NEET Chemistry NEET PYQ-Haloalkanes and Haloarenes

  • question_answer
    In a \[{{S}_{N}}2\] substitution reaction of the type \[R-Br+C{{l}^{-}}\xrightarrow[{}]{DNF}R-Cl+B{{r}^{-}},\]which one of the following has the highest relative rate?                                   [AIPMT (S) 2008]

    A) \[C{{H}_{3}}-C{{H}_{2}}-C{{H}_{2}}Br\]      

    B) \[C{{H}_{3}}-\underset{\begin{smallmatrix}  | \\  C{{H}_{3}} \end{smallmatrix}}{\mathop{CH}}\,-C{{H}_{2}}Br\]

    C) \[C{{H}_{3}}-\underset{\begin{smallmatrix}  | \\  C{{H}_{3}} \end{smallmatrix}}{\overset{\begin{smallmatrix}  C{{H}_{3}} \\  | \end{smallmatrix}}{\mathop{C}}}\,-C{{H}_{2}}Br\]      

    D) \[C{{H}_{3}}C{{H}_{2}}Br\]

    Correct Answer: D

    Solution :

    [d] Key Idea: The relative reactivity of alkyl halides towards \[{{S}_{N}}2\] reactions is as follows:
    Primary > Secondary > Tertiary
    However, if the primary alkyl halide or the nucleophile/base is sterically hindered the nucleophile will have difficulty to getting the back side of the a-carbon as a result of this, the elimination product will be predominant. Here \[C{{H}_{3}}C{{H}_{2}}Br\] is the least hindered, hence it has the highest relative rate towards \[{{S}_{N}}2\] reaction.


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