12th Class Chemistry Haloalkanes and Haloarenes / हैलोएल्केन तथा हैलोएरीन्स Question Bank

done Case Based - Haloalkanes and Haloarenes Total Questions - 25

• question_answer1)

  Directions: (Q. 1 to 5)

  Case I: Read the passage given below and answer the following questions.

  Anucleophilic substitution reaction that occurs by an \[{{S}_{N}}1\] mechanism proceeds in two steps. In the first step, the bond between the carbon atom and the leaving group breaks to produce a carbocation and most commonly, an anionic leaving group.

  In the second step, the carbocation reacts with the nucleophile to form the subsitution product. The formation of a carbocation is the slow or rate determining step.

  The rate of \[{{S}_{N}}1\] reactions decrease in the order \[3{}^\circ >2{}^\circ >1{}^\circ \], which is the reverse order observed in \[{{S}_{N}}2\] reactions. In contrast to \[{{S}_{N}}2\] reactions at sterogenic centers, which occur with inversion of configuration, an \[{{S}_{N}}1\]reaction gives a racemic mixture of enatiomers that has no optical rotation.

  In the following questions (Q. No. 1-25), a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices on the basis of the above passage.

  Assertion: Rate of ethanolysis of \[1{}^\circ \] halide by \[{{S}_{N}}2\] mechanism is fast.

  Reason: Carbocation is stablised by resonance.

  A)
  Assertion and reason both are correct statements and reason is correct explanation for assertion. done clear
  B)
  Assertion and reason both are correct statements but reason is not correct explanation for assertion. done clear
  C)
  Assertion is correct statement but reason is wrong statement. done clear
  D)
  Assertion is wrong statement but reason is correct statement. done clear
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• question_answer2)

  Assertion: \[{{S}_{N}}2\] reactions do not proceed with retention of configuration.

  Reason: \[{{S}_{N}}2\] reactions proceed in single step.

  A)
  Assertion and reason both are correct statements and reason is correct explanation for assertion. done clear
  B)
  Assertion and reason both are correct statements but reason is not correct explanation for assertion. done clear
  C)
  Assertion is correct statement but reason is wrong statement. done clear
  D)
  Assertion is wrong statement but reason is correct statement. done clear
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• question_answer3)

  Assertion: In an optically active alkyl halide nucleophilic substitution reaction gives a mixture of enantiomers.

  Reason: Reaction occurs by \[{{S}_{N}}1\] mechanism.

  A)
  Assertion and reason both are correct statements and reason is correct explanation for assertion. done clear
  B)
  Assertion and reason both are correct statements but reason is not correct explanation for assertion. done clear
  C)
  Assertion is correct statement but reason is wrong statement. done clear
  D)
  Assertion is wrong statement but reason is correct statement. done clear
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• question_answer4)

  Assertion: \[1{}^\circ \]allylic halides show higher reactivity in \[{{S}_{N}}1\] reactions than other \[1{}^\circ \] alkyl halides.

  Reason: Intermediate carbocation is stabalised by resonance.

  A)
  Assertion and reason both are correct statements and reason is correct explanation for assertion. done clear
  B)
  Assertion and reason both are correct statements but reason is not correct explanation for assertion. done clear
  C)
  Assertion is correct statement but reason is wrong statement. done clear
  D)
  Assertion is wrong statement but reason is correct statement. done clear
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• question_answer5)

  Assertion: \[{{S}_{N}}1\] reaction is carried out in the presence of polar protic solvent.

  Reason: A polar protic solvent decreases the stability of carbocation due to solvation.

  A)
  Assertion and reason both are correct statements and reason is correct explanation for assertion. done clear
  B)
  Assertion and reason both are correct statements but reason is not correct explanation for assertion. done clear
  C)
  Assertion is correct statement but reason is wrong statement. done clear
  D)
  Assertion is wrong statement but reason is correct statement. done clear
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• question_answer6)

  Directions: (Q. 6 to 10)

  Case II: Read the passage given below and answer the following questions.

  When haloalkanes with (\[\beta \]-hydrogen atom are boiled with alcoholic solution of KOH; they undergo elimination of hydrogen halide resulting in the formation of alkenes. These reactions are called (\[\beta \]-elimination reactions or dehydrohalogenation reactions. These reactions follow Saytzeff's rule. Substitution and therefore can engage in substitution or elimination reactions depending upon the alkyl halide and the reaction conditions.

  Among the following halides the most reactive towards alcoholic KOH is
  A)
  \[C{{H}_{2}}=CHBr\] done clear
  B)
  \[C{{H}_{3}}COC{{H}_{2}}C{{H}_{2}}Br\] done clear
  C)
  \[C{{H}_{3}}C{{H}_{2}}Br\] done clear
  D)
  \[C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}Br\] done clear
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• question_answer7)
  The general reaction, \[R-X\xrightarrow{aq.OH-}ROH+{{X}^{-}}\] is expected to follow decreasing order of reactivity as in
  A)
  \[t-BuI>t-BuBr>t-BuCl>t-BuF\] done clear
  B)
  \[t-BuF>t-BuCl>t-BuBr>t-BuI\] done clear
  C)
  \[t-BuBr>t-BuCl>t-BuI>t-BuF\] done clear
  D)
  \[t-BuF>t-BuCl>t-BuI>t-BuBr\] (t-Bu = tertiary Butyl group) done clear
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• question_answer8)
  Reaction of t-butyl bromide with sodium methoxide produces
  A)
  sodium t-butoxide done clear
  B)
  t -butyl methyl ether done clear
  C)
  iso-butane done clear
  D)
  iso-butylene. done clear
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• question_answer9)
  In the elimination reactions, the reactivity of alkyl halides follows the sequence
  A)
  R-F>R-Cl>R-Br>R-I done clear
  B)
  R-I>R-Br>R-Cl>R-F done clear
  C)
  R-I>R-F>R-Br>R-Cl done clear
  D)
  R-F>R-I>R-Br>R-Cl done clear
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• question_answer10)
  The ease of dehydrohalogenation of alkyl halide with alcoholic KOH is
  A)
  \[3{}^\circ <2{}^\circ <\text{ }1{}^\circ \] done clear
  B)
  \[3{}^\circ >2{}^\circ >\text{ }1{}^\circ \] done clear
  C)
  \[3{}^\circ <2{}^\circ >\text{ }1{}^\circ \] done clear
  D)
  \[3{}^\circ >2{}^\circ \text{ }<\text{ }1{}^\circ \] done clear
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• question_answer11)

  Directions: (Q. 11 to 15)

  Case III: Read the passage given below and answer the following questions.

  A chlorocompound [A] on reduction with Zn-Cu and ethanol gives the hydrocarbon [B] with five carbon atoms. When [A] is dissolved in dry ether and treated with sodium metal it gave 2, 2, 5, 5-tetramethylhexane. The treatment of [A] with alcoholic KCN gives compound [C].

  The compound [A] is
  A)
  1-chloro-2, 2-dimethylpropane done clear
  B)
  1-chloro-2, 2-dimethylbutane done clear
  C)
  1-chloro-2-methylbutane done clear
  D)
  2-chloro-2-methylbutane. done clear
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• question_answer12)
  The reaction of [C] with Na, \[{{C}_{2}}{{H}_{5}}OH\] is called
  A)
  \[{{\left( C{{H}_{3}} \right)}_{3}}CC{{H}_{2}}CON{{H}_{2}}\] done clear
  B)
  \[{{\left( C{{H}_{3}} \right)}_{3}}CN{{H}_{2}}\] done clear
  C)
  \[{{\left( C{{H}_{3}} \right)}_{3}}CC{{H}_{2}}C{{H}_{2}}N{{H}_{2}}\] done clear
  D)
  \[{{\left( C{{H}_{3}} \right)}_{2}}CHC{{H}_{2}}N{{H}_{2}}\] done clear
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• question_answer13)
  The reaction of [C] with Na, \[{{C}_{2}}{{H}_{5}}OH\]is called
  A)
  Gilman reaction done clear
  B)
  Mendius reaction done clear
  C)
  Grooves process done clear
  D)
  Swart's reaction. done clear
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• question_answer14)
  The reaction of [A] with aq. KOH will preferably favour
  A)
  \[{{S}_{N}}1\] mechanism done clear
  B)
  \[{{S}_{N}}2\] mechanism done clear
  C)
  E1 mechanism done clear
  D)
  E2 mechanism. done clear
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• question_answer15)
  Compound [B] is
  A)
  n-pentane done clear
  B)
  2, 2-dimethylpropane done clear
  C)
  2-methylbutane done clear
  D)
  none of these. done clear
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• question_answer16)

  Directions : (Q. 16 to 20)

  Case IV: Read the passage given below and answer the following questions.

  In haloalkanes, when a nucleophile stronger than the halide ion approaches the positively charged carbon atom of an alkyl halide, the halogen atom along with its bonding electron pair gets displaced and a new bond with the carbon and the nucleophile is formed. These reactions are called nucleophilic substitution reactions.

  In these reactions the atom or group of atoms which loses its bond from carbon and takes on an additional pair of electrons is called leaving group. Halide ions are good leaving groups. Some important nucleophilic substitution reactions of haloalkanes with common nucleophiles are given in the table below.

  Reagent Nucleophile Substitution product R-Nu Class of main product 1. NaOH or KOH or moist \[A{{g}_{2}}O\] \[^{-}OH\] ROH Alcohol 2. \[{{H}_{2}}O\] \[{{H}_{2}}O\] ROH Alcohol 3. NaI \[{{I}^{-}}\] R-I Alkyl iodide 4. \[R'N{{H}_{2}}\] \[R'\ddot{N}{{H}_{2}}\] RNHR' Sec. amine 5. KCN \[\overline{C}\equiv N:\] RCN Nitrile (cyanide) 6. \[KN{{O}_{2}}\] \[O=N-{{O}^{-}}\] R-O-N=O Alkyl nitrite

  In the following questions (Q. No. 16-20), a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices on the basis of the above passage.

  Assertion: Alkyl halides are hydrolysed to alcohols by moist silver oxide.

  Reason: RCl is hydrolysed to ROH easily but reactions slow down on addition of KI.

  A)
  Assertion and reason both are correct statements and reason is correct explanation for assertion. done clear
  B)
  Assertion and reason both are correct statements but reason is not correct explanation for assertion. done clear
  C)
  Assertion is correct statement but reason is wrong statement. done clear
  D)
  Assertion is wrong statement but reason is correct statement. done clear
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• question_answer17)

  Assertion: Alkyl halides form alkenes when heated above \[300{}^\circ C\].

  Reason: \[C{{H}_{3}}C{{H}_{2}}CI\] reacts slowly with strong base as compared to \[C{{H}_{3}}C{{H}_{2}}I\].

   
  A)
  Assertion and reason both are correct statements and reason is correct explanation for assertion. done clear
  B)
  Assertion and reason both are correct statements but reason is not correct explanation for assertion. done clear
  C)
  Assertion is correct statement but reason is wrong statement. done clear
  D)
  Assertion is wrong statement but reason is correct statement. done clear
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• question_answer18)

  Assertion: RBr reacts with \[AgN{{O}_{2}}\] to give nitroalkane.

  Reason: Silver nitrite (\[AgN{{O}_{2}}\]) is an ionic compound, therefore the negative charge on nitrogen is the attacking site.

  A)
  Assertion and reason both are correct statements and reason is correct explanation for assertion. done clear
  B)
  Assertion and reason both are correct statements but reason is not correct explanation for assertion. done clear
  C)
  Assertion is correct statement but reason is wrong statement. done clear
  D)
  Assertion is wrong statement but reason is correct statement. done clear
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• question_answer19)

  Assertion: The nucleophilic substitution of vinyl chloride is difficult than ethyl chloride.

  Reason: Vinyl group is electron donating group.

  A)
  Assertion and reason both are correct statements and reason is correct explanation for assertion. done clear
  B)
  Assertion and reason both are correct statements but reason is not correct explanation for assertion. done clear
  C)
  Assertion is correct statement but reason is wrong statement. done clear
  D)
  Assertion is wrong statement but reason is correct statement. done clear
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• question_answer20)

  Assertion: 2-Bromobutane on reaction with sodium ethoxide in ethanol gives 1-butene as the major product.

  Reason: 1-Butene is less stable than 2-butene.

  A)
  Assertion and reason both are correct statements and reason is correct explanation for assertion. done clear
  B)
  Assertion and reason both are correct statements but reason is not correct explanation for assertion. done clear
  C)
  Assertion is correct statement but reason is wrong statement. done clear
  D)
  Assertion is wrong statement but reason is correct statement. done clear
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• question_answer21)

  Directions: (Q. 21 to 25)

  Case V: Read the passage given below and answer the following questions.

  Haloarenes are less reactive than haloalkanes.

  The low reactivity ofhaloarenes can be attributed to

  - resonance effect

  - \[s{{p}^{2}}\] hybridisation of C-X bond

  - polarity of C-X bond

  - instability of phenyl cation (formed by self- ionisation of haloarene)

  - repulsion between the electron rich attacking nucleophiles and electron rich arenes.

  Reactivity of haloarenes can be increased or decreased by the presence of certain groups at certain positions for example, nitro \[\left( -N{{O}_{2}} \right)\]group at o/p positions increases the reactivity of haloarenes towards nucleophilc substitution reactions.

  Aryl halides are less reactive towards nucleophilic substitution reaction as compared to alkyl halides due to
  A)
  the formation of less stable carbonium ion done clear
  B)
  resonance stabilization done clear
  C)
  larger carbon-halogen bond done clear
  D)
  inductive effect. done clear
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• question_answer22)
  Which of the following aryl halides is the most reactive towards nucleophilic substitution?
  A)
  done clear
  B)
  done clear
  C)
  done clear
  D)
  done clear
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• question_answer23)
  Which one of the following will react fastest with aqueous NaOH?
  A)
             done clear
  B)
  done clear
  C)
  done clear
  D)
  done clear
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• question_answer24)

  Chlorobenzene can be converted into phenol by heating in aqueous sodium hydroxide solution at a temperature of 623 K and a pressure of 300 atm. However the rate of reaction can be increased by presence of certain groups in benzene ring. What will be the order of reactivity of following compounds towards the above substitution reaction?

  (i)

  (ii)

  (iii)

  A)
  (iii) > (ii) > (i) done clear
  B)
  (ii) > (iii) > (i) done clear
  C)
  (i) > (ii) > (iii) done clear
  D)
  (i) > (iii) > (ii) done clear
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• question_answer25)
  The reactivity of the compounds (i) MeBr, (ii) \[PhC{{H}_{2}}Br\], (iii) MeCl, (iv) \[p-MeO{{C}_{6}}{{H}_{4}}Br\] decreases as
  A)
  (i) > (ii) >(iii) > (iv) done clear
  B)
  (iv) > (ii) >(i) > (iii) done clear
  C)
  (iv) > (iii) >(i) > (ii) done clear
  D)
  (ii) > (i) >(iii) > (iv) done clear
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