Haloalkanes and Haloarenes Important Questions
Haloalkanes and Haloarenes Important Questions
Haloalkanes and Haloarenes Important Questions
GRADE : 12 CHEMISTRY
Chapter 10 : Haloalkanes and Haloarenes
1. A solution of KOH hydrolyses CH3CHClCH2CH3 and CH3CH2CH2CH2Cl. Which one of these is
more easily hydrolysed? (Delhi 2010)
Answer: In aq. KOH SN1 mechanism takes place and the formed carbocation is stabilized. Thus 2°
carbocation is more stable carbocation than 1° therefore
Answer:
3. Write the IUPAC name of the following compound : (CH3)3 CCH2Br (Delhi 2010)
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5., 6. Write the IUPAC name of the following compound : (All India 2010)
Answer:
10.In the following pair of compounds, which will react faster by SN1 mechanism and why?
Answer: reacts faster by SN1 mechanism as it is a tertiary halide and it produces a stable tertiary
carbocation.
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11. Draw the structure of major monohalogen product in the following reaction : (Comptt. Delhi
Answer:
12. Predict the order of reactivity of four isomeric bromobutanes in SN1 reaction.
Answer:
13. Predict the order of reactivity of the following compounds in SN1 reaction :
C6H5CH2Br, C6H5C(CH3) (C6H5)Br, C6H5CH(C6H5)Br, C6H5CH(CH3)Br (Comptt. Delhi 2012)
Answer:
C6H5C(CH3) (C6H5)Br > C6H5CH(C6H5)Br > C6H5CH(CH3)Br > C6H5CH2Br
14. Draw the structure of major monohalogen product in the following reaction : (Comptt. Delhi
Answer:
15. Give a chemical test to distinguish between C2H5Br and C6H5Br. (Comptt. All India 2012)
Answer: Both are heated with aqueous NaOH. C2H5Br gives ethanol and NaBr, which on reacting with
AgNO3, gives yellow precipitate of AgBr. C6H5Br does not respond to this test.
16.Arrange the following in increasing order of boiling point:
(i) CH3CH2CH2CH2Br (ii) (CH3)3.Br (iii) (CH3)2C.Br
Answer:
(CH3)2C.Br < (CH3)2.CHCH2.Br < CH3CH2CH2CH2Br
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17. Write the IUPAC name of the following : (Comptt. All India 2012)
Answer:
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24.Write the IUPAC name of the following compound: (All India 2013)
29. Identify the chiral molecule in the following pair : (All India 2014)
Answer:
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31. Which would undergo SN2 reaction faster in the following pair and why? (Delhi 2015)
Answer:
because the secondary carbocation formed is more stable than primary carbocation.
33. Out of
34. Write the structure of an isomer of compound C4H9Br which is most reactive towards SN1
reaction.
Answer:
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36. Write the structure of 2, 4-dinitrochlorobenzene.
Answer:
40. Which one in the following pairs of substances undergoes SN2 substitution reaction faster and
why?
Answer:
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41. Which one in the following pairs undergoes SN1 substitution reaction faster and why?
Answer:
Answer:
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(ii) Methyl magnesium bromide to 2-methylpropan-2-ol
Resonance leads to lowering of energy and hence greater stability. On the other hand, no such resonance is
possible in haloalkanes. Halogens directly attached to benzene ring are o, p-directing in electrophilic
subsitution reactions. This is due to greater electron density at these positions in resonance.
45. Chlorobenzene is extremely less reactive towards a nucleophilic substitution reaction. Give two
reasons for the same. (Delhi 2013)
Answer: The reasons are:
(i) Due to resonance/diagrammatic repre-sentation, C – Cl bond acquires a partial double bond character.
As a result, the C – Cl bond in chlorobenzene is shorter and hence stronger. Thus, cleavage of C – Cl bond
in benzene becomes difficult which makes it less reactive towards nucleophilic substition.
(ii) Due to repulsion between nucleophile and electron rich arenes.
46. (a) Why does p-dichlorobenzene have a higher m.p. than its o- and m-isomers?
(b) Why is (±)-Butan-2-ol optically inactive? (Delhi 2013)
Answer: (a) p-isomers are comparetively more symmetrical and fit closely in the crystal lattice, thus
require more heat to break these strong forces of attraction. Therefore higher melting point than o- and m-
isomers.
(b) (±)-Butan-2-ol is optically inactive because in racemic mix one type of rotation is cancelled by other.
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Answer: (i) In haloalkanes, the halogen atom is attached
to sp3-hybridized carbon while in haloarenes it is attached to sp2 -hybridized carbon whose size is smaller
than sp3 orbital carbon. Therefore C – Cl bond in chloro-benzene is shorter than alkyl chloride.
(ii) CHCl3 is stored in dark coloured bottles to cut off light because CHCl3is slowly oxidised by air in
presence of light to form an extremely poisonous gas, carbonyl chloride, popularly known as phosgene.
48. Give chemical tests to distinguish between the following pairs of compounds :
(a) Benzyl chloride and Chlorobenzene
(b) Chloroform and Carbon tetrachloride (Comptt. Delhi 2013)
Answer: (a) Chlorobenzene and Benzyl chloride :
Benzyl chloride is more reactive than chlorobenzene towards nucleophillic substitution reactions,
therefore, benzyl chloride on boiling with aqueous KOH produces benzyl alcohol and KCl.
The reaction mixture on acidification with dil. HNO3 followed by treatment with AgNO3 solution
produces white ppt. of AgCl due to formation of KCl
But chlorobenzene does not undergo hydrolysis under these mild conditions to give phenol and KCl.
(b) Chloroform and Carbon tetrachloride
By Carbylamine test: CCl4 does not give this reaction but chloroform gives this reaction and produces
offensive smell of phenyl isocyanide.
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49. Explain why :
(a) The dipole moment of chlorobenzene is lower than that of cyclohexyl chloride.
(b) Alkyl halides, though polar, are immiscible with water. (Comptt. All India 2013)
Answer: (a) Chlorobenzene has lower dipole moment than cyclohexyl chloride due to lower magnitude
of -ve charge on the Cl atom and shorter C – Cl distance. Due to greater S-character, a sp2-hybrid carbon
is more electronegative than a sp3-hybrid carbon. Therefore, the sp2-hybrid carbon of C – Cl bond in
chlorobenzene has less tendency to release electrons to Cl than a sp3 hybrid carbon of cyclohexyl chloride.
(b) Alkyl halides and polar molecules are held together by dipole-dipole interaction. The molecules of
H2O are held together by H- bonds. Since the new forces of attraction between water and alkyl halide
molecules are weaker than the forces of attraction already existing between alkyl halide-alkyl halide
molecules and water-water molecules, therefore alkyl halides are immiscible (not soluble) with water.
50. (i) Which alkyl halide from the following pair is chiral and undergoes faster SN2 reaction?
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Answer:
57. Which compound in each of the following pairs will react faster in SN2 reaction with —OH?
(i) CH3Br or CH3I (ii) (CH3)3 CCl or CH3Cl (Comptt. All India 2016)
Answer: (i) CH3I: Because Iodide is better leaving group than bromide.
(ii) CH3Cl : Carbon atom leaving group is less hindered.
58. Which alkyl halide from the following pair is (i) Chiral and (ii) undergoes SN2 reaction faster?
Answer:
59. Which alkyl halide from the following pair is (i) Chiral and (ii) undergoes SN1 reaction faster?
(a) (CH3)2CBr (b) CH3CH2CHBrCH3 (Comptt. Delhi 2017)
Answer: (i) 2-Bromobutane is chiral as the central C atom has all 4-different groups.
(ii) 3° Alkyl bromide i.e. (CH3)3CBr undergoes SN1 reaction faster due to more stability of 3° carbocation.
60. How will you carry out the following conversions :
(i) 2-Bromopropane to 1-bromopropane (ii) Benzene to p-chloronitrobenzene
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Answer:
(b) 1-Bromo-2-methylbutane having less steric hindrance, is thus more reactive towards SN2 reaction than
2-bromo-2-methyl butane (more steric hindrance).
62. How would you differentiate between S<subN1 and SN2 mechanisms of substitution reactions?
Give one example of each. (All India 2010)
Answer:
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63. Answer the following :
(i) Haloalkanes easily dissolve in organic solvents, why?
(ii) What is known as a racemic mixture? Give an example.
(iii) Of the two bromoderivatives, C6H5CH (CH3)Br and C6H5CH(C6H5)Br, which one is more
reactive in SN1 substitution reaction and why? (Delhi 2011)
Answer: (i) Because the new forces of attraction set up between haloalkanes and solvent molecules are of
the same strength as the forces of attraction being broken.
(ii) A mixture which contains the equal proportions of two enantiomers of a compound in equal
proportions is called racemic mixture
Example : (±) butan-2-ol
(iii) Since the reactivity of SN1 reactions increases as the stability of intermediate carbocation increases.
Of the two 2° bromides, the carbocation intermediate derived from C6H-CH(C6H5)Br i.e. C6H5CHC6H5 is
more stable as compared to the carbocation C6H+5CHCH3 obtained from C6H5CH(CH3)Br because it is
stabilized by two phenyl groups due to resonance.
64. Rearrange the compounds of each of the following sets in order of reactivity towards S N2
displacement :
(i) 2-Bromo-2-methylbutane, 1-Bromopentane, 2-Bromopentane
(ii) l-Bromo-3-methylbutane, 2-Bromo-2-methyl-butane, 3-Bromo-2-methylbutane
(iii) 1-Bromobutane, l-Bromo-2,2-dimethyl-propane, l-Bromo-2-methylbutane (All India 2011)
Answer:
(i) 1-Bromopentane > 2-Bromopentane > 2-Bromo-2-methylbutane
(ii) 1-Bromo-3-methylbutane > 3-Bromo-2 methylbutane > 2-Bromo-2-methylbutane
(iii) 1-Bromobutane > 1-Bromo-2-methylbutane > 1-Bromo-2, 2-dimethylpropane.
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65. Although chlorine is an electron withdrawing group, yet it is ortho-, para-directing in
electrophilic aromatic substitution reactions. Explain why it is so? (Delhi 2012)
Answer: Chlorine withdraws electrons through inductive effect and releases through resonance. Although
Cl shows -1 effect but through resonance, Cl tends to stabilize the intermediate carbocation and the effect
is more pronounced at ortho and para positions.
This can also be explained diagramatically as:
Answer:
(i) Chirality : The objects which are non-superimposable on their mirror image are said to be chiral and
this property is known as chirality for Butan-2-ol
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(iii) In halobenzene, halogen atom is attached to the sp2 hybrid carbon atom while in CH3-X halogen atom
is attached to sp3 hybrid carbon atom. Hence C-X bond length in halo benzene is smaller than CH3-X.
69. (a) Draw the structures of major monohalo products in each of the following reactions :
(b) Which halogen compound in each of the following pairs will react faster in SN2 reaction :
(i) CH3Br or CH3I
(ii) (CH3)3 C – Cl or CH3 – Cl (All India 2014)
Answer:
Answer: (a) (i) As I– ion is a good leaning group than Br, therefore reacts faster than CH3Br in SN2
reactions with -OH.
(ii) As 3° alkyl halides have more steric hindrance than 1° alkyl halide therefore CH3Cl undergoes SN2
reaction faster than 3° alkyl
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71. Give reasons : (a) n-Butyl bromide has higher boiling point than f-butyl bromide.
(b) Racemic mixture is optically inactive.
(c) The presence of nitro group (-NO2) at o/p positions increases the reactivity of haloarenes towards
nucleophilic substitution reactions. (Delhi 2015)
Answer: (a) n-Butyl bromide has higher boiling point than f-butyl bromide because it has larger surface
area hence have more Van der Waals’ forces.
(b) Rotation due to one enantiomer is cancelled by another enantiomer.
(c) The presence of nitro group (-NO2) at ortho and para positions withdraws the electron density’ from
benzene ring and thus facilitating the attack of nucleophile.
72. How can the following conversions be carried out :
(i) Aniline to bromobenzene (ii) Chlorobenzene to 2-chloroacetophenone
(iii) Chloroethane to butane (All India 2015)
Answer: (i) Aniline to bromobenzene
Answer:
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74. Name the following according to IUPAC system : (Comptt. Delhi 2015)
Answer:
(i) 2-Bromotoluene (ii) 1 chloro, 2, 2 dimethyl propane
75.Give the IUPAC names of the following compounds : (Comptt. All India 2015)
Answer:
to sp3-hybridized carbon while in haloarenes it is attached to sp2 -hybridized carbon whose size is smaller
than sp3 orbital carbon. Therefore C – Cl bond in chloro-benzene is shorter than alkyl chloride.
(ii) Dipole moment is the product of charge and inter nuclear distance. In case of cyclohexyl chloride
carbon—chlorine band length is longer than C—Cl band length in chlorobenzene. Hence dipole moments
of cyclohexyl chloride is more.
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(iii) SN1 reactions are accompanied by racemization in optically active alkyl halides because the
nucleophile will have an equal opportunity to attack on sp2 hybridised carbocation from either sides to
give a racemic mixture.
Example:
79. In the following pairs of the halogen compounds which compound undergoes faster S N1
reaction. (Comptt. Delhi 2016)
Answer:
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78. Write the major product(s) in the following: (All India 2016)
Answer:
80. What happens when (i) CH3—Cl is treated with aqueous KOH?
(ii) CH3—Cl is treated with KCN? (iii) CH3—Br is treated with Mg in the presence of dry ether?
Answer:
>
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Answer:
84. (a) Write the structural formula of A, B, C and D in the following sequence of reaction:
(b) Illustrate Sandmeyer’s reaction with the help of a suitable example. (Comptt. Delhi 2017)
Answer:
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(b) Sandmeyer’s reaction: The substitution of diazo group of benzene diazonium chloride by Chloro,
Bromo and Cyano group with the help of solution of CuCl dissolved in HC1, CuBr/HBr and CuCN/KCN
respectively is known as Sandmeyer’s reaction.
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