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BCECE Engineering BCECE Engineering Solved Paper-2015

  • question_answer
    The decreasing order of reactivity towards electrophilic addition of the following is (i) \[\text{CH }=\text{CH}\] (ii) \[\text{C}{{\text{H}}_{\text{2}}}=\text{C}{{\text{H}}_{\text{2}}}\]                 (iii) \[{{\text{H}}_{\text{2}}}\text{C}=\text{CH}\text{Cl}\] (iv) \[\text{H}\underset{\text{Cl}}{\mathop{\underset{|}{\mathop{\text{C}}}\,}}\,=\underset{\text{Cl}}{\mathop{\underset{|}{\mathop{\text{C}}}\,}}\,\text{H}\]

    A) \[\text{I }>\text{ II }>\text{ III }>\text{ IV}\]        

    B) \[~\text{II }>\text{ I }>\text{ III }>\text{ IV}\]

    C) \[\text{IV }>\text{ 111 }>\text{ I }>\text{ II}\]         

    D) \[~\text{IV }>\text{ III }>\text{ II }>\text{ I}\]

    Correct Answer: B

    Solution :

    The order of electrophilic addition (EA) is alkene > alkyne. But, if alkene has EWG (electron withdrawing group) reactivity of alkene is less than that of alkyne.                    ' Due to EWG electron density at \[\left( \text{C}=\text{C} \right)\] or nucleophilicity  decreases,  so  addition  of electrophile to \[\left( \text{C}=\text{C} \right)\] is slow. So, the decreasing order of EA is (II)-(alkene) > (I)-(alkyne) > (III)-(alkene with one EWG) > (IV)-(alkene with two EWGs).


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