the strength of an acid.
 The strength of an acid is expressed in terms of pKa value. The
lesser the pKa value the greater is the strength of an acid.

 In chloroacetic acid the electron withdrawing
effect (-I effect) of chlorine facilitates release of
proton in the form of H3O+ ion in aqueous
solution.
 Hence chloroacetic acid is a stronger acid than
acetic acid

By Dr Pramod R Padole
Example-2)
ii) Relative strength of acetic acid and formic acid.
 In acetic acid, the electron donating methyl group
(+I effect) opposes the release of proton in the form
of hydronium ion (H3O+) in aqueous solution. Hence
formic acid is a stronger acid
<
Stronger acid
Weaker acid

LOGO
b) Strength of base is directly
proportional to + I power of the group
present in base.
For example:
Relative strength of methyl amine and
ammonia:
 The strength of nitrogen containing base
depends on the availability of nitrogen lone pair
for protonation. The greater the availability of
nitrogen lone pair for protonation the greater is the
strength of base.

LOGO
 In terms of pKb values, the lesser the pKb values
the greater is the strength of base.
 Due to electron donating inductive effect
(+I effect) of methyl group, there is increase in
electron density on nitrogen which increases
availability of nitrogen lone pair for
protonation / donation. Hence methyl amine is
a stronger base than ammonia.

LOGO
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Mobility
or
displacement of
electrons in
multiple bond:
This phenomenon is known as Electromerization.

Mobility or displacement of electrons in multiple bond:
Introduction:
 It is a temporary effect observed in compounds
containing at least one π-bond or the compound that
undergoes addition reaction.
 In electromeric effect, π-bonded electrons are completely
transfer by leaving the octet of one atom when exposed to
reagent.
 This phenomenon is known as Electromerization.
 Due to such electron transfer, one atom lose an electron, get
positive charge (+), while other gain electron, get negative
charge (-). This electron transfer is shown by arrow ( )
whose head shows the position of transfer of electrons.
A B A B
Attacking reagent
Removal of reagent

LOGO
Q.1) Define Electromeric effect with suitable examples. (S-12, W-16 & S-18, 2 Mark)
Q.2) Explain Electromeric effect with suitable example. (S-13, W-13 & S-17, 4 Mark)
Q.3) What is electromeric effect? (S-16,1 Mark)
(ii) Electromeric Effect:
(Mobility or Migration or displacement of electrons
in the multiple covalent bond)

LOGO
Electromeric Effect:
Defination:
 The temporary complete transfer of π-bonded electron pair
from one atom to another atom joined by multiple bond in
presence of an attacking suitable reagent (such as
electrophile or nucleophile) is called as Electromeric effect.
 Salient features:
i) It occurs in unsaturated compounds (e.g. C=C or C≡C bond).
ii) It is a temporary effect. (W-12, ½ Mark)
iii) It is related to migration or transfer of electrons in π-bond
and observed only in presence of attacking suitable
(proper) reagent.
iv) It is observed only when the atoms are joined together by
multiple (double or triple bond) covalent bond with same
or different electronegativity.

LOGO
 Salient features:
v) In this effect, the complete transfer of electron shared pair
with leaving the octet of one of the two atoms.
vi) It occurs in presence of an attacking reagent.
vii) Due to this effect, one atom becomes negatively (-) charged.
viii) Effect of electromeric effect falls off, when attacking reagent
is removed (It may be called polarisability of multiple bond).
ix) Electromeric effect can be relayed from one atom to another,
if they are joined by alternate single and double bonds.
x) It is denoted by curved arrow ( ) which shows the direction
of electron shifting.
xi) It is represented / designated by ‘E’.

LOGO
xii) Representation / Examples:
a) Electromeric effect in alkene:
b) Electromeric effect in Carbon chain (Relayed of
π-electrons from one atom to another):
[ Note: As soon as the attacking reagent is removed, the normal electronic condition is restored.]
A B A B
Attacking reagent
Removal of reagent

LOGO
On the basis of the direction of the electron movement,
the electronic effect is classified in two types:

LOGO
(i) Positive electromeric effect
(+E effect):
When the transfer of π-electrons occurs towards
the atom where the attacking reagent
(electrophile) attaches, then the effect is called
positive electromeric effect (+E effect).
For example: Addition of acids to alkenes:
C = C + H C C
H

LOGO
(ii) Negative electromeric effect
(-E effect):
When the transfer of π -electrons occurs away
from the atom where the attacking reagent
(nucleophile) attaches, then the effect is called
negative electromeric effect (-E effect).
For example: Addition of cyanide ion to carbonyl
compounds.
C = O + CN C O
CN

LOGO
Resonance
& Resonance Effect:

LOGO
Resonance:
The double bond keep on changing their
positions and this is called Resonance.
Resonance is also called mesomerism.
I II
Resonance structure of Benzene Resonance hybrid

LOGO
Resonance:
I II
Resonance structure of Benzene Resonance hybrid

LOGO
Necessary conditions for resonance:
1) All the resonating structures must have the same
arrangement of atomic nuclei.
2) The resonating structures must have the same
number of paired and unpaired electrons.
However, they differ in the way of distribution of
electrons.
3) The energies of the various limiting structures
must be same or nearly the same.
4) Resonating structures must be planar.
5) All the resonating structures do not contribute equal
to the real molecule and hence only the major
contributing forms are used while representing a
resonance hybrid.

LOGO
How do you know when to draw
resonance structures?
pramodpadole@gmail.com By Dr Pramod R Padole

LOGO
Mesomeric effect or
Resonance Effect:
permanent effect and they effect the physical as well as chemical
property of the compound
Resonance refers
to delocalization of electrons in a given system.
Resonance effect or Mesomeric effect is
the electron donating or withdrawing nature of
a substitutent due to resonance.

LOGO
Resonance Effect or Mesomeric effect:
 The effect caused by the phenomenon of resonance or
mesomerism in molecule is called resonance effect or
mesomeric effect.
Defination:
 Resonance effects can either donate or withdraw
electron density, depending on whether, they place
a ‘+ve’ or ‘–ve’ charge on Carbon atoms of the
benzene ring.
 Resonance effect is a permanent effect and observed only
in the molecules or ions with substituent containing
conjugated system of π-bonds or conjugated system of π-
bond and lone pair of electrons.
 Thus π-electrons are involved in resonance effect.
Q.1) Explain the term: Resonance effect with suitable example. (S-16, 2 Mark)
Q.2) Define Mesomeric effect with suitable examples. (S-18, 2 Mark)

LOGO
Resonance Effect or Mesomeric effect:

LOGO
pramodpadole@gmail.com By Dr Pramod R Padole
Types of resonance effect:
Positive
Resonance
Effect
(+ R effect):
Resonance
Effect
Negative
Resonance
Effect
(- R effect):

Positive Resonance Effect (+ R effect):
 +R effect is electron donating, when
resonance structures place a ‘-ve’ charge on
carbon atoms of the benzene ring.
 +R effect is observed when Z-atom having a
lone pair of electrons except, alkyl group(R-),
is directly bonded to the benzene ring.
Z = N / O / X The atom of the group is directly attached to
the benzene ring is called as Key atom
Cl Br I OH OR NH2 etc.
Z = , , , , , ,

Positive Resonance Effect (+ R effect):
 +R effect is electron donating, when
resonance structures place a ‘-ve’ charge on
OH
I II III
Phenol
Resonance structures of Phenol
Three R.S. place a -ve charge on C-atoms in the ring



:
:
OH
:
OH
:
OH
:
OH
:

Resonance hybrid

Negative Resonance Effect (- R effect):
 -R effect is electron withdrawing, when
resonance structures place a ‘+ve’ charge on
 –R effect is observed when π-bond between Y & Z
and Z is more electronegative than Y.
Z is more EN-atom than Y
(i.e. Z > Y )
C– atom is key atom
& N is more EN
than C-atom
In these groups, the key atom (-Y=Z) is
positively charged are joined to the more
electronegative atom (Z) by multiple bonds
(double or triple bonds).

Negative Resonance Effect (- R effect):
 -R effect is electron withdrawing, when
resonance structures place a ‘+ve’ charge on
C
I II III
Benzoic acid
Resonance structures of benzoic acid

 
Resonance hybrid
O
HO
C
O
HO
C
O
HO
C
O
HO
C
O
HO


Applications of resonance effect:
Stabilities of molecules and ions
1
Unexpected bond lengths in
molecules and ions
2
Acidity of phenols and
basicity of amines
3
Mechanism of reactions
4
Resonance effect is useful to explain:
Note that:
I) Bond length of C-C bond = 1.54 Ao
II) Bond length of C=C bond = 1.34 – 1.37 Ao
III) Bond length of all C-C bonds in Benzene = 1.39 Ao

LOGO
Hyperconjugation or
No bond resonance or
Hyperconjugative Effect:
The idea of hyperconjugation
was put forward by Baker and
Nathan in 1935.
CH2 H
.
.
C
H C
CH2 H
H
H
H
1 -free radical
0
3- equivalent forms

Hyperconjugation:
 The delocalization of σ-electrons or lone pair of electrons into
adjacent π-orbital or p-orbital is called hyperconjugation.
 It occurs due to overlapping of σ-bonding orbital or the orbital
containing a lone pair with adjacent π-orbital or p-orbital. It is also
known as "no bond resonance" or "Baker-Nathan effect".
CH2 H
.
.
C
H C
CH2 H
H
H
H
1 -free radical
0
3- equivalent forms
Q.1) Explain the term: Hyperconjugation or No Bond Resonance or Hyperconjugative effect
with suitable example. (W-12, S-14 & W-14, 4 Mark)
Q.2) Delocalization of σ-bond electrons with π-bond electron is called as: (W-17, ½ Mark)
(a) Inductive effect (b) Resonance effect (c) Hyperconjugative effect (d) Electromeric effect.
Q.3) Explain: Hyperconjugation (W-18, 2 Mark)

Hyperconjugation:
 The relative stability of carbonium ions or free radicals can be
explained by the numbers of hyperconjugation structures.
 Such structures are obtained by shifting the bonding electrons from
an adjacent C-H bond to the electron-deficient carbon.
 In this way, the positive charge originally on carbon is shifted
(dispersed) to the hydrogen.
 This manner of electron release by assuming no-bond character in
the adjacent C-H bond is called Hyperconjugation or No-bond
resonance.

Hyperconjugation:
Defination:
1) The interaction of σ-electrons of C-H bond
with π- electrons of the double bond is called
hyperconjugation.
Or
2) The electron release through σ →π conjugation
is known as Hyperconjugation effect.
Or
3) Delocalization of σ-bond electrons with
π-bond electron is called as Hyperconjugation
effect.
CH2 H
.
.
C
H C
CH2 H
H
H
H
1 -free radical
0
3- equivalent forms

Hyperconjugation:
 In general, more number of hyperconjugative structures
(no-bond resonance structures), more the stability of the
species.
For example-1):
 The ability of alkyl groups to release electrons via
hyperconjugative effect increases the stability of free radical.
 In general, more number of hyperconjugative structures; the
greater is the stability of free radical.
 Hence the decreasing order of stability of alkyl free
radical is,

Hyperconjugation:
CH2 H
.
.
C
CH3
C
CH2 H
CH3
H
H
.
2 -free radical
0
6- equivalent forms
CH2 H
.
.
C
H C
CH2 H
H
H
H
1 -free radical
0
3- equivalent forms
C
H
H
H
1 - methyl free radical
0
No - equivalent forms
.

Hyperconjugation:
 The ability of alkyl groups to release electrons via hyperconjugative
effect increases the stability of carbocation.
 In general, more number of hyperconjugative structures; the greater is
the stability of carbocation.
 Hence the decreasing order of stability of alkyl carbocations is,
C
CH2-H
CH3
CH3
C
CH2 H
CH3
CH3
+
+
3 -butyl carbonium ion
0
9 equivalent forms

Hyperconjugation:
 Applications of hyperconjugation:
The concept of hyperconjugation is used to explain relative stabilities of alkenes,
carbocations and free radicals. The greater the number of hyper conjugative structures
the greater is the stability of the molecule or ion.
C
CH2-H
CH3
C
CH2
H
CH3
H
H
+
+
2 -carbonium ion
0
6- equivalent forms
C
CH2-H
H C
CH2
H
H
H
H
+
+
1 -carbonium ion
0
3- equivalent forms
C
H
H
H
+
1 - methyl carbonium ion
0
No - equivalent forms

LOGO
Electronic Displacements by Dr Pramod R. Padole
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Sem I A) Electronic displacements by Dr Pramod R Padole

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