Haloalkanes haloarenes bounce back.pdf

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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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

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

52. 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

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

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

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

144. Predict the product of following reactions

145. predict the product of reaction

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

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.

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