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Which alkyl halide from the following pairs would you expect to react more rapidly by S_(N)2 mechanism ? Explain your answer. CH_(3)CH_(2)CH_(2)CH_(2) " or " CH_(3)CH_(2) - underset(Br)underset(|)CH - CH_(3) (ii) CH_(3)CH_(2)underset(Br)underset(|)CHCH_(3)" or " H_(3)C - underset(CH_(3))underset(|)overset(CH_(3))overset(|)C-Br (iii) CH_(3)underset(CH_(3))underset(|)CHCH_(2)CH_(2) Br " or " CH_(3)CH_(2)underset(CH_(3))underset(|)CHCH_(2)Br |
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Answer» Solution :(i) `CH_(3)CH_(2)CH_(2)Br " isa " 1^(@) " ALKYL halide while " CH_(3)CH_(2) - CHBR - CH_(3) " is a " 2^(@) ` alkyl halide . There willbe more steric hindrance in `2^(@)` alkyl halides than in `1^(@) ` alkyl halides. Therefore, `CH_(3), CH_(2)CH_(2) Br` will react faster than `CH_(3)CH_(2) - CHBr - CH_(3) " in " S_(N) 2` reaction . (ii) `CH_(3)CH_(2) - CHBr - CH_(3) " is " 2^(@) ` alkyl halide while `(CH_(3))_(3)C - Br " is " 3^(@) ` alkyl halide . Due to lesser steric hindrance, `2^(@) ` alkyl halides react faster than `3^(@)` alkyl halides. Therefore, `CH_(3)CH_(2) - CHBr - CH_(3) ` will reacter than `(CH_(3))_(3)CBr " in " S_(N) 2` reaction . (iii) `overset(4)CH_(3) - UNDERSET(I)underset(CH_(3))underset(|)overset(3)(CH) - overset(2)CH_(2)overset(1)CH_(2)Br "" overset(4)CH_(3)overset(3)CH_(2)- underset(II)underset(CH_(3))underset(|)overset(2)(CH) - overset(1)CH_(2)Br` Both I and II are `1^(@) ` alkyl halides . But in alkyl halide (II) , the `CH_(3) " group at " C_(2) `is CLOSER to the Br atom while in alkyl halide (I) , the `CH_(3)" group at " C_(3)` is little away from the Br atom. As a result , alkylhalide (II) will suffer greater steric hindrance than alkyl halide (I) . Therefore , `(CH_(3))_(2) CHCH_(2)CH_(2) Br` willreact faster than `CH_(3)CH_(2)CH(CH_(3))Br " in " S_(N)2` reaction . |
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