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

done Case Based (MCQs) - Haloalkanes and Haloarenes Total Questions - 15

• question_answer1)

  Direction (1-5): The alkyi halides are extremely important reagents because they undergo a large variety of reactions that make them valuable in organic synthesis. This arises from the fact that the halide ion is a weak base and is readily displaced by stronger bases. This type of reaction is an example of nucleophilic substitution \[\left( {{S}_{N}} \right)\] since the attacking reagent is a nucleophilic reagent. This type of heterolytic reaction in solution can take place by two different mechanisms, unimolecular \[\left( {{S}_{N}}1 \right)\] or bimolecular \[\left( {{S}_{N}}2 \right)\].

  \[{{S}_{N}}1\] reaction is a two step process and in the first step, R-X ionises to give carbocation (slow process). In the second step, the nucleophile attacks the carbocation from either side to form the product (fast process). \[{{S}_{N}}1\] reaction is favoured by heavy (bulky) groups on the carbon atom attached to halogens. \[{{S}_{N}}2\] reaction is a one stage process and two molecules simultaneously undergo covalency change in the rate determining step. In the transition state, the groups OH and X are collinear and on the opposite sides of attacked carbon. \[{{S}_{N}}2\] mechanism always lead to inversion of product. \[{{S}_{N}}2\]   reaction is favoured by small groups on the carbon atom attached to halogen. (I.L. Finar, Organic Chemistry Vol I. The fundamental principles Ch. 5, Sixth Edition).

  Which among the following will not give \[{{S}_{N}}^{1}\] reaction?
  A)
    done clear
  B)
  \[{{\left( C{{H}_{3}} \right)}_{3}}C-Br\] done clear
  C)
  \[C{{H}_{3}}C{{H}_{2}}-I\] done clear
  D)
  done clear
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• question_answer2)
  Which one of the following compounds is most readily hydrolysed by \[{{S}_{N}}^{1}\] mechanism?
  A)
  \[C{{H}_{3}}CH=CHCI\] done clear
  B)
  \[{{\left( {{C}_{6}}{{H}_{5}} \right)}_{3}}CCl\] done clear
  C)
  \[ClC{{H}_{2}}CH=C{{H}_{2}}\] done clear
  D)
  done clear
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• question_answer3)

  The main product formed in the following reaction is :

  A)
  done clear
  B)
  done clear
  C)
  done clear
  D)
  done clear
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• question_answer4)
  Which of the following is an example of \[{{S}_{N}}^{2}\] reaction?
  A)
  \[C{{H}_{3}}Br+O{{H}^{-}}\to C{{H}_{3}}OH+B{{r}^{-}}\] done clear
  B)
  \[C{{H}_{3}}C{{H}_{2}}OH\to C{{H}_{2}}=C{{H}_{2}}\] done clear
  C)
  \[{{\left( C{{H}_{3}} \right)}_{2}}CHBr+O{{H}^{-}}\to {{\left( C{{H}_{3}} \right)}_{2}}CHOH+B{{r}^{-}}\] done clear
  D)
  \[{{\left( C{{H}_{3}} \right)}_{3}}CBr+O{{H}^{-}}\to {{\left( C{{H}_{3}} \right)}_{3}}COH+B{{r}^{-}}\] done clear
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• question_answer5)
  An optically active halide when allowed to react with \[C{{N}^{-}}\], gives a racemic mixture. The halide is most likely to be :
  A)
  primary             done clear
  B)
  secondary done clear
  C)
  tertiary              done clear
  D)
  none of these. done clear
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• question_answer6)

  Direction (6 - 10): In both alkyi halides and aryl halides, the halogen (X) atom is attached directly to the carbon atom. They are expected to exhibit similar reactivity. However, aryl halides are comparatively very little reactive, particularly towards nucleophilic substitution reactions. For example, hydrolysis of ethyl chloride occurs by simply boiling with aqueous KOH. On the other hand, the alkaline hydrolysis of chlorobenzene requires a very high temperature (623 K) as well as a very high pressure.

  Among the following, which has weakest C - X bond?
  A)
  Benzyl bromide   done clear
  B)
  Bromobenzene done clear
  C)
  Vinyl bromide     done clear
  D)
  Benzyl chloride. done clear
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• question_answer7)
  The halide which does not give any precipitate when warmed with alcoholic \[AgN{{O}_{3}}\] solution is :
  A)
  Chlorobenzene    done clear
  B)
  Benzyl chloride done clear
  C)
  Allyl chloride      done clear
  D)
  Tert-butyl chloride. done clear
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• question_answer8)
  Benzene reacts with \[C{{l}_{2}}\] in the presence of \[FeC{{l}_{3}}\] (and absence of sun light) to form :
  A)
  Benzyl chloride   done clear
  B)
  Benzyl chloride done clear
  C)
  Chlorobenzene    done clear
  D)
  Benzene hexachloride. done clear
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• question_answer9)
  Friedel Craft's reaction of bromobenzene with methyl iodide gives :
  A)
  o-Bromotoluene done clear
  B)
  p-Bromotoluene done clear
  C)
  o- and p- Bromotoluene done clear
  D)
  m-Bromotoluen. done clear
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• question_answer10)

  Aryl halides are less reactive towards nucleophile substitution reactions than alkyi halides due to :

  [A] formation of less stable carbocation

  [B] resonance stabilisation

  [C] longer carbon-halogen bond

  [D] inductive effect

  [E] \[s{{p}^{2}}\]hybridised carbon attached to halogen.

   
  A)
  A, B                              done clear
  B)
  B, E done clear
  C)
  B, C                              done clear
  D)
  A, E. done clear
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• question_answer11)

  Direction (11 - 15): A number of methods of preparation of alkyl halides are available in literature. Alkyi halides are best prepared from alcohols, which are easily accessible. The hydroxyl group of an alcohol is replaced by halogen on reaction with concentrated halogen acids, phosphorus babies orthionyl chloride (preferred method). Free radical chlorination of alkanes give a complex mixture of mono- and polyhaloalkanes, which are difficult to separate as pure compounds. An alkene is converted to corresponding alkyl halide by reaction with hydrogen halide. Markownikoff rule is followed in case of unsymmetrical alkene. Alkyl iodides are often prepared by the reaction of alkyl chloride/bromides with Nal in dry acetone (Finkelstein reaction).

  (Source : N.C.E.R.T. book)

  Identify the respective major monohalo product in the following ;

  and

   
  A)
  done clear
  B)
  done clear
  C)
    done clear
  D)
  done clear
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• question_answer12)

  The major product-obtained in the following reaction

  ; is

  A)
     done clear
  B)
  done clear
  C)
    done clear
  D)
  done clear
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• question_answer13)

  Identify the respective major product obtained in the following reactions :

  \[C{{H}_{3}}C{{H}_{2}}CH=C{{H}_{2}}\xrightarrow{HBr,\,Peroxide}\]and \[C{{H}_{3}}CH=C{{\left( C{{H}_{3}} \right)}_{2}}+HBr\to \]

   
  A)
  \[C{{H}_{3}}C{{H}_{2}}Ch\left( Br \right)C{{H}_{3}}\]and \[C{{H}_{3}}C{{H}_{2}}C\left( Br \right){{\left( C{{H}_{3}} \right)}_{2}}\] done clear
  B)
  \[C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}C{{H}_{2}}Br\] and \[C{{H}_{3}}-CH\left( Br \right)CH\left( C{{H}_{3}} \right){{  }_{2}}\] done clear
  C)
  \[C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}C{{H}_{2}}Br\] and \[C{{H}_{3}}C{{H}_{2}}C\left( Br \right){{\left( C{{H}_{3}} \right)}_{2}}\] done clear
  D)
  None of these. done clear
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• question_answer14)
  Which of the following- isomer of \[{{C}_{5}}{{H}_{12}}\] will give four monochloro products on free radical chlorination?
  A)
      done clear
  B)
  done clear
  C)
                  done clear
  D)
  done clear
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• question_answer15)

  Identify the major monohalo product

  A)
  done clear
  B)
  (b) done clear
  C)
  done clear
  D)
  done clear
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