Organic reaction mechanism and intermediates their stabilities fully discussed.

1. An Overview of Organic Reactions
Why this chapter?Why this chapter?
To understand organic and/or biochemistry, it is
necessary to know:
-What occurs
-Why and how chemical reactions take place
We will see how a reaction can be described

2. 2
Kinds of Organic Reactions
 In general, we look at what occurs and try to learn how it
happens
 Common patterns describe the changes
◦ Addition reactions – two molecules combine
◦ Elimination reactions – one molecule splits into two

3. 3
◦ Substitution – parts from two molecules exchange
◦ Rearrangement reactions – a molecule undergoes
changes in the way its atoms are connected

4. What kind of reaction is the transformation shown below?What kind of reaction is the transformation shown below?
1. an elimination reaction
2. a rearrangement reaction
3. a substitution reaction
4. an addition reaction
5. none of these
+ HCl
Cl
Learning Check:

5. What kind of reaction is the transformation shown below?What kind of reaction is the transformation shown below?
1. an elimination reaction
2. a rearrangement reaction
3. a substitution reaction
4. an addition reaction
5. none of these
+ HCl
Cl
Solution:

6. 6
How Organic Reactions Occur?
Mechanisms
 In a clock the hands move but the mechanism behind the
face is what causes the movement
 In an organic reaction, we see the transformation that
has occurred.The mechanism describes the steps
behind the changes that we can observe
 Reactions occur in defined steps that lead from reactant
to product

7. 7
Steps in Mechanisms
We classify the types of steps in a sequence
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

8. 8
Types of Steps in Reaction Mechanisms
 Bond formation or breakage can be symmetrical or unsymetrical
 Symmetrical- homolytic
 Unsymmetrical- heterolytic
Bond Breaking
Bond MakingBond Making

9. 9
Indicating Steps in Mechanisms
 Curved arrows indicate breaking and
forming of bonds
 Arrowheads with a “half” head (“fish-
hook”) indicate homolytic and
homogenic steps (called ‘radical
processes’)
 Arrowheads with a complete head
indicate heterolytic and heterogenic
steps (called ‘polar processes’)

10. 10
Radical Reactions
 Not as common as polar 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

11. 11
 Three types of steps
◦ Initiation – homolytic formation of two reactive species
with unpaired electrons
 Example – formation of Cl atoms form Cl2 and light
◦ Propagation – reaction with molecule to generate radical
 Example - reaction of chlorine atom with methane to
give HCl and CH3
.
Steps in Radical Substitution

12. 12
Steps in Radical Substitution:
Monochlorination of Methane
Initiation
Propagation

13. 13
Steps in Radical Substitution
Termination
With excess concentration of Cl2 present continued
reaction is probable with formation of dichloro,
trichloro, and tetrachloro methanes.

14. In a radical chain reaction, what would be the bestIn a radical chain reaction, what would be the best
description of the following reaction?description of the following reaction?
HH33C• + •Cl CH→C• + •Cl CH→ 33ClCl
1. propagation
2. elimination
3. initiation
4. termination
5. substitution
Learning Check:

15. In a radical chain reaction, what would be the bestIn a radical chain reaction, what would be the best
description of the following reaction?description of the following reaction?
HH33C• + •Cl CH→C• + •Cl CH→ 33ClCl
1. propagation
2. elimination
3. initiation
4. termination
5. substitution
Solution:

16. 16
Radical Substitution: With >1 kind of H
When there is
• >1 type of H then there is
• >1 option for radical formation and therefore
• >1 option for a monohalogenation product.
C
H
H
H
C C
H
H
H
H
H
C
H
H
H
C
H
H
C
Cl
H
H
C
H
H
H
C
Cl
H
C
H
H
H
+ Cl2
hv +
1-chloropropane 2-chloropropane

17. In the reaction of ClIn the reaction of Cl22 with 2-methylbutane, how manywith 2-methylbutane, how many
monochlorinatedmonochlorinated isomers are produced?isomers are produced?
1. 2
2. 3
3. 4
4. 5
5. 6
C
H
C C
H
H
H
H
H
C
H
H
H
C
HH H
+ Cl2
hv
Learning Check:

18. In the reaction of ClIn the reaction of Cl22 with 2-methylbutane, how manywith 2-methylbutane, how many
monochlorinatedmonochlorinated isomers are produced?isomers are produced?
1. 2
2. 3
3. 4
4. 5
5. 6
C
H
C C
H
H
H
H
H
C
H
H
H
C
HH H
+ Cl2
hv
Solution:

19. 19
Polar Reactions
Molecules can contain local unsymmetrical electron
distributions due to differences in electro negativities
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

21. 21

22. 22

23. 23
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

24. p. 144
Polarizability
Bonds inherently polar already can be made more
polar by reactions with acids or bases.

25. p. 144
Polarizability
2.55
2.58
2.55
2.66
Bonds not inherently polar can be polarizable as
interactions with solvent or other polar molecules
effect the electron distribution.
Large atoms with loosely held electrons are more polarizable
than small atoms with few tightly held electrons.
So: SS is more polarizable than OO
II is more polarizable than ClCl

26. 26
Generalized Polar Reactions
An electrophileelectrophile, an electron-poor species, combines
with a nucleophilenucleophile, an electron-rich species
An electrophile is a Lewis acid
A nucleophile is a Lewis base
The combination is indicate with a curved arrow from
nucleophile to electrophile

28. p. 146
Learning Check:
Which of the following is likely to be a nucleophile
and which an electrophile?

29. p. 146
Solution:
Which of the following is likely to be a nucleophile
and which an electrophile?
E EN N

30. p. 146
Is BF3 is likely to be a nucleophile or an electrophile?
Learning Check:

31. p. 146
Is BF3 is likely to be a nucleophile or an electrophile?
Solution:
E

32. Which of the following is expected to be theWhich of the following is expected to be the
worst nucleophile?worst nucleophile?
1. NH3
2. H2O
3. BH3
4. ethylene
5. (CH3) 3P
Learning Check:

33. Which of the following is expected to be theWhich of the following is expected to be the
worst nucleophile?worst nucleophile?
1. NH3
2. H2O
3. BH3
4. ethylene
5. (CH3) 3P
Solution:

34. 34
An Example of a Polar Reaction:
Addition of HBr to Ethylene
 HBr adds to the π part of C-C double bond
 The π bond is e-
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

35. 35
Mechanism of Addition of
HBr to Ethylene
HBr electrophile is attacked by π 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
σ 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

36. 36

38. p. 142
Learning Check:
What product would you expect?

39. p. 142
Solution:
What product would you expect?
Br

40. 40
Using Curved Arrows in Polar
Reaction Mechanisms
Curved arrows are a way to keep track
of changes in bonding in polar reaction
The arrows track “electron movement”
Electrons always move in pairs
Charges change during the reaction
One curved arrow corresponds to one
step in a reaction mechanism

41. 41
Rules for Using Curved Arrows
The arrow (electrons) goes from the nucleophilic
reaction site (Nu: or Nu:Nu: or Nu:--
) to the electrophilic
reaction site (sink, E or Esink, E or E++
)
The nucleophilic site can be neutral or negative

42. 42
The nucleophilic site can be negative or neutral
Rules for Using Curved ArrowsRules for Using Curved Arrows

43. 43
The electrophilic site can be positive or
neutral

44. 44
The octet rule must be followed
The hydrogen already has two e-s so when
another pair moves in the 2 already owned have
to leave.

45. What is the role of the alkene in the reactionWhat is the role of the alkene in the reaction
above?above?
1. electrophile
2. nucleophile
3. free radical
4. catalyst
5. Lewis acid
+ H3O + H2O
Learning Check:

46. What is the role of the alkene in the reactionWhat is the role of the alkene in the reaction
above?above?
1. electrophile
2. nucleophile
3. free radical
4. catalyst
5. Lewis acid
+ H3O + H2O
Solution:

47. Learning Check:
Add curved arrows to indicate the flow of electrons:

48. Solution:
Add curved arrows to indicate the flow of electrons:

49. Add curved arrows to indicate the flow of electrons:
Learning Check:

50. 50
Add curved arrows to indicate the flow of electrons:
Solution:

51. p. 142
Learning Check:
What carbocation intermediate is consistent with the
product formed? Propose a mechanism. (Add curved
arrows to indicate the flow of electrons.)

52. Reaction MechanismReaction Mechanism
• Detailed description of sequence
of steps involved in group from reactants
to products.
• Reactant intermediate
product

53. Bond Cleavage
−+
→ B:AB:A A : B A– : B+
+vely
charged ion – carbocation
-vely
charged ion – carbaanion
Heterolytic Cleavage
Homolytic Cleavage
⋅⋅
+→ BAB:A
Free radicals.

54. Carbonium ion
• Planar – sp2
hybridised bond
angle 120o
• Has six electrons
• Stabilized by resonance or inductive
effect or hyperconjugation
C
Empty unhybridised
p-orbital
sp2
Hybridisation of
carbon
Planar Strucutre of carbnion
+

55. Examples of carbonium ion
Benzyl cation
CH2 CH2 CH2
etc
+
+ +
C H 2 C H C H 2 C H 2 C H C H 2
+ +
S ta b ilis e d th r o u g h
r e s o n a n c e
A lly l c a t io n
H2C CH+
Vinyl cation
no resonance hence unstable.

56. Stability of Cabocation
(i) By inductive effect
The resonance effect is always more predominant
than the inductive effect in stabilizing an ion.
CH3
C
CH3
CH3
CH3
C
CH3
H
> > H C
CH3
H
> >
3° 2°
+ +
1°
+

57. Stability of Cabocation
(ii) By hyperconjugation
H3C — C
CH3
CH2
— H
H3
C — C
CH3
CH2
H
H3C — C
CH2
H
C
CH3
CH3
CH3
CH2H
+
+
+
etc. +
Thus, tertiary carbocation is more stable than
secondary and so on.

58. Carbanion
• Pyramidal - sp3
hybridised
bond angle 109.28
• Has eight electrons
• Stabilized by resonance or by
inductive effect.
. . sp3 hybrid orbital
containing lone pair
Tetrahedral structure of carboanion

59. Stability of Carbanion
(i) By resonance
H
-
H
-
H
Cyclopentadienyl carbanion

60. Stability of Carbanion
(ii) By inductive
CH3
C H
CH3
CH3
CH3
C
CH3
H
C
CH3
H
3° 2° 1°

61. Stability of Carbanion
(iii) Electron-donating groups destabilize a
carbanion while electron-withdrawing groups
stabilize it.
N O 2 3O C H
>
−
2C H −
2C H

62. Free Radical
• Planar or Pyramidal
• Has seven electrons
• Stabilized by resonance or by inductive
effect.
• Order of stability of free radical 3o
>2o
> 1o
C(
Unhybridised orbital
containing odd electron
120oC
sp2
hybridised carbon
Planar Sturcutre
+

63. Classification of Reagents
Nucleophilic Reagents (Nucleophiles)
• Attacks the positive end of a polar bond or nucleus-
loving is known as nucleophile.
• Generally, negatively charged or electron rich
species are nucleophilic.
3 3 2 3e.g. OH , OCH , CN , I , CH COO , NH , CH
Θ
− − − − − −
− −
+
2 3 3 2H O, NH , NH — NH
N..
..
N H 3 ,C H 3 — O — C H 3 ,
..
. .
..
C 2 H 5 — O H ,
. .
..H 2 O ,
• All nucleophiles are in general Lewis bases.

64. Classification of Reagents
Electrophilic Reagents (Electrophiles)
• Attacks a region of high electron density
or electron-loving is known as
electrophile.
• All positively charged or electron deficient
species are electrophilic.
3 2
H , CH , NO , Cl , Br , Ag+ + + + + +

65. Classification of Reagents
• Neutral reagents which contain an
electron-deficient atom are also
electrophiles.
AlCl3, SO3, BF3, SOCl2, POCl3, FeCl3, ZnCl2
• All electrophiles are in general Lewis acids.

66. Carbenes
• Divalent carbon compound.
• Carbon atom is linked to two adjacent
groups by covalent bonding.
• A carbene is neutral and possesses two
free electrons, i.e. a total of six electrons.
• Electron deficient.

67. Carbenes
Carbene is of two types
(i) Singlet carbene:
(ii)Triplet carbene:
Triplet carbene is more stable
than single carbene.
CH2 hybridisation sp2
it is v-shaped
CH2 hybridisation sp
it is linear shaped

68. Types of Organic Reactions
Substitution Addition
Elimination Rearrangement
Condensation Isomerisation

69. Types of Organic Reactions
Substitution Reaction
Replacement of an atom or group by other atom of
group
Nucleophilic substitution:
R X OH R OH X− −
− + → +
SN1 Reaction: Unimolecular nucleophilic
substitution reaction.

70. Types of Organic Reaction- SN1
Reaction
CH3 — C — CH2Cl
CH3
CH3
OH
–
slow
CH3 — C — CH2
CH3
CH3
SN1 +
CH3 — C — CH2
CH3
CH3
CH3 — C — CH2 – CH3
CH3
+
+
1, 2-Methyl anion
shift
Fast OH
–
CH3
— C — CH2
CH3
OH
CH3
(1)
(2)

71. Types of Organic Reaction - SN2
Reaction
SN2 Reaction: This is called bimolecular
nucleophilic substitution and it is one-step process.
H — C — Br + OH
CH2CH3
CH3
–
OH C Br
H
CH3
.
CH2CH3
Fast
HO — C — H
CH3
CH2CH3
δ– δ–
Transition state
unstable
slow

72. Addition Reactions
The reagent often adds to
bond and the π bond is converted into bond.
Can be electrophilic addition or nucleophilic addition.
C C , C O or C N− ≡ − > = − ≡
C C ,> = <
Cl2
2 2 2 2
CCl4
CH CH Cl CH CH Cl= → −
OH2
+
H
OH
+
(Hydration)
σ

73. Elimination Reactions
Two groups on adjacent atoms are lost as a double
bond is formed.
CH3 – CH – CH – CH3
OH H
Conc. H2SO4
– H2
O
CH3 — CH CH – CH3
We divide elimination reactions into three classes.
(1) E1 (2) E1 CB (3) E2

74. Rearrangement
• Migration of a group takes place within the same
molecule.
C6
H5
C6H5
C = N
OH
C6
H5
— C — N — C6
H5
O
H
H
ether
+
→
O H
H +
(Beckmann rearrangement)
(Dehydration and rearrangement)

75. Condensation
H3C — C — CH3 + H3C — C — CH3
O
H3C — C — CH — C — CH3
CH3O O
dil.
NaOH, ∆
Two molecules of same or different reactants
combine to give a new product with the elimination
of simple byproducts like H2O, NH3, etc,

76. Isomerisation
C
H
H3C
C
H
CH3
C
H
H3C
C
CH3
H
hν

77. Class Test

78. Class Exercise – 1Class Exercise – 1
Select the most stable carbocation among
the following.
CH3
CH3
HC
+
6 5 3(C H ) C+
3 2 2CH CH CH
+
3 3(CH ) C+
(a)
(b)
(c) (d)

79. Solution:
C — C6H5
C6H5
C6H5
C+ +
This carbocation is highly stabilized through resonance
with three benzene rings.
Hence answer is (b).

80. Class Exercise - 2
Which of the following is an addition reaction?
3 3CH CH CH
|
Br
OH
2 3
Alcohol
CH CH CH
−
→ =
ν
+ →
h
3 3 2 3 2CH CH Cl CH CH Cl
(a)
(b)
− −
+ → +3 2 3 2CH CH Br CN CH CH CN Br(c)
= + →3 2 3 3
|
Br
CH CH CH H CH C H CHBr(d)

81. Solution:
H3C — CH — CH3
Br
Alcohol
H2C CH — CH3
H3C — CH3 + Cl2
CH3CH2Cl + HCl
H3C — CH2Br + CN H3C — CH2CN + Br
H3C CH CH2 + HBr H3C CH CH3
Br
OH
–
(Elimination)
hν
–
(Substitution)
Substitution
–
Addition
Hence answer is (d).

82. Class Exercise - 3
Which of the following is the most effective
group in stabilizing a free radical
inductively?
(a) F (b) I
(c) Br (d) Cl

83. Solution:
Since free radical is electron deficient, any
substituent with more electron releasing
and less electron withdrawing ability will
stabilize the radical inductively.
The decreasing order of electronegativity of
halogens is: F > Cl > Br > I
Hence answer is (b).

84. Class Exercise - 4
Which of the following is not a nucleophile?
(a) CN–
(b) BF3
(c) RNH2 (d) OH–

85. Solution:
Among the following, BF3 is only
electron deficient. Hence, it will not
act as a nucleophile.
Hence answer is (b).

86. Class Exercise - 5
Which of the following is the correct order
regarding –I effect of the substituents?
(a) –NR2 > –OR > –F
(b) –NR2 > –OR < –F
(c) –NR2 < –OR < –F
(d) –OR > –NR2 > –F

87. Solution:
–I effect increases with electronegativity of
atom.
The decreasing order of electronegativity is
F > O > N
∴ The correct order for –I effect is
–NR2 < –OR < –F
Hence answer is (c).

88. Class Exercise - 6
The least stable carbonium ion is
(a) (b)
(c) (d)
+
3 2H CCH
+
6 5 2 2C H — CH — CH
+
6 5 2C H — CH
+
6 5 6 5C H — CH — C H

89. Solution:
Among the following, (a) is stabilized
through +I effect and (b) is
destabilized through –I effect of
phenyl ring. Other two are stabilized
through resonance.
Hence answer is (c).

90. Class Exercise - 7
Arrange the following ions in the decreasing order
of stability.
2HC
+
CH3
CH3
CH3
+
+
+
(a) (b) (c) (d)

91. Solution:
CH2
+
. It is a primary cation.
Hence, minimum stability.
CH3
CH3
+
and
+
(c) (b)
are secondary cations.
Hence, stabilized through +I effect of –CH3 group which
decreases with distance. (c) is more stable as compared to
(b).
(d) is most stable as it is tertiary cation and stabilized
through +I effect of –CH3 group and hyper conjugation.
∴ The order is (d) > (c) > (b) > (a)

92. Class Exercise - 8
Arrange the following radicals in order of
their decreasing stability
=
   
3 2 3 3 6 5 2 2 2CH CH , (CH ) C, C H CH , CH CH CH

93. Solution:
Radicals are stabilised through electron releasing
resonance and inductive effect.
CH2CH2
etc.
More resonating structure
H2C CH — CH2 H2C — CH CH2

94. Solution:
One resonating structure, although both are primary
radicals.
Among and , later is a tertiary
radical. Hence, more stable.
The decreasing order of stability is
H3C — CH2 (CH3)3C
C6H5CH2 > H2C CH — CH2 > (CH3)3C > H3CCH2