Semester - I B) Reactive Intermediates by Dr Pramod R Padole

1
B.Sc. First year
Students
Semester – I
Reactive Intermediates:
by
Dr Pramod R Padole

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

Company
LOGO
Most organic reactions occur through a intermediate, these are
generally short lived and highly reactive.

Short lived and highly reactive

Important reactive intermediates are:
Carbocations
(Carbonium Ions):
Carbanions:
Free Radicals
or Carbon Free Radicals:

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

III B) Reactive Intermediates:
First Carbocations (Carbonium Ions):
Q.1) What are carbocations? How are they obtained? Write their stability order.
(S-11, 5 Mark)
Q.2) Define: Carbocations (Carbonium ion) with examples. (S-12 & S-17, 2 Mark)
Q.3) What are carbocations? (W-13 & W-17, 1 Mark)
Q.4) What are Carbocations? Give two methods of their formation.
(S-14 & W-15, 4 Mark)
Q.5) Carbocations have _____ geometry. (S-14, ½ Mark)
Q.6) Explain the stability of carbocations on the basis of inductive effect and
resonance effect. (W-14, W-16 & W-17, 4 Mark)
Q.7) What are Carbocations? Give one method of their formation and
stability order of alkyl carbocations. (S-15, 5 Mark)
Q.8) Organic ions which containing positively charged carbon atoms are
called _______. (W-15, ½ Mark)
Q.9) The species containing positively charged carbon centre are called _______.
(W-16, ½ Mark)
Q.10) Define: Carbocation. Explain its stability on the basis of inductive effect.
(W-18, 4 Mark)

Carbocations (Carbonium Ions):
Defination:
 Organic ions which contain a positively
charged carbon atom are called carbocations
or carbonium ions.
Or
A species contain the positively charged
carbon is called carbocations or carbonium
ions.

Salient features:
i) It is a positively charged carbon atom having
six electrons in its outermost shell or valence
shell.
ii) Generally, it undergoes sp2 hybridization.

iii) In carbocation, the carbon atom is sp2 - hybridised.
It uses three sp2 orbitals to form three σ-bonds with other
atoms or group of atoms. An empty (vacant) p-orbital lies
perpendicular to the plane of σ- bonds.
Thus, it has trigonal planar geometry or flat ion and bond
angle is 1200.
iv) It is an electron deficient (electrophile).
v) It combines with electron rich species.
vi) The order of stability of carbonium ions are as, (W-12, 2 Mark)

vii) The order of reactivity of carbonium ions are as,
primary ( 10) > secondary (20) > tertiary (30)
viii) It is represented as
ix) Formation of carbonium ion:
C
+
or R
+
Heterolysis
C X C + X
Carbocation
; X is more EN than C

x) For example,
Methods of Generation or Formation:
 Carbocations can be generated (formed) in a variety of
ways.
(i) Heterolysis of alkyl halides:
(ii) Protonation of alkenes:

x) For example,
Methods of Generation or Formation:
 Carbocations can be generated (formed) in a variety of
ways.
(iii) Protonation followed by dehydration of alcohols:
xi) Stability ( or Reactivity):
The stability of carbonium ions depends upon:
(i) Inductive effect:
(ii) Resonance effect
(iii) Hyperconjugation:

Stability of Carbocation depends on
Inductive effect:
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.

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

pramodpadole@gmail.com 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



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


Inductive effect:
(i.e. on the basis of + I effect & -I effect)
(a) Electron donating alkyl groups or atoms,
i.e., +I effect, increases the stability of
carbonium ion (by decreasing the intensity of the
‘+ve’ charge on the carbon atom).
The order of stability (less reactivity) of
carbonium ion can be explained on the basis of
‘+ve’ charge present on carbon.
e.g. Stability of carbonium ions are,
30 > 20 > 10

Inductive effect:
Stability of carbonium ions are,
30 > 20 > 10
CH3
CH2CH3
CH3CHCH3
C
CH3
CH3
CH3
Methyl carbocation
Ethylcarbocation
Isopropyl carbocation
t-Butyl carbocation
30
20 10
10
The electron donating alkyl groups disperse the positive (+) charge present on carbon.
The greater the number of alkyl groups, the greater is the dispersal of positive (+) charge and
the greater is the stability of carbocation.
Thus, tertiary (3°) carbocation is more stable than a secondary (2°) carbocation which in turn is
more stable than a primary (1°) carbocation. (30 > 20 > 10 )
The reactivity of carbonium ion is, 10 > 20 > 30

C
Cl
H
H
C
Cl
H
Cl
more stable than
Inductive effect:
(b) Electron withdrawing groups or atoms,
i.e., -I effect, decrease the stability of carbonium ion.

Resonance effect:
 Carbocations involving resonance are more stable than
those which do not involve resonance.
The resonance delocalisation of positive charge
increases the stability of carbocation.
 For example, allyl carbocation is more stable than n-propyl
carbocation.
HC CH CH2 CH2 CH CH2 
CH2 C CH2
Resonance structures of allyl carbocation Resonance hybrid
CH3 CH2 CH2
n-propyl carbocation
No Resonance

Resonance effect:
 Carbocations involving resonance are more stable than
those which do not involve resonance.
The resonance delocalisation of positive charge
increases the stability of carbocation.
 For example, allyl carbocation is more stable than n-propyl
carbocation.
No Resonance
CH2 CH2 CH2 CH2
Phenyl carbocation
While Benzyl carbocation is stabilized by resonance
Resonance structures of Benzyl carbocation
I II III
Hence Benzyl carbocation is more stable than phenyl carbocation

Hyperconjugation:
:
 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.
 In general, more number of hyperconjugative structures;
the greater is the stability of carbocation
 Hence the decreasing order of stability of alkyl carbocations
is,

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

B)
Carbanions:
Negatively charged carbon atom
LOGO

LOGO
B) Carbanions:
Q.1) Define Carbanion with example. (S-11, W-11 & W-12, 2 Mark)
Q.2) What is carbanion? Discuss it’s structure. (S-13, 4 Mark)
Q.3) The organic ion which has unshared pair of electrons on central
carbon atom is called _______. (W-14, ½ Mark)
Q.4) What are Carbanions? Give two methods of their formation.
(S-14 & W-19, 4 Mark)
Q.5) What is Carbanion ion? Give one preparation of it and explain the
structure of carbanion ion. (S-16, 4 Mark)
Q.6) The organic ion containing negatively charged carbon center is
called Carbanion. (W-17, ½ Mark)
Q.7) What are Carbanions? Explain the stability of carbanions on the
basis of resonance effect and inductive effect. (S-19, 4 Mark)

LOGO
B) Carbanions:
 Organic ions which contain a negatively charged
carbon atom are called carbanions.
Or
A species contain the negatively charged carbon are
called carbanions.
Or
The organic ion containing negatively charged carbon
center is called Carbanion.
Or
The organic ion which has unshared pair of electrons
on central carbon atom is called Carbanion.
Defination:
Defination:

LOGO
Salient Features Carbanions:
i) It is a negatively charged carbon atom having eight
electrons in its outermost shell or valence shell.

LOGO
iii) In carbanion, the carbon atom is sp3 – hybridized state.
It uses three sp3 hybrid orbitals; to form three σ-bonds
with other atoms or group of atoms.
The fourth sp3 orbital contains an unshared electron pair
(the nonbonding electron pair).
Thus, it has distorted tetrahedral ion geometry (appears to
have a pyramidal structure) and bond angle is <109.280.
iv) It is an electron rich (nuclophile).
v) It combines with electron deficient species.
vi) The order of stability of carbanions is as, ( 10 > 20 > 30 )

LOGO
vi) The order of stability of carbanions is as, ( 10 > 20 > 30 )
vii) The order of reactivity of carbanions are
tertiary (30) > secondary (20) > primary ( 10)
CH3
CH3CH2
CH3CHCH3 C
CH3
CH3
CH3
Methyl carbanion Ethyl carbanion Isopropyl carbanion
t-Butyl carbanion
(10
) (10
) (20
)
(30
)
C or R

LOGO
ix) Formation of carbanion ion:
x) For example,
Methods of Generation:
 Carbanions are generated (formed) by following reactions
i) Heterolysis:
ii) Abstraction of proton by base:
H C C Na H C C Na
Acetylide carbanion
Heterolysis

pramodpadole@gmail.com By Dr Pramod R Padole
Stability of Carbanions ( or Reactivity of Carbanions):
The stability of carbanions depends upon-
(i) Inductive effect,
(ii) Resonance effect and
(iii)s-character of orbital
(i) Inductive effect:
 Electron donating (releasing) alkyl groups or atoms,
i.e., + I effect, decreases the stability of carbanion and
electron withdrawing groups or atoms,
i.e., - I effect, increases the stability of carbanion.
 The order of stability ( Less Reactivity) of carbanion can be
explain on the basis of negative charge present on carbon.

 a) Electron donating (releasing) alkyl groups or atoms, i.e., + I
effect, increases the density of negative charge present on
carbon.
 Greater the number of alkyl groups on carbon carries negative
charge; greater will be the intensity of negative charge carbon
and hence less stable the carbanion.
 Hence, tertiary (30) carbanion is less stable (more reactive) than
secondary (20) carbanion, which is less stable than primary (10)
carbanion.
 So, reactivity of carbanion is 30 > 20 > 1
CH3
CH3CH2
CH3CHCH3 C
CH3
CH3
CH3
Methyl carbanion Ethyl carbanion Isopropyl carbanion
t-Butyl carbanion
(10
) (10
) (20
)
(30
)

 b) Electron withdrawing groups or atoms,
i.e., - I effect, increases the stability of
carbanion by decreasing the negative charge.
C
H
NO2
NO2 C
H
NO2
H
more stable than

Stability of Carbanions depends on Resonance effect:
 Carbanions involving resonance are more stable than those which do
not involve resonance.
 The resonance delocalisation of negaive charge increases the stability
of carbocation.
 For example, allyl carbanion is more stable than n-propyl carbanion

Stability of Carbanions depends on Resonance effect:
 Carbanions involving resonance are more stable than those which do
not involve resonance.
 The resonance delocalisation of negaive charge increases the stability
of carbocation.
 For example, Benzyl carbanion is more stable than Phenyl carbanion
CH2 CH2 CH2 CH2
Resonance structures of benzyl carbanion
No Resonance
Phenyl carbanion
While Benzyl carbanion is stabilized by resonance

LOGO
iii) s-character of the orbital:
 Stability of the carbanion increases with
increasing s-character of the orbital.
 Example:
R C=C C=C-H
R
R
C CH2
R
R
R
50 % s-character 33 % s-character 25 % s-character
sp-hybridization sp - hybridization sp - hybridization
2 3
(More stable) (Less stable)

Company
LOGO
C) Free Radicals
contain, seven or an odd electron.
Carbon Free Radical is basically a Carbon atom
with an unpaired valence electron.
It has sp2 hybridisation and is planar in shape.

C) Free Radicals
Q.1) Define Free radical with example. (W-11, 2 Mark)
Q.2) What is/are free radicals? Explain their stability on the basis of resonance effect.
(S-13, 4 Mark)
Q.3) Free radicals are produced by _______ fission. (W-13, ½ Mark)
Q.4) The species containing odd electron or unpaired electron is/ are called as _______.
(S-15, W-17 & S-19, ½ Mark)
Q.5) Define Free radicals. Give two methods of generation of free radicals. ( W-16, 4 Mark)
Q.6) What are free radicals? Explain the stability of free radicals on the basis of resonance and
hyperconjugation effect. (W-17 & S-19, 4 Mark)
Q.7) What are Free radicals? Give two methods of generation. (W-18, 4 Mark)
Q.8) ______ contain, an odd electron. ( W-19, ½ Mark)
(a) Carbocation (b) Carbanion (c) Free radical (d) None of these
Q.9) Define Free radical. (W-19, 1 Mark)
Q.10) Explain the stability order for free radicals. (W-19, 4 Mark)

C) Free Radicals or Carbon Free Radicals:
Defination:
The species (atom or group of atoms) having an
odd or unpaired electron are called free radicals.
Salient features:
i) They are neutral radicals having seven or odd electrons in its
outermost shell or valence shell.

iii) It has trigonal planar geometry ( or some time pyramidal
geometry) and bond angle is < 1200.
iv) It is an electron rich species.
v) Generally, it combines with another free radical.
vi) The order of stability of free radicals is as,
CH3
CH2CH3
CH3CHCH3
C
CH3
CH3
CH3
Methyl free readical
(10
)
Ethyl free radical
(10
)
Isopropyl free radical
(20
)
t-Butyl free radical
(30
)

The order of stability of free radicals is as,
vii) The order of reactivity of free radicals are,
primary ( 10) > secondary (20) > tertiary (30)
x) The reaction involved in free radicals are called free radical
reactions.
CH3
CH2CH3
CH3CHCH3
C
CH3
CH3
CH3
Methyl free readical
(10
)
Ethyl free radical
(10
)
Isopropyl free radical
(20
)
t-Butyl free radical
(30
)

ix) Formation of carbon free radical:
x) For example,
 Free radicals are generated by following methods-
C C C C
U.V. light
Homolysis
. .
+
Free radicals

Stability of Free radicals depends on Resonance effect:
The stability of free radicals is influenced by (i) resonance effect
and (ii) hyperconjugative effect.
(i) Resonance effect
Free radicals get stabilised by resonance due to delocalisation
of the odd or unpaired electron.
e.g. allyl free radical, benzyl free radical, etc.
CH2 CH CH2 CH2 CH CH2
CH2 CH2 CH2 CH2
Resonance structures of benzyl free radicals
I II
Resonance structures of allyl free radicals
I II III
CH3 CH2 CH2
n-propyl F.R.
No Resonance
Less stable
No Resonance
Phenyl F.R.

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:
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
.

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