1. 1
Late Ku. Durga K. Banmeru Science College,
LONAR DIST. BULDANA (Maharashtra), India.
Section –II
Unit-3
Electronicdisplacements
“Reactive Intermediates”
B. Sc. Ist year Sem-Ist
Subject:- Chemistry
2. 2
Dr. Suryakant B. Borul
(M.Sc., M.Phil., Ph.D.)
Head Of Department
Department of Chemistry
Late Ku. Durga K. Banmeru Science College,
Lonar
Teacher Profile
3. B) Reactive Intermediates
Q.-What are reactive intermediates?
Ans- Definition- “The short lived fragments or species obtained in homolytic or
heterolytic bond fission are called as reactive intermediates.”
Ex. Carbocations, Carbanions and Free radicals.
The organic compounds undergoes heterolytic bond fission gives carbocations
and carbanions.
4. Carbocations (Carbonium ions)
Ans-
“The positively charge carries carbon atoms containing six electrons in its
valence shell is called as carbocation or carbonium ion.”
Examples-
Q.-Explain the terms Carbocations. OR
What is carbocation? How it is formed?
5. Characteristics of Carbocations
It is positive charge carries carbon atom.
Six electrons in its valence shell.
Carbocation carbon is in sp2 hybridization state.
It is planner ion.
It acts as electrophiles.
It attack on electron rich species i.e. on nucleophiles.
It is classified into primary, secondary, tertiary carbocation
Stability order as –
tertiary > secondary > primary > Methyl carbocation
It is product of heterolytic bond fission of organic compounds.
6. Formation of Carbocations
1. Heterolysis of Alkyl Halides-
C
H3 C
CH3
CH3
Br C
H3 C
+
CH3
CH3
Br
-
+
t - Butyl bromide t - Butyl Carbocation
2. Dehydration of Alcohols-
C
H3 CH2 OH + H
+
C
H3 CH2 OH2
+
C
H3 CH2
+
+ O
H2
ethyl alcohol ethyl carbocation
7. 3. Protonation of Alkenes-
C
H2 CH2 + H
+
C
H3 CH2
+
ethy l carbocation
Ethylene
4. Decomposition of Diazo salt-
+
C
H3 N
+
NCl
-
C
H3 N
+
N CH3
+
N2
methyl diazonium chloride Methy l carbocation
8. Stability of Carbocations
Stability of carbocation is influenced by-
A. Inductive effect-
• The electron releasing group i. e. + effect such as alkyl groups stabilize
the carbocation by dispersing the positive charge.
• Maximum number such group alkyl groups attached to positive charge
carbon greater the dispersal of positive charge and increases the stability
of carbocation.
• Thus tertiary carbocation is more stable than secondary carbocation,
which in turn is more stable than primary carbocation.
9. • -I effect due to electron withdrawing groups destabilize the carbocation.
C
H3 C
+
CH3
CH3
C
H3 CH
+
CH3 C
H3 CH2
+
CH3
+
> > >
t - butyl isopropyl ethyl methyl
Tertiary Secondary Primary Methyl
carbocation carbocation carbocation carbocation.
10. B. Resonance-
Q- Explain the stability of Allyl carbocation and n-propyl carbocation.
Ans-
• The carbocation which involved resonance are more stable than those not
involve in resonance.
• In resonance delocalization of positive charge increases the stability of
carbocation.
• Ex.- Allyl carbocation is more stable than n-propyl carbocation, because
allyl carbocation involved in resonance while n-propyl carbocation do not
involve in resonance.
11. C
H3 CH2 CH2
+
No Resonance Occur
n-Propyl carbocation-
Allyl carbocation-
Ex- Benzyl carbocation -
CH2
+
CH
+
CH2
CH
+
CH2
C
H
+
CH2
Benzyl carbocation is stabilized by resonance.
Thus, Allyl carbocation is more stable than n-propyl carbocation.
12. C. Hyperconjugation-
• Allyl group release electrons via hyperconjugation effect increases the
stability of carbocation.
• Greater the number of hyperconjugative structures the greater its stability
of carbocations.
• The decreasing order of stability of allyl carbocation as-
C
H3 C
+
CH3
CH3
C
H3 CH
+
CH3 C
H3 CH2
+
CH3
+
> > >
t - butyl isopropyl ethyl methyl
13. Recations of Carbocations
1. Combination with carbocations-
2. Elimination of Proton-
C
H3 C
CH3
CH3
OH
C
H3 C
+
CH3
CH3
O
H
-
+
t - Buty l alcohol
t - Buty l Carbocation
Nucleophile
+
C
H3 CH2
+
C
H2 CH2 H
+
Ethyl carbocation Ethylene Proton
14. 3. Addition to double bond to produce higher carbocation-
C
H3 C
CH3
CH3
CH2 C
+
CH3
C
H3
C
H3 C
+
CH3
CH3
+
t - Buty l Carbocation
C
H2
CH3
CH3
4. Rearrangement to produce more stable carbocation-
CH2 CH2
+
C
H3 CH
+
C
H3 CH3
( less stable ) ( more stable )
1 2
0 0
15. Carbanion
Ans-
“The negatively charge carries carbon atoms containing eight electrons in
its valence shell is called as carbanion.”
Examples-
Q.-Explain the terms Carbanions. OR
What is carbanion? How it is formed?
C
H3 C
-
CH3
CH3
t - Butyl Carbanion
CH3
-
Methyl carbanion
C
H3 CH2
+
ethyl carbanion
CH2
-
Benzyl carbanion
16. Characteristics of Carbanions
It is negative charge carries carbon atom.
Eight electrons in its valence shell.
Carbanion carbon is in sp3 hybridization state.
It is electron rich center.
It acts as nucleophiles.
It attack on electron deficient species i.e. on electrophiles.
It is classified into primary, secondary, tertiary carbanion
Stability order as –
Methyl > primary > secondary > tertiary carbocation
It is product of heterolytic bond fission of organic compounds.
17. Formation of Carbanion
1. Heterolysis -
C
H3 C
CH3
CH3
Br C
H3 C
+
CH3
CH3
Br
-
+
t - Butyl bromide t - Butyl Carbocation
2. Dehydration of Alcohols-
C
H3 CH2 OH + H
+
C
H3 CH2 OH2
+
C
H3 CH2
+
+ O
H2
ethyl alcohol ethyl carbocation
18. Stability of Carbocations
Stability of carbocation is influenced by-
A. Inductive effect-
• The electron releasing group i. e. + effect such as alkyl groups stabilize
the carbocation by dispersing the positive charge.
• Maximum number such group alkyl groups attached to positive charge
carbon greater the dispersal of positive charge and increases the stability
of carbocation.
• Thus tertiary carbocation is more stable than secondary carbocation,
which in turn is more stable than primary carbocation.
19. • -I effect due to electron withdrawing groups destabilize the carbocation.
C
H3 C
+
CH3
CH3
C
H3 CH
+
CH3 C
H3 CH2
+
CH3
+
> > >
t - butyl isopropyl ethyl methyl
Tertiary Secondary Primary Methyl
carbocation carbocation carbocation carbocation.
20. B. Resonance-
Q- Explain the stability of Allyl carbocation and n-propyl carbocation.
Ans-
• The carbocation which involved resonance are more stable than those not
involve in resonance.
• In resonance delocalization of positive charge increases the stability of
carbocation.
• Ex.- Allyl carbocation is more stable than n-propyl carbocation, because
allyl carbocation involved in resonance while n-propyl carbocation do not
involve in resonance.
21. C
H3 CH2 CH2
+
No Resonance Occur
n-Propyl carbocation-
Allyl carbocation-
Ex- Benzyl carbocation -
CH2
+
CH
+
CH2
CH
+
CH2
C
H
+
CH2
Benzyl carbocation is stabilized by resonance.
Thus, Allyl carbocation is more stable than n-propyl carbocation.
22. C. Hyperconjugation-
• Alkyl group release electrons via hyperconjugation effect increases the
stability of carbocation.
• Greater the number of hyperconjugative structures the greater its stability
of carbocations.
• The decreasing order of stability of alkyl carbocation as-
C
H3 C
+
CH3
CH3
C
H3 CH
+
CH3 C
H3 CH2
+
CH3
+
> > >
t - butyl isopropyl ethyl methyl
23. 1. The tertiary carbocation is more stable than secondary carbocation and
secondary carbocation more stable than primary carbocation.
2. The stability order as –
Tertiary > Secondary > Primary > Methyl
Ex-. t-Butyl > sec-Propyl > Ethyl > Methyl
3. The carbocations which gives maximum structure of hyperconjugation which
are more stable.
24. t-butyl carbocation gives nine hyperconjugative structures
sec-propyl carbocation gives six hyperconjugative structures
4
5
25. ethyl carbocation gives three hyperconjugative structures
Thus t-carbocation is more stable than primary carbocation.
26. Recations of Carbocations
1. Combination with carbocations-
2. Elimination of Proton-
C
H3 C
CH3
CH3
OH
C
H3 C
+
CH3
CH3
O
H
-
+
t - Buty l alcohol
t - Buty l Carbocation
Nucleophile
+
C
H3 CH2
+
C
H2 CH2 H
+
Ethyl carbocation Ethylene Proton
27. 3. Addition to double bond to produce higher carbocation-
C
H3 C
CH3
CH3
CH2 C
+
CH3
C
H3
C
H3 C
+
CH3
CH3
+
t - Buty l Carbocation
C
H2
CH3
CH3
4. Rearrangement to produce more stable carbocation-
CH2 CH2
+
C
H3 CH
+
C
H3 CH3
( less stable ) ( more stable )
1 2
0 0
28. Carbanion
Ans-
“The negatively charge carries carbon atoms containing eight electrons in
its valence shell is called as carbanion.”
Examples-
Q.-Explain the terms Carbanions. OR
What is carbanion? How it is formed?
C
H3 C
-
CH3
CH3
t - Butyl Carbanion
CH3
-
Methyl carbanion
C
H3 CH2
+
ethyl carbanion
CH2
-
Benzyl carbanion
B) Reactive Intermediates
29. Characteristics of Carbanions
It is negative charge carries carbon atom.
Eight electrons in its valence shell.
Carbanion carbon is in sp3 hybridization state.
It is electron rich center.
It acts as nucleophiles.
It attack on electron deficient species i.e. on electrophiles.
It is classified into primary, secondary, tertiary carbanion
Stability order as –
Methyl > primary > secondary > tertiary carbocation
It is product of heterolytic bond fission of organic compounds.
30. Formation of Carbanion
1. Heterolysis -
2. Abstraction of Proton-
+
Na
C
H Na
+
Sodium Acetylene Acetylide Carbanion
C
-
C
H
CH2 CHO
H
+
O
H
-
+ O
H2
C
H2
-
CHO
2. Decomposition of anion-
O
-
O
C
H3 CH3
-
CO2
+
31. Stability of Carbanion
Stability of carbanion is influenced by-
A. Inductive effect-
• The electron releasing group such as alkyl groups decrease stabilize the
carbanion by intensifying the negative charge.
• The electron withdrawing group i. e. -I effect increase stabilize the
carbanion by dispersal of the negative charge.
• The stability order as –
Methyl > Primary > Secondary > Tertiary
Ex-. Methyl > Ethyl > sec-Propyl > t-Butyl
32. B. Resonance-
• Carbanions stabilized by resonance due to delocalization of the negative
charge.
• Ex.– benzyl carbocation is more stable than ethyl carbanion.
CH2
-
CH
-
CH2
CH
-
CH2
C
H
-
CH2
C
H3 CH2
-
No Resonance
Resonance structures of Benzyl carbanion
33. Reactions of Carbanion
1. Combination with proton or cation -
C
H3 CH2
-
+ H
+
C
H3 CH3
Ethyl carbanion Proton Ethane
2. Additions to multiple bonds to form an anion or new carbanion-
C
H3 C
O
H + C
H2
-
C
O
H C
H3 C
O
-
H
CH2 CH
O
Carbanion New carbanion
34. Free Radicals
Ans-
“The highly reactive odd or unpaired electron containing charge less
species is called as free radical.”
Examples-
Q.-Explain the terms Free Radicals. OR
What is Free Radical? How it is formed?
B) Reactive Intermediates
35. Characteristics of Free radical
It is charge less species.
It carries odd or unpaired electron in its valence shell.
The central carbon atom of free radicals is in sp2 hybridized state.
It is attack on free radicals and combine with free radicals.
It has planer structure.
It is classified into Primary, Secondary, & tertiary free radicals.
Stability order as Tertiary > Secondary > Primary free radicals.
36. Formation of Free Radicals
1. Thermal Decomposition -
C
H3 CH2
C
H3 CH2
Pb
CH2 CH3
CH2 CH3
C
H3 CH2
4 + Pb
i
ii
Tetraethyl lead
Ethyl free radical
H5C6
O
O
O
C6H5
O H5C6 C
O
O C6H5 + CO2
Benzoyl peroxide Phenyl freeradical
Due to heating above molecule undergoes homolysis to form free radicals
37. 2. Photochemical Decomposition -
i
ii
In presence of light compound undergoes homolysis to from free radicals
Cl Cl Cl
2
homolysis
Chlorine free radical
C
H3
C
H3
O C CH2
CH3
+ CH3
Acetyl free radical
methyl free radical
Acetadehyde
38. Stability of Free Radicals
1. Resonance-
• According to resonance theory those molecule gives maximum resonating
structures which are more stable.
• Free radicals get stability by resonance due to delocalization of the
unpaired electron.
• Ex. Allyl free radicals–
Benzyl free radical-
C
H2 CH CH2 C
H2 CH CH2
I II
CH2
CH
CH2
CH
CH2
C
H
CH2
I II III IV
Resonance structures of benzyl free radicals
In above two examples benzyl free radicals more stable than allyl free radicals.
39. 2. Hyperconjugation Effect-
• The alkyl groups are electron releasing groups.
• The presence of alkyl group attached to free radical carbon increases the
stability of free radical.
• Hence the decreasing order of stability of free radical is
• Presence of alkyl group increases the hyperconjugation
structures.
40. Reactions of Free Radicals
1. Combination with other free radicals-
2. Addition to multiple bond to form a new free radicals-
3. Disproportionation-
