Haloalkanes and haloarennes

1. Haloalkanes Haloarenes
chemistry

2. Sakshi Vora
IIT - Roorkee
10th, 12th CBSE State Topper
7+ Years of Teaching Experience
KVPY fellow

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9. SAKSHI

10. Haloalkanes
Haloarenes

11. Alkyl Halide
⇒ Halogen derivative of alkanes
Ex :- R - X ( X = -F , -Cl , - Br, - I )
⇒ Alkyl halides are electrophilic in nature

12. Physical
Properties

13. Physical Properties
● C-X bond is polar in nature
● Alkyl halides are insoluble in H2O because they cannot form bond with
H2O.
● These are completely soluble in organic solvents.
● M.P & B.P. α molecular weight.
● For same alkyl group the order of B.P. is RI > RBr > RCl > RF
● Polarity order is RF > RCI > RBr > RI
● Reactivity order is RI > RBr > RCl > RF

14. Practice
Questions

15. Among the following compounds, which one has the shortest
C - Cl bond ?
[Sep. 04, 2020 (II)]
A.
B.
C.
D.

16. Reactions
of
Alkyl halide

17. Reactions shown by alkyl halides
1. Substitution reaction
2. Elimination reaction

18. Substitution
Reactions

19. Nucleophilic Substitution Reaction (SN)
When a Nucleophile (Strong) substitutes another nucleophile (weak)

20. SN2
Reactions

21. SN2 Mechanism

22. Substitution Nucleophilic bimolecular SN2

23. Important
points

24. Important points about SN2
1. One step mechanism
2. No carbocation intermediate is formed
3. No rearrangement is possible
4. A pentavalent transition state is involved
5. Bimolecular reaction
6. Backside attack of nucleophile takes place
7. Inversion takes place

25. SN1
Reactions

26. Mechanism of SN1 Reaction

27. Substitution Nucleophilic Unimolecular Reaction

28. Important points about SN1 reaction
1. Two step mechanism
2. Carbocation intermediate is formed
3. Rearrangement is possible
4. First order reaction
5. Nucleophile can attack from both sides

29. Factors
affecting SN
reactions

30. Factors for SN mechn : -
1. Structure of Substrate
2. Nature of leaving group
3. Nature of Nucleophile
4. Nature of solvent.

31. More the ease of dissociation,
more would be SN1
More stable carbocation, more will be SN1

32. Structure of
Substrate

33. Structure of substrate SN1
A.
B.
C.
D.

34. More the stability of TS by
EWG, more will be SN2
More the steric hindrance, lower SN2

35. Structure of substrate SN2

36. Structure of substrate SN2

37. Structure of substrate SN2

38. Important
points

39. 3o ⟶ always SN1
1O, CH3 X⟶ always SN2
2o ⟶ can show both SN1 & SN2

40. Nature of
Leaving group

41. Nature of leaving group
In both SN1 and SN2,
RDS includes breaking of C-Leaving group,
Thus
better the leaving group, faster will be the rate

42. Nature of
Nucleophile

43. Nature of nucleophile
SN1: Nucleophile is not involved in RDS, no effect of concentration,
size or nucleophilicity of nucleophile
SN2: SN2 increases by
❏ Higher concentration of nucleophile
❏ Small size of nucleophile
❏ Higher ability of nucleophile to give electrons (nucleophilicity)

44. Nature of
solvent

45. Nature of Solvent
For SN1
Polar Protic Solvent - favors dissociation of bond along with
stabilization of ions hence SN1 increases in PPS

46. Nature of solvent
SN2: Polar protic solvent will solvate the nucleophile thereby
decreasing its nucleophilicity,
Thus a polar aprotic solvent will be required

47. Important
Points

48. SN2 SN1
1o > 2o > 3o 3o > 2o > 1o
R - I > R - Br > R - Cl R - I > R - Br > R - Cl
Strong/small/ high
concentration of
Nucleophile
Poor Nucleophile
(H2O, ROH, RCOOH etc)
PAS
(Acetone, DMF, DMSO,
TMF)
PPS
(H2O, ROH) etc

49. Important
Points

50. Important point
➔ In most of the SN1 reactions nucleophile is not taken
SOLVENT itself behaves like a Nucleophile.
➔ Bridge head halides, aryl halides and vinyl halides never show SN1
and SN2
➔ Neopentyl halide never shows SN2 due to crowding

51. Practice
Questions

52. Predict the rate of SN1 and SN2

53. Predict the rate of SN1 and SN2

54. Predict the rate of SN1 and SN2

55. Predict the rate of SN1 and SN2

56. The mechanism of SN1 reaction is given as :
[Sep. 03, 2020 (I)]
A student writes general characteristics based on the given
mechanism as :

57. 1. The reaction is favoured by weak nucleophiles.
2. R would be easily formed if the substituents are bulky.
3. The reaction is accompanied by racemization.
4. The reaction is favoured by non-polar solvents.
Which observations are correct ?
A. (1) and (2)
B. (1) and (3)
C. (1) , (2) and (3)
D. (2) and (4)

58. KI in acetone, undergoes SN2 reaction with each of P, Q, R and S.
The rates of the reaction vary as
[Adv. 2013]
A. P > Q > R > S
B. S > P > R > Q
C. P > R > Q > S
D. R > P > S > Q

59. An ‘Assertion’ and a ‘Reason’ are given below. Choose the correct
answer from the following options :
Assertion (A) : Vinyl halides do not undergo nucleophilic
substitution easily.
Reason (R) : Even though the intermediate carbocation is
stabilized by loosely held p - electrons, the cleavage is difficult
because of strong bonding.
A. Both (A) and (R) are wrong statements.
B. Both (A) and ® are correct statements and (R) is the correct
explanation of (A)
C. Both (A) and ® are correct statements but (R) is not the correct
explanation of (A).
D. (A) is a correct statement but (R) is a wrong statement.
[April. 12, 2019 (II)]

60. [Sep. 03, 2020 (II)]
The decreasing order of reactivity of the following compounds
towards nucleophilic substitution (SN2) is :
A. (II) > (III) > (I) > (IV)
B. (II) > (III) > (IV) > (I)
C. (III) > (II) > (IV) > (I)
D. (IV) > (II) > (III) > (I)

61. Stereochemistry
of
SN1 reactions

62. Stereochemistry of SN1 reactions
In SN1 reaction partial racemisation takes place, slightly invented product
dominates.

63. Stereochemistry of SN2
Configuration of bonds at a chiral centre may or may not change

64. Methods of
preparation
of
Haloalkanes

65. Preparation
from alkanes

66. By Halogenation of Alkanes
Halogenation of alkanes takes place by free radical mechanism.
R - H + X - X R - X + H - X
h𝝂

67. Reed’s Reaction
RH + SO2Cl2 ----------------> RCl + HCl + SO2

68. Notes
● Formation of Radical is r. d. S
● Chain Reaction
● Oxidation Reaction of (R - H)

69. SO2Cl2
hv
Predict the product of the reaction

70. Preparation from
alkenes by
Hydrohalogenation

71. From alkene: Hydrohalogenation

72. Practice Questions

73. The decreasing order of reactivity of the following organic
molecules towards AgNO3 solution is :
A. (C) > (D) > (A) > (B)
B. (A) > (B) > (D) > (C)
C. (A) > (B) > (C) > (D)
D. (B) > (A) > (C) > (D)
[Sep. 04, 2020 (I)]

74. Which of the following compounds will form the precipitate with
aq. AgNO3 solution most readily ?
[Sep. 04, 2020 (II)]
A.
B.
C.
D.

75. Preparation from
alcohols

76. From Alcohol :-
(i) Using dry H - X :-
R - OH + H - X R - X + H2O
Anhydrous ZnCl2
Δ, 300oC

77. Mechanism

78. Important points

79. Important points:
● Mechanism is SN1
● CARBOCATION INTERMEDIATE IS FORMED
● The reactivity order of HX is - HI > HBr > HCl
● Rearrangement is possible
● Order of rate of reaction for ROH
3o > 2o > 1o < CH3 - OH
SN1 SN1 SN2 SN2

80. Use of Catalyst :
HI - Usually no catalyst required
HBr - H2SO4 as catalyst
HCl - LUCAS REAGENT Conc HCl - Anhyd ZnCl2

81. 3o Alcohol ⟶ Immediate turbidity
2o Alcohol ⟶ 5 to 7 min far turbidity
1o Alcohol ⟶ No turbidity until heating is not done
Important points:

82. Predict the products on reaction with Lucas reagent

83. Reaction of SOCl2 (Darzen Reaction)
(a) R - OH + SOCl2 ⟶ R - Cl + SO2 (g) + HCl (g)
Thionyl
Chloride

84. SNi Reactions

85. Mechanism of Darzen reaction

86. Note
(a) Reaction of ROH & SOCl2 is SNi mechn
(b) Retained product is obtained
(c) In presence of weak base (like pyridine mechanism is SN2 &
inversion takes place

87. predict the products of darzen reaction

88. Finkelstein Reaction

89. Finkelstein Reaction
R - Br or R - Cl + Kl R - I + KCl
In this reaction only exchange takes place and the reaction is called
as Halogen exchange reaction
Acetone

90. 1. Reaction occurs due to formation of ppt because NaCl is
insoluble in acetone
2. The mechanism involves SN2
3. Inversion product is formed
Important Points

91. Predict the products of the reaction

92. Swarts Reaction

93. Swarts reaction
1. AgF is taken in polar aprotic solvent DMF
2. SN2 mechanism is followed
R - Br or R - Cl R - F
Ag F/ Δ
or Hg2F2 / Δ

94. predict the product of the reaction

95. Grove’s Reaction

96. Grove’s Process
Reaction of alcohol with PCl3 or red P/Cl2
(a) Using PCl3 :- 3ROH + PCl3 ⟶ 3R - Cl + H3PO3
(b) Using PCl5 : - ROH + PCl5 ⟶ R - Cl + HCl + POCl3

97. Mechanism

98. It follows SN2 mechanism
Inverted product is formed
Important points

99. Practice Questions

100. Predict the products of the reaction
o
||
R - C - OH + PCl5
O
+ PCl5

101. Predict the products of the reaction
o
||
R - C - Cl
+ PCl5
+ PCl5

102. Ambidentate
Nucleophiles

103. Ambidentate Nucleophile :-
Nucleophiles having more than one electron donating sites.

104. Reaction with KCN :
R - X + KCN ⟶ R - C ≡ N + KX
Alkane nitrile

105. Reaction with AgCN :
R - X + AgCN ⟶ R - N ≡ C + AgX

106. Reaction with KNO2 :

107. SN NGP
Neighbouring
group
participation

108. SN - NGP in SN reactions
(i) Anchimeric assistance
(ii) Compound must have leaving group along with internal nucleophilic site
(iii) Both leaving group and the interval Nucleophilic site must be anti

109. SN - NGP in SN reactions
(iv) NGP increases rate of reaction
(v) For NGP concentration of external nucleophile must be low
(vi) Tendery of NGP decreases if leaving tendency is usually high.

110. Group showing NGP
(viii) Atoms/ group which show NGP are:
- Cl, - NR2 - OR - COOR
- Br - NHR - O - - C = C
- I - NH2 - COO - SR

111. Predict the product of the reaction
aq.
NaOH
(2. eq.)

112. Elimination
Reactions

113. 1,2 Elimination Reactions
When both eliminating atoms / groups eliminate from vicinal sites (1,2)

114. E2
Elimination
Reaction

115. Mechanism of E2 elimination

116. Important points of E2 elimination
Important points of E2 elimination
NOTE
❖ No carbocation Intermediate
❖ One step reaction
❖ No rearrangement possible
❖ A pentavalent C - TS is involved
❖ Anti elimination

117. E1
Elimination
Reaction

118. Mechanism of E1 Elimination

119. Notes of E1 Elimination
NOTE
Two step mechanism
Carbocation Intermediate
Rearrangement possible
Formation of cation is r.d.s

120. E1cb
Elimination
Reaction

121. Mechanism of E1cb Elimination

122. Important points about E1cb elimination
NOTE
Two step mechanism
Carbanion intermediate
No rearrangement possible
Step II is r.d.s due to lower conc of Conjugate base

123. Factors affecting
Elimination
Reaction

124. Factors affecting elimination mechn
(a) Structure of reactant
(b) Nature of leaving group
(c) Nature of base
(d) Nature of solvent
(e) Effect of temp

125. Structure of alkyl
halide

126. Structure of alkyl halide: E1 Elimination
Higher the ease of dissociation / higher the stability of carbocation
higher would be rate of E1 mechn
3o > 2o > 1o

127. More the ease of dissociation,
more would be E1
More stable carbocation, more will be E1

128. Structure of alkyl halide: E2 Elimination
Since TS contains alkene like character,
hence higher the stability of alkene, higher would be stability of T.S
hence higher would be the rate of E2

129. More the stability of TS (alkene
like character), more will be E2

130. Structure of alkyl halide: E1cb Elimination
⇒ 𝛃 - Anion stabilizing group (EWG) must be present for E1 cb in R - X
- C - C -
H H
EWG X
For E1 - CB

131. Nature of leaving
group

132. Nature of leaving group E1 and E2
Since r.d.s of both E1 & E2 involves breaking of bond,
hence on increasing leaving tendency rate of both
Mechn increases

133. Nature of leaving group E1cb
For E 1 - CB
Very Bad leaving group
(-F, -OH, -OR etc)

134. Nature of base

135. Nature of Base
(For E1 )
Since r.d.s of E1 does not involve Base
hence No effect on rate of conc / size / basic strength of base
Usually weak base is used

136. Nature of Base
For E2
Since Base is involved in rds of E2 hence rate increases at
⇒ higher conc. and small size
⇒ high Basic strength

137. Important Base
(Caustic potash)
Alcoholic NaOH
Alcoholic KOH
NaNH2
RONa
ONa
N

138. Effect of solvent

139. Solvents
(For E1)
Polar Protic Solvent (H2O ROH, HCOOH etc)
(For E2)
PAS (DMSO, DMF)
(E1 cb)
PPS

140. Important
Reactions

141. Dehydrohalogenation
Reaction

142. Follows E2 mechanism
The TS should have alkene like character
Dehydrohalogenation Reaction

143. Practice Questions

144. Predict the product of following reactions

145. predict the product of reaction

146. Important points

147. Note : -
Use of Bulky Bases favor formation of Hofmann product as a
major product
Cl
Alc. KOH
t-BuOK

148. Note : -
1. A leaving group with strong I effect and poor leaving tendency on
elimination gives hoffmann alkene as a major product.
2. Reaction mechanism follows E2a mechn
3. Such leaving groups are
- F, - NR3, - SR2 etc

149. Substitution vs Elimination : -
1. CH3 - X always show SN2
2. Higher temp favors elimination
3. Aq. NaOH and Aq.KOH shows SN2 mechn
4. Alc. KOH and Alc. NaOH shows E2 mechn
5. Use of Bulkcier Base gives higher yield of elimination (E2) product
6. On increasing Branching elimination mechn gets favored.

150. Factors for elimination Mechn
SN2 SN1 E1 E2
(i) 1o > 2o > 3o (i) 3o > 2o > 1o (i) 3o > 2o > 1o (i) 3o > 2o > 1o
(ii) R-I >R-Br> R-Cl (ii) R-I >R-Br>
R-Cl
(ii) R-I >R-Br> R-
Cl
(ii) R-I >R-Br> R-
Cl
Strong nucleophile
NaI, NaSR, NaBr,
NaCN, NaN3
Poor nucleophile
Water, alcohol
Poor base
Water, alcohol
Strong base
NaOH, NaOR,
NaNH2, t-BuO-K+
Polar Aprotic solvent Polar protic
solvent
Polar protic solvent
HEAT
Polar aprotic
solvent
HEAT

151. Practice Questions

152. The major product in the following reaction is :
[Sep. 03, 2020 (II)]
A.
B.
C.
D.

153. The major product obtained from E2 - elimination
of 3 - bromo - 2 - fluoropentane is :
[Sep. 02, 2020 (II)]
A.
B.
C.
D.

154. Consider the reaction sequence given below :
Which of the following statements is true ?
A. Changing the base from OH to OR will have no effect on
reaction (2).
B. Changing the concentration of base will have no effect on
reaction (1).
C. Doubling the concentration of base will double the rate of
both the reactions.
D. Changing the concentration of base will have no effect on
reaction (2)
[Sep. 02, 2020 (II)]

155. The decreasing order of reactivity towards dehydrohalogenation
(E1) reaction of the following compounds is:
[Jan. 08, 2020 (I)]
A. D > B > C > A
B. B > D > A > C
C. B > D > C > A
D. B > A > D > C

156. The major product of the following reaction is :
[Jan. 12, 2019 (II)]
A.
B.
C.
D.

157. The major product of the following reaction is :
[2018]
A.
B.
C.
D.

158. The major product obtained in the following reaction is :
[2017]
A.
B.
C.
D.

159. Telegram Channel
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162. Bratin Mondal
100 %ile
Amaiya singhal
100 %ile
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163. SAKSHI
12th / Drop
11th / 9, 10
SAKSHI