DUMET Medical DUMET Medical Solved Paper-2010

  • question_answer
    Decreasing order of reactivity in Williamson ether synthesis of the following is (I) \[M{{e}_{3}}CC{{H}_{2}}Br\],       (II) \[C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}Br\], (III) \[C{{H}_{2}}=CHC{{H}_{2}}Cl\], (IV) \[C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}Cl\]

    A)  \[III>II>IV>I\] 

    B)  \[I>II>IV>III\]

    C)  \[II>III>IV>I\]  

    D)  \[I>III>II>IV\]

    Correct Answer: A

    Solution :

    Williamsons ether synthesis involves \[{{S}_{N}}2\]pathway, ie, involves formation of transition state and back side attack. Thus, less hindered alkyl halides react readily in Williamsons synthesis. The order of hindrance is Allyl halide \[<{{1}^{o}}<{{2}^{o}}<{{3}^{o}}\] As the size of halide ion increases, reactivity towards \[{{S}_{N}}2\]reaction increases. Thus, bromides are more reactive than their corresponding chlorides towards Williamsons ether synthesis. Hence, the decreasing order of reactivity towards Williamsons ether synthesis is \[C{{H}_{2}}=\underset{III}{\mathop{C}}\,HC{{H}_{2}}Cl>C\underset{II}{\mathop{{{H}_{2}}C}}\,{{H}_{2}}Br\] \[<C{{H}_{3}}\underset{IV}{\mathop{C{{H}_{2}}C{{H}_{2}}}}\,Cl>M{{e}_{3}}\underset{I}{\mathop{CC{{H}_{2}}}}\,Br\]


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